Checks
Check | Short description |
---|---|
S1??? | Code simplifications |
S1000 | Use plain channel send or receive instead of single-case select |
S1001 | Replace for loop with call to copy |
S1002 | Omit comparison with boolean constant |
S1003 | Replace call to strings.Index with strings.Contains |
S1004 | Replace call to bytes.Compare with bytes.Equal |
S1005 | Drop unnecessary use of the blank identifier |
S1006 | Use for { ... } for infinite loops |
S1007 | Simplify regular expression by using raw string literal |
S1008 | Simplify returning boolean expression |
S1009 | Omit redundant nil check on slices |
S1010 | Omit default slice index |
S1011 | Use a single append to concatenate two slices |
S1012 | Replace time.Now().Sub(x) with time.Since(x) |
S1016 | Use a type conversion instead of manually copying struct fields |
S1017 | Replace manual trimming with strings.TrimPrefix |
S1018 | Use copy for sliding elements |
S1019 | Simplify make call by omitting redundant arguments |
S1020 | Omit redundant nil check in type assertion |
S1021 | Merge variable declaration and assignment |
S1023 | Omit redundant control flow |
S1024 | Replace x.Sub(time.Now()) with time.Until(x) |
S1025 | Don't use fmt.Sprintf("%s", x) unnecessarily |
S1028 | Simplify error construction with fmt.Errorf |
S1029 | Range over the string directly |
S1030 | Use bytes.Buffer.String or bytes.Buffer.Bytes |
S1031 | Omit redundant nil check around loop |
S1032 | Use sort.Ints(x), sort.Float64s(x), and sort.Strings(x) |
S1033 | Unnecessary guard around call to delete |
S1034 | Use result of type assertion to simplify cases |
S1035 | Redundant call to net/http.CanonicalHeaderKey in method call on net/http.Header |
S1036 | Unnecessary guard around map access |
S1037 | Elaborate way of sleeping |
S1038 | Unnecessarily complex way of printing formatted string |
S1039 | Unnecessary use of fmt.Sprint |
S1040 | Type assertion to current type |
SA1??? | Various misuses of the standard library |
SA1000 | Invalid regular expression |
SA1001 | Invalid template |
SA1002 | Invalid format in time.Parse |
SA1003 | Unsupported argument to functions in encoding/binary |
SA1004 | Suspiciously small untyped constant in time.Sleep |
SA1005 | Invalid first argument to exec.Command |
SA1006 | Printf with dynamic first argument and no further arguments |
SA1007 | Invalid URL in net/url.Parse |
SA1008 | Non-canonical key in http.Header map |
SA1010 | (*regexp.Regexp).FindAll called with n == 0, which will always return zero results |
SA1011 | Various methods in the strings package expect valid UTF-8, but invalid input is provided |
SA1012 | A nil context.Context is being passed to a function, consider using context.TODO instead |
SA1013 | io.Seeker.Seek is being called with the whence constant as the first argument, but it should be the second |
SA1014 | Non-pointer value passed to Unmarshal or Decode |
SA1015 | Using time.Tick in a way that will leak. Consider using time.NewTicker, and only use time.Tick in tests, commands and endless functions |
SA1016 | Trapping a signal that cannot be trapped |
SA1017 | Channels used with os/signal.Notify should be buffered |
SA1018 | strings.Replace called with n == 0, which does nothing |
SA1019 | Using a deprecated function, variable, constant or field |
SA1020 | Using an invalid host:port pair with a net.Listen-related function |
SA1021 | Using bytes.Equal to compare two net.IP |
SA1023 | Modifying the buffer in an io.Writer implementation |
SA1024 | A string cutset contains duplicate characters |
SA1025 | It is not possible to use (*time.Timer).Reset's return value correctly |
SA1026 | Cannot marshal channels or functions |
SA1027 | Atomic access to 64-bit variable must be 64-bit aligned |
SA1028 | sort.Slice can only be used on slices |
SA1029 | Inappropriate key in call to context.WithValue |
SA1030 | Invalid argument in call to a strconv function |
SA2??? | Concurrency issues |
SA2000 | sync.WaitGroup.Add called inside the goroutine, leading to a race condition |
SA2001 | Empty critical section, did you mean to defer the unlock? |
SA2002 | Called testing.T.FailNow or SkipNow in a goroutine, which isn't allowed |
SA2003 | Deferred Lock right after locking, likely meant to defer Unlock instead |
SA3??? | Testing issues |
SA3000 | TestMain doesn't call os.Exit, hiding test failures |
SA3001 | Assigning to b.N in benchmarks distorts the results |
SA4??? | Code that isn't really doing anything |
SA4000 | Boolean expression has identical expressions on both sides |
SA4001 | &*x gets simplified to x, it does not copy x |
SA4003 | Comparing unsigned values against negative values is pointless |
SA4004 | The loop exits unconditionally after one iteration |
SA4006 | A value assigned to a variable is never read before being overwritten. Forgotten error check or dead code? |
SA4008 | The variable in the loop condition never changes, are you incrementing the wrong variable? |
SA4009 | A function argument is overwritten before its first use |
SA4010 | The result of append will never be observed anywhere |
SA4011 | Break statement with no effect. Did you mean to break out of an outer loop? |
SA4012 | Comparing a value against NaN even though no value is equal to NaN |
SA4013 | Negating a boolean twice (!!b) is the same as writing b. This is either redundant, or a typo. |
SA4014 | An if/else if chain has repeated conditions and no side-effects; if the condition didn't match the first time, it won't match the second time, either |
SA4015 | Calling functions like math.Ceil on floats converted from integers doesn't do anything useful |
SA4016 | Certain bitwise operations, such as x ^ 0, do not do anything useful |
SA4017 | A pure function's return value is discarded, making the call pointless |
SA4018 | Self-assignment of variables |
SA4019 | Multiple, identical build constraints in the same file |
SA4020 | Unreachable case clause in a type switch |
SA4021 | x = append(y) is equivalent to x = y |
SA4022 | Comparing the address of a variable against nil |
SA4023 | Impossible comparison of interface value with untyped nil |
SA4024 | Checking for impossible return value from a builtin function |
SA4025 | Integer division of literals that results in zero |
SA5??? | Correctness issues |
SA5000 | Assignment to nil map |
SA5001 | Defering Close before checking for a possible error |
SA5002 | The empty for loop (for {}) spins and can block the scheduler |
SA5003 | Defers in infinite loops will never execute |
SA5004 | for { select { ... with an empty default branch spins |
SA5005 | The finalizer references the finalized object, preventing garbage collection |
SA5007 | Infinite recursive call |
SA5008 | Invalid struct tag |
SA5009 | Invalid Printf call |
SA5010 | Impossible type assertion |
SA5011 | Possible nil pointer dereference |
SA5012 | Passing odd-sized slice to function expecting even size |
SA6??? | Performance issues |
SA6000 | Using regexp.Match or related in a loop, should use regexp.Compile |
SA6001 | Missing an optimization opportunity when indexing maps by byte slices |
SA6002 | Storing non-pointer values in sync.Pool allocates memory |
SA6003 | Converting a string to a slice of runes before ranging over it |
SA6005 | Inefficient string comparison with strings.ToLower or strings.ToUpper |
SA9??? | Dubious code constructs that have a high probability of being wrong |
SA9001 | Defers in range loops may not run when you expect them to |
SA9002 | Using a non-octal os.FileMode that looks like it was meant to be in octal. |
SA9003 | Empty body in an if or else branch |
SA9004 | Only the first constant has an explicit type |
SA9005 | Trying to marshal a struct with no public fields nor custom marshaling |
SA9006 | Dubious bit shifting of a fixed size integer value |
ST1??? | Stylistic issues |
ST1000 | Incorrect or missing package comment |
ST1001 | Dot imports are discouraged |
ST1003 | Poorly chosen identifier |
ST1005 | Incorrectly formatted error string |
ST1006 | Poorly chosen receiver name |
ST1008 | A function's error value should be its last return value |
ST1011 | Poorly chosen name for variable of type time.Duration |
ST1012 | Poorly chosen name for error variable |
ST1013 | Should use constants for HTTP error codes, not magic numbers |
ST1015 | A switch's default case should be the first or last case |
ST1016 | Use consistent method receiver names |
ST1017 | Don't use Yoda conditions |
ST1018 | Avoid zero-width and control characters in string literals |
ST1019 | Importing the same package multiple times |
ST1020 | The documentation of an exported function should start with the function's name |
ST1021 | The documentation of an exported type should start with type's name |
ST1022 | The documentation of an exported variable or constant should start with variable's name |
S1000 – Use plain channel send or receive instead of single-case select
Select statements with a single case can be replaced with a simple send or receive.
Before:
select {
case x := <-ch:
fmt.Println(x)
}
After:
x := <-ch
fmt.Println(x)
- Available since
- 2017.1
S1001 – Replace for loop with call to copy
Use copy() for copying elements from one slice to another.
Before:
for i, x := range src {
dst[i] = x
}
After:
copy(dst, src)
- Available since
- 2017.1
S1002 – Omit comparison with boolean constant
Before:
if x == true {}
After:
if x {}
- Available since
- 2017.1
S1003 – Replace call to strings.Index with strings.Contains
Before:
if strings.Index(x, y) != -1 {}
After:
if strings.Contains(x, y) {}
- Available since
- 2017.1
S1004 – Replace call to bytes.Compare with bytes.Equal
Before:
if bytes.Compare(x, y) == 0 {}
After:
if bytes.Equal(x, y) {}
- Available since
- 2017.1
S1005 – Drop unnecessary use of the blank identifier
In many cases, assigning to the blank identifier is unnecessary.
Before:
for _ = range s {}
x, _ = someMap[key]
_ = <-ch
After:
for range s{}
x = someMap[key]
<-ch
- Available since
- 2017.1
S1006 – Use for { ... } for infinite loops
For infinite loops, using for { ... } is the most idiomatic choice.
- Available since
- 2017.1
S1007 – Simplify regular expression by using raw string literal
Raw string literals use ` instead of " and do not support any escape sequences. This means that the backslash (\) can be used freely, without the need of escaping.
Since regular expressions have their own escape sequences, raw strings can improve their readability.
Before:
regexp.Compile("\\A(\\w+) profile: total \\d+\\n\\z")
After:
regexp.Compile(`\A(\w+) profile: total \d+\n\z`)
- Available since
- 2017.1
S1008 – Simplify returning boolean expression
Before:
if <expr> {
return true
}
return false
After:
return <expr>
- Available since
- 2017.1
S1009 – Omit redundant nil check on slices
The len function is defined for all slices, even nil ones, which have a length of zero. It is not necessary to check if a slice is not nil before checking that its length is not zero.
Before:
if x != nil && len(x) != 0 {}
After:
if len(x) != 0 {}
- Available since
- 2017.1
S1010 – Omit default slice index
When slicing, the second index defaults to the length of the value, making s[n:len(s)] and s[n:] equivalent.
- Available since
- 2017.1
S1011 – Use a single append to concatenate two slices
Before:
for _, e := range y {
x = append(x, e)
}
After:
x = append(x, y...)
- Available since
- 2017.1
S1012 – Replace time.Now().Sub(x) with time.Since(x)
The time.Since helper has the same effect as using time.Now().Sub(x) but is easier to read.
Before:
time.Now().Sub(x)
After:
time.Since(x)
- Available since
- 2017.1
S1016 – Use a type conversion instead of manually copying struct fields
Two struct types with identical fields can be converted between each other. In older versions of Go, the fields had to have identical struct tags. Since Go 1.8, however, struct tags are ignored during conversions. It is thus not necessary to manually copy every field individually.
Before:
var x T1
y := T2{
Field1: x.Field1,
Field2: x.Field2,
}
After:
var x T1
y := T2(x)
- Available since
- 2017.1
S1017 – Replace manual trimming with strings.TrimPrefix
Instead of using strings.HasPrefix and manual slicing, use the strings.TrimPrefix function. If the string doesn't start with the prefix, the original string will be returned. Using strings.TrimPrefix reduces complexity, and avoids common bugs, such as off-by-one mistakes.
Before:
if strings.HasPrefix(str, prefix) {
str = str[len(prefix):]
}
After:
str = strings.TrimPrefix(str, prefix)
- Available since
- 2017.1
S1018 – Use copy for sliding elements
copy() permits using the same source and destination slice, even with overlapping ranges. This makes it ideal for sliding elements in a slice.
Before:
for i := 0; i < n; i++ {
bs[i] = bs[offset+i]
}
After:
copy(bs[:n], bs[offset:])
- Available since
- 2017.1
S1019 – Simplify make call by omitting redundant arguments
The make function has default values for the length and capacity arguments. For channels and maps, the length defaults to zero. Additionally, for slices the capacity defaults to the length.
- Available since
- 2017.1
S1020 – Omit redundant nil check in type assertion
Before:
if _, ok := i.(T); ok && i != nil {}
After:
if _, ok := i.(T); ok {}
- Available since
- 2017.1
S1021 – Merge variable declaration and assignment
Before:
var x uint
x = 1
After:
var x uint = 1
- Available since
- 2017.1
S1023 – Omit redundant control flow
Functions that have no return value do not need a return statement as the final statement of the function.
Switches in Go do not have automatic fallthrough, unlike languages like C. It is not necessary to have a break statement as the final statement in a case block.
- Available since
- 2017.1
S1024 – Replace x.Sub(time.Now()) with time.Until(x)
The time.Until helper has the same effect as using x.Sub(time.Now()) but is easier to read.
Before:
x.Sub(time.Now())
After:
time.Until(x)
- Available since
- 2017.1
S1025 – Don't use fmt.Sprintf("%s", x) unnecessarily
In many instances, there are easier and more efficient ways of getting a value's string representation. Whenever a value's underlying type is a string already, or the type has a String method, they should be used directly.
Given the following shared definitions
type T1 string
type T2 int
func (T2) String() string { return "Hello, world" }
var x string
var y T1
var z T2
we can simplify the following
fmt.Sprintf("%s", x)
fmt.Sprintf("%s", y)
fmt.Sprintf("%s", z)
to
x
string(y)
z.String()
- Available since
- 2017.1
S1028 – Simplify error construction with fmt.Errorf
Before:
errors.New(fmt.Sprintf(...))
After:
fmt.Errorf(...)
- Available since
- 2017.1
S1029 – Range over the string directly
Ranging over a string will yield byte offsets and runes. If the offset isn't used, this is functionally equivalent to converting the string to a slice of runes and ranging over that. Ranging directly over the string will be more performant, however, as it avoids allocating a new slice, the size of which depends on the length of the string.
Before:
for _, r := range []rune(s) {}
After:
for _, r := range s {}
- Available since
- 2017.1
S1030 – Use bytes.Buffer.String or bytes.Buffer.Bytes
bytes.Buffer has both a String and a Bytes method. It is almost never necessary to use string(buf.Bytes()) or []byte(buf.String()) – simply use the other method.
The only exception to this are map lookups. Due to a compiler optimization, m[string(buf.Bytes())] is more efficient than m[buf.String()].
- Available since
- 2017.1
S1031 – Omit redundant nil check around loop
You can use range on nil slices and maps, the loop will simply never execute. This makes an additional nil check around the loop unnecessary.
Before:
if s != nil {
for _, x := range s {
...
}
}
After:
for _, x := range s {
...
}
- Available since
- 2017.1
S1032 – Use sort.Ints(x), sort.Float64s(x), and sort.Strings(x)
The sort.Ints, sort.Float64s and sort.Strings functions are easier to read than sort.Sort(sort.IntSlice(x)), sort.Sort(sort.Float64Slice(x)) and sort.Sort(sort.StringSlice(x)).
Before:
sort.Sort(sort.StringSlice(x))
After:
sort.Strings(x)
- Available since
- 2019.1
S1033 – Unnecessary guard around call to delete
Calling delete on a nil map is a no-op.
- Available since
- 2019.2
S1034 – Use result of type assertion to simplify cases
- Available since
- 2019.2
S1035 – Redundant call to net/http.CanonicalHeaderKey in method call on net/http.Header
The methods on net/http.Header, namely Add, Del, Get and Set, already canonicalize the given header name.
- Available since
- 2020.1
S1036 – Unnecessary guard around map access
When accessing a map key that doesn't exist yet, one receives a zero value. Often, the zero value is a suitable value, for example when using append or doing integer math.
The following
if _, ok := m["foo"]; ok {
m["foo"] = append(m["foo"], "bar")
} else {
m["foo"] = []string{"bar"}
}
can be simplified to
m["foo"] = append(m["foo"], "bar")
and
if _, ok := m2["k"]; ok {
m2["k"] += 4
} else {
m2["k"] = 4
}
can be simplified to
m["k"] += 4
- Available since
- 2020.1
S1037 – Elaborate way of sleeping
Using a select statement with a single case receiving from the result of time.After is a very elaborate way of sleeping that can much simpler be expressed with a simple call to time.Sleep.
- Available since
- 2020.1
S1038 – Unnecessarily complex way of printing formatted string
Instead of using fmt.Print(fmt.Sprintf(...)), one can use fmt.Printf(...).
- Available since
- 2020.1
S1039 – Unnecessary use of fmt.Sprint
Calling fmt.Sprint with a single string argument is unnecessary and identical to using the string directly.
- Available since
- 2020.1
S1040 – Type assertion to current type
The type assertion 'x.(SomeInterface)', when 'x' already has type 'SomeInterface', can only fail if 'x' is nil. Usually, this is left-over code from when 'x' had a different type and you can safely delete the type assertion. If you want to check that 'x' is not nil, consider being explicit and using an actual 'if x == nil' comparison instead of relying on the type assertion panicking.
- Available since
- Unreleased
SA1000 – Invalid regular expression
- Available since
- 2017.1
SA1001 – Invalid template
- Available since
- 2017.1
SA1002 – Invalid format in time.Parse
- Available since
- 2017.1
SA1003 – Unsupported argument to functions in encoding/binary
The encoding/binary package can only serialize types with known sizes. This precludes the use of the int and uint types, as their sizes differ on different architectures. Furthermore, it doesn't support serializing maps, channels, strings, or functions.
Before Go 1.8, bool wasn't supported, either.
- Available since
- 2017.1
SA1004 – Suspiciously small untyped constant in time.Sleep
The time.Sleep function takes a time.Duration as its only argument. Durations are expressed in nanoseconds. Thus, calling time.Sleep(1) will sleep for 1 nanosecond. This is a common source of bugs, as sleep functions in other languages often accept seconds or milliseconds.
The time package provides constants such as time.Second to express large durations. These can be combined with arithmetic to express arbitrary durations, for example '5 * time.Second' for 5 seconds.
If you truly meant to sleep for a tiny amount of time, use 'n * time.Nanosecond' to signal to Staticcheck that you did mean to sleep for some amount of nanoseconds.
- Available since
- 2017.1
SA1005 – Invalid first argument to exec.Command
os/exec runs programs directly (using variants of the fork and exec system calls on Unix systems). This shouldn't be confused with running a command in a shell. The shell will allow for features such as input redirection, pipes, and general scripting. The shell is also responsible for splitting the user's input into a program name and its arguments. For example, the equivalent to
ls / /tmp
would be
exec.Command("ls", "/", "/tmp")
If you want to run a command in a shell, consider using something like the following – but be aware that not all systems, particularly Windows, will have a /bin/sh program:
exec.Command("/bin/sh", "-c", "ls | grep Awesome")
- Available since
- 2017.1
SA1006 – Printf with dynamic first argument and no further arguments
Using fmt.Printf with a dynamic first argument can lead to unexpected output. The first argument is a format string, where certain character combinations have special meaning. If, for example, a user were to enter a string such as
Interest rate: 5%
and you printed it with
fmt.Printf(s)
it would lead to the following output:
Interest rate: 5%!(NOVERB).
Similarly, forming the first parameter via string concatenation with user input should be avoided for the same reason. When printing user input, either use a variant of fmt.Print, or use the %s Printf verb and pass the string as an argument.
- Available since
- 2017.1
SA1007 – Invalid URL in net/url.Parse
- Available since
- 2017.1
SA1008 – Non-canonical key in http.Header map
Keys in http.Header maps are canonical, meaning they follow a specific combination of uppercase and lowercase letters. Methods such as http.Header.Add and http.Header.Del convert inputs into this canonical form before manipulating the map.
When manipulating http.Header maps directly, as opposed to using the provided methods, care should be taken to stick to canonical form in order to avoid inconsistencies. The following piece of code demonstrates one such inconsistency:
h := http.Header{}
h["etag"] = []string{"1234"}
h.Add("etag", "5678")
fmt.Println(h)
// Output:
// map[Etag:[5678] etag:[1234]]
The easiest way of obtaining the canonical form of a key is to use http.CanonicalHeaderKey.
- Available since
- 2017.1
SA1010 – (*regexp.Regexp).FindAll called with n == 0, which will always return zero results
If n >= 0, the function returns at most n matches/submatches. To return all results, specify a negative number.
- Available since
- 2017.1
SA1011 – Various methods in the strings package expect valid UTF-8, but invalid input is provided
- Available since
- 2017.1
SA1012 – A nil context.Context is being passed to a function, consider using context.TODO instead
- Available since
- 2017.1
SA1013 – io.Seeker.Seek is being called with the whence constant as the first argument, but it should be the second
- Available since
- 2017.1
SA1014 – Non-pointer value passed to Unmarshal or Decode
- Available since
- 2017.1
SA1015 – Using time.Tick in a way that will leak. Consider using time.NewTicker, and only use time.Tick in tests, commands and endless functions
- Available since
- 2017.1
SA1016 – Trapping a signal that cannot be trapped
Not all signals can be intercepted by a process. Speficially, on UNIX-like systems, the syscall.SIGKILL and syscall.SIGSTOP signals are never passed to the process, but instead handled directly by the kernel. It is therefore pointless to try and handle these signals.
- Available since
- 2017.1
SA1017 – Channels used with os/signal.Notify should be buffered
The os/signal package uses non-blocking channel sends when delivering signals. If the receiving end of the channel isn't ready and the channel is either unbuffered or full, the signal will be dropped. To avoid missing signals, the channel should be buffered and of the appropriate size. For a channel used for notification of just one signal value, a buffer of size 1 is sufficient.
- Available since
- 2017.1
SA1018 – strings.Replace called with n == 0, which does nothing
With n == 0, zero instances will be replaced. To replace all instances, use a negative number, or use strings.ReplaceAll.
- Available since
- 2017.1
SA1019 – Using a deprecated function, variable, constant or field
- Available since
- 2017.1
SA1020 – Using an invalid host:port pair with a net.Listen-related function
- Available since
- 2017.1
SA1021 – Using bytes.Equal to compare two net.IP
A net.IP stores an IPv4 or IPv6 address as a slice of bytes. The length of the slice for an IPv4 address, however, can be either 4 or 16 bytes long, using different ways of representing IPv4 addresses. In order to correctly compare two net.IPs, the net.IP.Equal method should be used, as it takes both representations into account.
- Available since
- 2017.1
SA1023 – Modifying the buffer in an io.Writer implementation
Write must not modify the slice data, even temporarily.
- Available since
- 2017.1
SA1024 – A string cutset contains duplicate characters
The strings.TrimLeft and strings.TrimRight functions take cutsets, not prefixes. A cutset is treated as a set of characters to remove from a string. For example,
strings.TrimLeft("42133word", "1234"))
will result in the string "word" – any characters that are 1, 2, 3 or 4 are cut from the left of the string.
In order to remove one string from another, use strings.TrimPrefix instead.
- Available since
- 2017.1
SA1025 – It is not possible to use (*time.Timer).Reset's return value correctly
- Available since
- 2019.1
SA1026 – Cannot marshal channels or functions
- Available since
- 2019.2
SA1027 – Atomic access to 64-bit variable must be 64-bit aligned
On ARM, x86-32, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically. The first word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned.
You can use the structlayout tool to inspect the alignment of fields in a struct.
- Available since
- 2019.2
SA1028 – sort.Slice can only be used on slices
The first argument of sort.Slice must be a slice.
- Available since
- 2020.1
SA1029 – Inappropriate key in call to context.WithValue
The provided key must be comparable and should not be of type string or any other built-in type to avoid collisions between packages using context. Users of WithValue should define their own types for keys.
To avoid allocating when assigning to an interface{}, context keys often have concrete type struct{}. Alternatively, exported context key variables' static type should be a pointer or interface.
- Available since
- 2020.1
SA1030 – Invalid argument in call to a strconv function
This check validates the format, number base and bit size arguments of the various parsing and formatting functions in strconv.
- Available since
- Unreleased
SA2000 – sync.WaitGroup.Add called inside the goroutine, leading to a race condition
- Available since
- 2017.1
SA2001 – Empty critical section, did you mean to defer the unlock?
Empty critical sections of the kind
mu.Lock()
mu.Unlock()
are very often a typo, and the following was intended instead:
mu.Lock()
defer mu.Unlock()
Do note that sometimes empty critical sections can be useful, as a form of signaling to wait on another goroutine. Many times, there are simpler ways of achieving the same effect. When that isn't the case, the code should be amply commented to avoid confusion. Combining such comments with a //lint:ignore directive can be used to suppress this rare false positive.
- Available since
- 2017.1
SA2002 – Called testing.T.FailNow or SkipNow in a goroutine, which isn't allowed
- Available since
- 2017.1
SA2003 – Deferred Lock right after locking, likely meant to defer Unlock instead
- Available since
- 2017.1
SA3000 – TestMain doesn't call os.Exit, hiding test failures
Test executables (and in turn 'go test') exit with a non-zero status code if any tests failed. When specifying your own TestMain function, it is your responsibility to arrange for this, by calling os.Exit with the correct code. The correct code is returned by (*testing.M).Run, so the usual way of implementing TestMain is to end it with os.Exit(m.Run()).
- Available since
- 2017.1
SA3001 – Assigning to b.N in benchmarks distorts the results
The testing package dynamically sets b.N to improve the reliability of benchmarks and uses it in computations to determine the duration of a single operation. Benchmark code must not alter b.N as this would falsify results.
- Available since
- 2017.1
SA4000 – Boolean expression has identical expressions on both sides
- Available since
- 2017.1
SA4001 – &*x gets simplified to x, it does not copy x
- Available since
- 2017.1
SA4003 – Comparing unsigned values against negative values is pointless
- Available since
- 2017.1
SA4004 – The loop exits unconditionally after one iteration
- Available since
- 2017.1
SA4006 – A value assigned to a variable is never read before being overwritten. Forgotten error check or dead code?
- Available since
- 2017.1
SA4008 – The variable in the loop condition never changes, are you incrementing the wrong variable?
- Available since
- 2017.1
SA4009 – A function argument is overwritten before its first use
- Available since
- 2017.1
SA4010 – The result of append will never be observed anywhere
- Available since
- 2017.1
SA4011 – Break statement with no effect. Did you mean to break out of an outer loop?
- Available since
- 2017.1
SA4012 – Comparing a value against NaN even though no value is equal to NaN
- Available since
- 2017.1
SA4013 – Negating a boolean twice (!!b) is the same as writing b. This is either redundant, or a typo.
- Available since
- 2017.1
SA4014 – An if/else if chain has repeated conditions and no side-effects; if the condition didn't match the first time, it won't match the second time, either
- Available since
- 2017.1
SA4015 – Calling functions like math.Ceil on floats converted from integers doesn't do anything useful
- Available since
- 2017.1
SA4016 – Certain bitwise operations, such as x ^ 0, do not do anything useful
- Available since
- 2017.1
SA4017 – A pure function's return value is discarded, making the call pointless
- Available since
- 2017.1
SA4018 – Self-assignment of variables
- Available since
- 2017.1
SA4019 – Multiple, identical build constraints in the same file
- Available since
- 2017.1
SA4020 – Unreachable case clause in a type switch
In a type switch like the following
type T struct{}
func (T) Read(b []byte) (int, error) { return 0, nil }
var v interface{} = T{}
switch v.(type) {
case io.Reader:
// ...
case T:
// unreachable
}
the second case clause can never be reached because T implements io.Reader and case clauses are evaluated in source order.
Another example:
type T struct{}
func (T) Read(b []byte) (int, error) { return 0, nil }
func (T) Close() error { return nil }
var v interface{} = T{}
switch v.(type) {
case io.Reader:
// ...
case io.ReadCloser:
// unreachable
}
Even though T has a Close method and thus implements io.ReadCloser, io.Reader will always match first. The method set of io.Reader is a subset of io.ReadCloser. Thus it is impossible to match the second case without matching the first case.
Structurally equivalent interfaces
A special case of the previous example are structurally identical interfaces. Given these declarations
type T error
type V error
func doSomething() error {
err, ok := doAnotherThing()
if ok {
return T(err)
}
return U(err)
}
the following type switch will have an unreachable case clause:
switch doSomething().(type) {
case T:
// ...
case V:
// unreachable
}
T will always match before V because they are structurally equivalent and therefore doSomething()'s return value implements both.
- Available since
- 2019.2
SA4021 – x = append(y) is equivalent to x = y
- Available since
- 2019.2
SA4022 – Comparing the address of a variable against nil
Code such as 'if &x == nil' is meaningless, because taking the address of a variable always yields a non-nil pointer.
- Available since
- 2020.1
SA4023 – Impossible comparison of interface value with untyped nil
Under the covers, interfaces are implemented as two elements, a type T and a value V. V is a concrete value such as an int, struct or pointer, never an interface itself, and has type T. For instance, if we store the int value 3 in an interface, the resulting interface value has, schematically, (T=int, V=3). The value V is also known as the interface's dynamic value, since a given interface variable might hold different values V (and corresponding types T) during the execution of the program.
An interface value is nil only if the V and T are both unset, (T=nil, V is not set), In particular, a nil interface will always hold a nil type. If we store a nil pointer of type *int inside an interface value, the inner type will be *int regardless of the value of the pointer: (T=*int, V=nil). Such an interface value will therefore be non-nil even when the pointer value V inside is nil.
This situation can be confusing, and arises when a nil value is stored inside an interface value such as an error return:
func returnsError() error {
var p *MyError = nil
if bad() {
p = ErrBad
}
return p // Will always return a non-nil error.
}
If all goes well, the function returns a nil p, so the return value is an error interface value holding (T=*MyError, V=nil). This means that if the caller compares the returned error to nil, it will always look as if there was an error even if nothing bad happened. To return a proper nil error to the caller, the function must return an explicit nil:
func returnsError() error {
if bad() {
return ErrBad
}
return nil
}
It's a good idea for functions that return errors always to use the error type in their signature (as we did above) rather than a concrete type such as *MyError, to help guarantee the error is created correctly. As an example, os.Open returns an error even though, if not nil, it's always of concrete type *os.PathError.
Similar situations to those described here can arise whenever interfaces are used. Just keep in mind that if any concrete value has been stored in the interface, the interface will not be nil. For more information, see The Laws of Reflection (https://golang.org/doc/articles/laws_of_reflection.html).
This text has been copied from https://golang.org/doc/faq#nil_error, licensed under the Creative Commons Attribution 3.0 License.
- Available since
- 2020.2
SA4024 – Checking for impossible return value from a builtin function
Return values the len and cap builtins cannot be negative.
See https://golang.org/pkg/builtin/#len and https://golang.org/pkg/builtin/#cap.
Example:
if len(slice) < 0 {
fmt.Println("unreachable code")
}
- Available since
- Unreleased
SA4025 – Integer division of literals that results in zero
When dividing two integer constants, the result will also be an integer. Thus, a division such as '2 / 3' results in '0'. This is true for all of the following examples:
_ = 2 / 3 const _ = 2 / 3 const _ float64 = 2 / 3 _ = float64(2 / 3)
Staticcheck will flag such divisions if both sides of the division are integer literals, as it is highly unlikely that the division was intended to truncate to zero. Staticcheck will not flag integer division involving named constants, to avoid noisy positives.
- Available since
- Unreleased
SA5000 – Assignment to nil map
- Available since
- 2017.1
SA5001 – Defering Close before checking for a possible error
- Available since
- 2017.1
SA5002 – The empty for loop (for {}) spins and can block the scheduler
- Available since
- 2017.1
SA5003 – Defers in infinite loops will never execute
Defers are scoped to the surrounding function, not the surrounding block. In a function that never returns, i.e. one containing an infinite loop, defers will never execute.
- Available since
- 2017.1
SA5004 – for { select { ... with an empty default branch spins
- Available since
- 2017.1
SA5005 – The finalizer references the finalized object, preventing garbage collection
A finalizer is a function associated with an object that runs when the garbage collector is ready to collect said object, that is when the object is no longer referenced by anything.
If the finalizer references the object, however, it will always remain as the final reference to that object, preventing the garbage collector from collecting the object. The finalizer will never run, and the object will never be collected, leading to a memory leak. That is why the finalizer should instead use its first argument to operate on the object. That way, the number of references can temporarily go to zero before the object is being passed to the finalizer.
- Available since
- 2017.1
SA5007 – Infinite recursive call
A function that calls itself recursively needs to have an exit condition. Otherwise it will recurse forever, until the system runs out of memory.
This issue can be caused by simple bugs such as forgetting to add an exit condition. It can also happen "on purpose". Some languages have tail call optimization which makes certain infinite recursive calls safe to use. Go, however, does not implement TCO, and as such a loop should be used instead.
- Available since
- 2017.1
SA5008 – Invalid struct tag
- Available since
- 2019.2
SA5009 – Invalid Printf call
- Available since
- 2019.2
SA5010 – Impossible type assertion
Some type assertions can be statically proven to be impossible. This is the case when the method sets of both arguments of the type assertion conflict with each other, for example by containing the same method with different signatures.
The Go compiler already applies this check when asserting from an interface value to a concrete type. If the concrete type misses methods from the interface, or if function signatures don't match, then the type assertion can never succeed.
This check applies the same logic when asserting from one interface to another. If both interface types contain the same method but with different signatures, then the type assertion can never succeed, either.
- Available since
- 2020.1
SA5011 – Possible nil pointer dereference
A pointer is being dereferenced unconditionally, while also being checked against nil in another place. This suggests that the pointer may be nil and dereferencing it may panic. This is commonly a result of improperly ordered code or missing return statements. Consider the following examples:
func fn(x *int) {
fmt.Println(*x)
// This nil check is equally important for the previous dereference
if x != nil {
foo(*x)
}
}
func TestFoo(t *testing.T) {
x := compute()
if x == nil {
t.Errorf("nil pointer received")
}
// t.Errorf does not abort the test, so if x is nil, the next line will panic.
foo(*x)
}
Staticcheck tries to deduce which functions abort control flow. For example, it is aware that a function will not continue execution after a call to panic or log.Fatal. However, sometimes this detection fails, in particular in the presence of conditionals. Consider the following example:
func Log(msg string, level int) {
fmt.Println(msg)
if level == levelFatal {
os.Exit(1)
}
}
func Fatal(msg string) {
Log(msg, levelFatal)
}
func fn(x *int) {
if x == nil {
Fatal("unexpected nil pointer")
}
fmt.Println(*x)
}
Staticcheck will flag the dereference of x, even though it is perfectly safe. Staticcheck is not able to deduce that a call to Fatal will exit the program. For the time being, the easiest workaround is to modify the definition of Fatal like so:
func Fatal(msg string) {
Log(msg, levelFatal)
panic("unreachable")
}
We also hard-code functions from common logging packages such as logrus. Please file an issue if we're missing support for a popular package.
- Available since
- 2020.1
SA5012 – Passing odd-sized slice to function expecting even size
Some functions that take slices as parameters expect the slices to have an even number of elements. Often, these functions treat elements in a slice as pairs. For example, strings.NewReplacer takes pairs of old and new strings, and calling it with an odd number of elements would be an error.
- Available since
- 2020.2
SA6000 – Using regexp.Match or related in a loop, should use regexp.Compile
- Available since
- 2017.1
SA6001 – Missing an optimization opportunity when indexing maps by byte slices
Map keys must be comparable, which precludes the use of byte slices. This usually leads to using string keys and converting byte slices to strings.
Normally, a conversion of a byte slice to a string needs to copy the data and causes allocations. The compiler, however, recognizes m[string(b)] and uses the data of b directly, without copying it, because it knows that the data can't change during the map lookup. This leads to the counter-intuitive situation that
k := string(b)
println(m[k])
println(m[k])
will be less efficient than
println(m[string(b)])
println(m[string(b)])
because the first version needs to copy and allocate, while the second one does not.
For some history on this optimization, check out commit f5f5a8b6209f84961687d993b93ea0d397f5d5bf in the Go repository.
- Available since
- 2017.1
SA6002 – Storing non-pointer values in sync.Pool allocates memory
A sync.Pool is used to avoid unnecessary allocations and reduce the amount of work the garbage collector has to do.
When passing a value that is not a pointer to a function that accepts an interface, the value needs to be placed on the heap, which means an additional allocation. Slices are a common thing to put in sync.Pools, and they're structs with 3 fields (length, capacity, and a pointer to an array). In order to avoid the extra allocation, one should store a pointer to the slice instead.
See the comments on https://go-review.googlesource.com/c/go/+/24371 that discuss this problem.
- Available since
- 2017.1
SA6003 – Converting a string to a slice of runes before ranging over it
You may want to loop over the runes in a string. Instead of converting the string to a slice of runes and looping over that, you can loop over the string itself. That is,
for _, r := range s {}
and
for _, r := range []rune(s) {}
will yield the same values. The first version, however, will be faster and avoid unnecessary memory allocations.
Do note that if you are interested in the indices, ranging over a string and over a slice of runes will yield different indices. The first one yields byte offsets, while the second one yields indices in the slice of runes.
- Available since
- 2017.1
SA6005 – Inefficient string comparison with strings.ToLower or strings.ToUpper
Converting two strings to the same case and comparing them like so
if strings.ToLower(s1) == strings.ToLower(s2) {
...
}
is significantly more expensive than comparing them with strings.EqualFold(s1, s2). This is due to memory usage as well as computational complexity.
strings.ToLower will have to allocate memory for the new strings, as well as convert both strings fully, even if they differ on the very first byte. strings.EqualFold, on the other hand, compares the strings one character at a time. It doesn't need to create two intermediate strings and can return as soon as the first non-matching character has been found.
For a more in-depth explanation of this issue, see https://blog.digitalocean.com/how-to-efficiently-compare-strings-in-go/
- Available since
- 2019.2
SA9001 – Defers in range loops may not run when you expect them to
- Available since
- 2017.1
SA9002 – Using a non-octal os.FileMode that looks like it was meant to be in octal.
- Available since
- 2017.1
SA9003 – Empty body in an if or else branch
- Available since
- 2017.1
SA9004 – Only the first constant has an explicit type
In a constant declaration such as the following:
const (
First byte = 1
Second = 2
)
the constant Second does not have the same type as the constant First. This construct shouldn't be confused with
const (
First byte = iota
Second
)
where First and Second do indeed have the same type. The type is only passed on when no explicit value is assigned to the constant.
When declaring enumerations with explicit values it is therefore important not to write
const (
EnumFirst EnumType = 1
EnumSecond = 2
EnumThird = 3
)
This discrepancy in types can cause various confusing behaviors and bugs.
Wrong type in variable declarations
The most obvious issue with such incorrect enumerations expresses itself as a compile error:
package pkg
const (
EnumFirst uint8 = 1
EnumSecond = 2
)
func fn(useFirst bool) {
x := EnumSecond
if useFirst {
x = EnumFirst
}
}
fails to compile with
./const.go:11:5: cannot use EnumFirst (type uint8) as type int in assignment
Losing method sets
A more subtle issue occurs with types that have methods and optional interfaces. Consider the following:
package main
import "fmt"
type Enum int
func (e Enum) String() string {
return "an enum"
}
const (
EnumFirst Enum = 1
EnumSecond = 2
)
func main() {
fmt.Println(EnumFirst)
fmt.Println(EnumSecond)
}
This code will output
an enum
2
as EnumSecond has no explicit type, and thus defaults to int.
- Available since
- 2019.1
SA9005 – Trying to marshal a struct with no public fields nor custom marshaling
The encoding/json and encoding/xml packages only operate on exported fields in structs, not unexported ones. It is usually an error to try to (un)marshal structs that only consist of unexported fields.
This check will not flag calls involving types that define custom marshaling behavior, e.g. via MarshalJSON methods. It will also not flag empty structs.
- Available since
- 2019.2
SA9006 – Dubious bit shifting of a fixed size integer value
Bit shifting a value past its size will always clear the value.
For instance:
v := int8(42)
v >>= 8
will always result in 0.
This check flags bit shifiting operations on fixed size integer values only. That is, int, uint and uintptr are never flagged to avoid potential false positives in somewhat exotic but valid bit twiddling tricks:
// Clear any value above 32 bits if integers are more than 32 bits.
func f(i int) int {
v := i >> 32
v = v << 32
return i-v
}
- Available since
- 2020.2
ST1000 – Incorrect or missing package comment non-default
Packages must have a package comment that is formatted according to the guidelines laid out in https://github.com/golang/go/wiki/CodeReviewComments#package-comments.
- Available since
- 2019.1
ST1001 – Dot imports are discouraged
Dot imports that aren't in external test packages are discouraged.
The dot_import_whitelist option can be used to whitelist certain imports.
Quoting Go Code Review Comments:
The import . form can be useful in tests that, due to circular dependencies, cannot be made part of the package being tested: package foo_test import ( "bar/testutil" // also imports "foo" . "foo" ) In this case, the test file cannot be in package foo because it uses bar/testutil, which imports foo. So we use the 'import .' form to let the file pretend to be part of package foo even though it is not. Except for this one case, do not use import . in your programs. It makes the programs much harder to read because it is unclear whether a name like Quux is a top-level identifier in the current package or in an imported package.
- Available since
- 2019.1
- Options
ST1003 – Poorly chosen identifier non-default
Identifiers, such as variable and package names, follow certain rules.
See the following links for details:
- https://golang.org/doc/effective_go.html#package-names
- https://golang.org/doc/effective_go.html#mixed-caps
- https://github.com/golang/go/wiki/CodeReviewComments#initialisms
- https://github.com/golang/go/wiki/CodeReviewComments#variable-names
- Available since
- 2019.1
- Options
ST1005 – Incorrectly formatted error string
Error strings follow a set of guidelines to ensure uniformity and good composability.
Quoting Go Code Review Comments:
Error strings should not be capitalized (unless beginning with proper nouns or acronyms) or end with punctuation, since they are usually printed following other context. That is, use fmt.Errorf("something bad") not fmt.Errorf("Something bad"), so that log.Printf("Reading %s: %v", filename, err) formats without a spurious capital letter mid-message.
- Available since
- 2019.1
ST1006 – Poorly chosen receiver name
Quoting Go Code Review Comments:
The name of a method's receiver should be a reflection of its identity; often a one or two letter abbreviation of its type suffices (such as "c" or "cl" for "Client"). Don't use generic names such as "me", "this" or "self", identifiers typical of object-oriented languages that place more emphasis on methods as opposed to functions. The name need not be as descriptive as that of a method argument, as its role is obvious and serves no documentary purpose. It can be very short as it will appear on almost every line of every method of the type; familiarity admits brevity. Be consistent, too: if you call the receiver "c" in one method, don't call it "cl" in another.
- Available since
- 2019.1
ST1008 – A function's error value should be its last return value
A function's error value should be its last return value.
- Available since
- 2019.1
ST1011 – Poorly chosen name for variable of type time.Duration
time.Duration values represent an amount of time, which is represented as a count of nanoseconds. An expression like 5 * time.Microsecond yields the value 5000. It is therefore not appropriate to suffix a variable of type time.Duration with any time unit, such as Msec or Milli.
- Available since
- 2019.1
ST1012 – Poorly chosen name for error variable
Error variables that are part of an API should be called errFoo or ErrFoo.
- Available since
- 2019.1
ST1013 – Should use constants for HTTP error codes, not magic numbers
HTTP has a tremendous number of status codes. While some of those are well known (200, 400, 404, 500), most of them are not. The net/http package provides constants for all status codes that are part of the various specifications. It is recommended to use these constants instead of hard-coding magic numbers, to vastly improve the readability of your code.
- Available since
- 2019.1
- Options
ST1015 – A switch's default case should be the first or last case
- Available since
- 2019.1
ST1016 – Use consistent method receiver names non-default
- Available since
- 2019.1
ST1017 – Don't use Yoda conditions
Yoda conditions are conditions of the kind 'if 42 == x', where the literal is on the left side of the comparison. These are a common idiom in languages in which assignment is an expression, to avoid bugs of the kind 'if (x = 42)'. In Go, which doesn't allow for this kind of bug, we prefer the more idiomatic 'if x == 42'.
- Available since
- 2019.2
ST1018 – Avoid zero-width and control characters in string literals
- Available since
- 2019.2
ST1019 – Importing the same package multiple times
Go allows importing the same package multiple times, as long as different import aliases are being used. That is, the following bit of code is valid:
import (
"fmt"
fumpt "fmt"
format "fmt"
_ "fmt"
)
However, this is very rarely done on purpose. Usually, it is a sign of code that got refactored, accidentally adding duplicate import statements. It is also a rarely known feature, which may contribute to confusion.
Do note that sometimes, this feature may be used intentionally (see for example https://github.com/golang/go/commit/3409ce39bfd7584523b7a8c150a310cea92d879d) – if you want to allow this pattern in your code base, you're advised to disable this check.
- Available since
- 2020.1
ST1020 – The documentation of an exported function should start with the function's name non-default
Doc comments work best as complete sentences, which allow a wide variety of automated presentations. The first sentence should be a one-sentence summary that starts with the name being declared.
If every doc comment begins with the name of the item it describes, you can use the doc subcommand of the go tool and run the output through grep.
See https://golang.org/doc/effective_go.html#commentary for more information on how to write good documentation.
- Available since
- 2020.1
ST1021 – The documentation of an exported type should start with type's name non-default
Doc comments work best as complete sentences, which allow a wide variety of automated presentations. The first sentence should be a one-sentence summary that starts with the name being declared.
If every doc comment begins with the name of the item it describes, you can use the doc subcommand of the go tool and run the output through grep.
See https://golang.org/doc/effective_go.html#commentary for more information on how to write good documentation.
- Available since
- 2020.1
ST1022 – The documentation of an exported variable or constant should start with variable's name non-default
Doc comments work best as complete sentences, which allow a wide variety of automated presentations. The first sentence should be a one-sentence summary that starts with the name being declared.
If every doc comment begins with the name of the item it describes, you can use the doc subcommand of the go tool and run the output through grep.
See https://golang.org/doc/effective_go.html#commentary for more information on how to write good documentation.
- Available since
- 2020.1