STDERRFILENO

STDERR_FILENO is a POSIX macro that expands to the file descriptor number assigned to the standard error stream (stderr). It is typically defined as 2 and is used by low-level I/O routines that operate on file descriptors rather than FILE* streams.

In POSIX systems, STDERR_FILENO is declared in unistd.h alongside the other standard file descriptors, with STDIN_FILENO (0) and STDOUT_FILENO (1) being the counterparts. The macro provides a portable way to refer to the standard error stream in functions such as read, write, and dup2, enabling redirection or inspection of error output without using standard I/O library wrappers.

Common uses include redirecting standard error to a file or another descriptor by manipulating file descriptors directly. For example, a program might duplicate a file descriptor onto STDERR_FILENO to funnel error messages to that target, or use dup2 to redirect stderr to a log file descriptor. This approach is particularly relevant in shell programs, daemons, or libraries that perform low-level I/O control.

Portability notes: while STDERR_FILENO is standard in POSIX, it may not be defined in non-POSIX environments or certain Windows C libraries. In such cases, equivalent behavior might be achieved through other mechanisms, such as using explicit file descriptors (often 2) or using platform-specific APIs. When writing portable code, consult the target platform’s headers to determine whether STDERR_FILENO is available and appropriate.

See also: STDIN_FILENO, STDOUT_FILENO, unistd.h, dup2, write, read.