osStderr

osStderr refers to the standard error stream in the Go programming language’s standard library, exposed as the exported variable os.Stderr. It is a pointer to an os.File that represents the process’s standard error file descriptor and is initialized to the system’s real standard error when the program starts.

As an *os.File, os.Stderr implements the io.Writer interface, allowing error messages to be written using standard I/O patterns. It is commonly used with the fmt and log packages, for example fmt.Fprintln(os.Stderr, "error message") or log.SetOutput(os.Stderr). The stream typically corresponds to the console in a terminal, but it can be redirected by the operating system or by the program itself.

os.Stderr is distinct from os.Stdin and os.Stdout, which represent standard input and standard output respectively. It is generally a shared, global resource and, while it is possible to replace it at runtime by assigning a different *os.File value, doing so should be done with care because it affects all code that writes to the standard error stream.

In practice, os.Stderr is used to separate error output from regular program output, which aids debugging and logging, especially in environments where stdout and stderr are redirected to different destinations. For testing, developers may temporarily redirect os.Stderr to capture error messages, then restore the original stream afterward.

See also: os.Stdin, os.Stdout, log, fmt, and the concept of file descriptors and streams in Go.