#include <stdio.h> void perror(const char *s); #include <errno.h> const char * const sys_errlist[]; int sys_nerr; int errno; /* Not really declared this way; see errno(3) */
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sys_errlist, sys_nerr:
From glibc 2.19 to 2.31:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_BSD_SOURCE
First (if s is not NULL and *s is not a null byte ('\0')), the argument string s is printed, followed by a colon and a blank. Then an error message corresponding to the current value of errno and a new-line.
To be of most use, the argument string should include the name of the function that incurred the error.
The global error list sys_errlist[], which can be indexed by errno, can be used to obtain the error message without the newline. The largest message number provided in the table is sys_nerr-1. Be careful when directly accessing this list, because new error values may not have been added to sys_errlist[]. The use of sys_errlist[] is nowadays deprecated; use strerror(3) instead.
When a system call fails, it usually returns -1 and sets the variable errno to a value describing what went wrong. (These values can be found in <errno.h>.) Many library functions do likewise. The function perror() serves to translate this error code into human-readable form. Note that errno is undefined after a successful system call or library function call: this call may well change this variable, even though it succeeds, for example because it internally used some other library function that failed. Thus, if a failing call is not immediately followed by a call to perror(), the value of errno should be saved.
| Interface | Attribute | Value |
| perror() | Thread safety | MT-Safe race:stderr |
The externals sys_nerr and sys_errlist derive from BSD, but are not specified in POSIX.1.