perror - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Signature – void perror(const char *message);

Semantics – prints to stderr: the message (or nothing if NULL or empty), then ": " and the strerror(errno) text, then newline (manual, Error Messages). It does not change stderr’s orientation – if the stream is not yet oriented, glibc writes through a dup of the descriptor to avoid orienting it (manual safety note). Because it reads the current errno, call it immediately after the failure, or save errno first – every intervening library call may overwrite it.

When to prefer what – perror is portable ISO C; the manual’s critique is that the format is fixed: no program-name prefix, no location, no custom text after the message. error() for GNU-style output, fprintf(stderr, "%s: %s\n", prog, strerror(errno)) when needing both, and strerror_r/snprintf("%m") where the static strerror buffer would be a race.