PERROR(3) Linux Programmer's Manual PERROR(3)

perror - print a system error message

#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:
Since glibc 2.19:
Glibc 2.19 and earlier:

The perror() function produces a message on standard error describing the last error encountered during a call to a system or library function.
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.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
perror () Thread safety MT-Safe race:stderr

perror(), errno: POSIX.1-2001, POSIX.1-2008, C89, C99, 4.3BSD.
The externals sys_nerr and sys_errlist derive from BSD, but are not specified in POSIX.1.

The externals sys_nerr and sys_errlist are defined by glibc, but in <stdio.h>.

err(3), errno(3), error(3), strerror(3)