errcheck

errcheck is a program for checking for unchecked errors in Go code.

It is forked from https://github.com/kisielk/errcheck but unlike the original allows you to write defer foo.Close().

Why a fork

defer h.Close() is idiomatic Go, which appears regularly in the standard library and text books. If h is a read only descriptor, then nothing useful can be done if the Close fails.

This has been raised a number of times upstream, but these issues have been closed, as the author believes that these errors should always be checked, and if necessary deferred calls to Close should be wrapped in a lambda.

This unecessarily complicates code, and means that the Go standard library will fail errcheck.

The Author is, of course, entitled to his opinion. But those who disagree with him have the right to fork and do otherwise.

Install

go install github.com/getkalido/errcheck@latest

errcheck requires Go 1.18 or newer.

Use

For basic usage, just give the package path of interest as the first argument:

errcheck github.com/kisielk/errcheck/testdata

To check all packages beneath the current directory:

errcheck ./...

Or check all packages in your $GOPATH and $GOROOT:

errcheck all

errcheck also recognizes the following command-line options:

The -tags flag takes a space-separated list of build tags, just like go build. If you are using any custom build tags in your code base, you may need to specify the relevant tags here.

The -asserts flag enables checking for ignored type assertion results. It takes no arguments.

The -blank flag enables checking for assignments of errors to the blank identifier. It takes no arguments.

The -abspath flag prints the absolute paths to files with unchecked errors.

The -mod flag sets the module download mode to use: readonly or vendor.

go/analysis

The package provides Analyzer instance that can be used with go/analysis API.

Currently supported flags are blank, assert, exclude, and excludeonly. Just as the API itself, the analyzer is experimental and may change in the future.

Excluding functions

Use the -exclude flag to specify a path to a file containing a list of functions to be excluded.

errcheck -exclude errcheck_excludes.txt path/to/package

The file should contain one function signature per line. The format for function signatures is package.FunctionName while for methods it's (package.Receiver).MethodName for value receivers and (*package.Receiver).MethodName for pointer receivers. If the function name is followed by string of form (TYPE), then the the function call is excluded only if the type of the first argument is TYPE. It also accepts a special suffix (os.Stdout) and (os.Stderr), which excludes the function only when the first argument is a literal os.Stdout or os.Stderr.

An example of an exclude file is:

io.Copy(*bytes.Buffer)
io.Copy(os.Stdout)
os.ReadFile

// Sometimes we don't care if a HTTP request fails.
(*net/http.Client).Do

By default, the exclude list is combined with an internal list for functions in the Go standard library that have an error return type but are documented to never return an error. To disable the built-in exclude list, pass the -excludeonly flag.

Run errcheck in -verbose mode to see the resulting list of added excludes.

When using vendored dependencies, specify the full import path. For example:

• Your project's import path is example.com/yourpkg
• You've vendored example.net/fmt2 as vendor/example.net/fmt2
• You want to exclude fmt2.Println from error checking

In this case, add this line to your exclude file:

example.com/yourpkg/vendor/example.net/fmt2.Println

Empty lines and lines starting with // are ignored.

The deprecated method

The -ignore flag takes a comma-separated list of pairs of the form package:regex. For each package, the regex describes which functions to ignore within that package. The package may be omitted to have the regex apply to all packages.

For example, you may wish to ignore common operations like Read and Write:

errcheck -ignore '[rR]ead|[wW]rite' path/to/package

or you may wish to ignore common functions like the print variants in fmt:

errcheck -ignore 'fmt:[FS]?[Pp]rint*' path/to/package

The -ignorepkg flag takes a comma-separated list of package import paths to ignore:

errcheck -ignorepkg 'fmt,encoding/binary' path/to/package

Note that this is equivalent to:

errcheck -ignore 'fmt:.*,encoding/binary:.*' path/to/package

If a regex is provided for a package pkg via -ignore, and pkg also appears in the list of packages passed to -ignorepkg, the latter takes precedence; that is, all functions within pkg will be ignored.

Note that by default the fmt package is ignored entirely, unless a regex is specified for it. To disable this, specify a regex that matches nothing:

errcheck -ignore 'fmt:a^' path/to/package

The -ignoretests flag disables checking of _test.go files. It takes no arguments.

The -ignoregenerated flag disables checking of generated source code. It takes no arguments.

Exit Codes

errcheck returns 1 if any problems were found in the checked files. It returns 2 if there were any other failures.

Editor Integration

Emacs

go-errcheck.el integrates errcheck with Emacs by providing a go-errcheck command and customizable variables to automatically pass flags to errcheck.

Vim

vim-go can run errcheck via both its :GoErrCheck and :GoMetaLinter commands.