void perror(const char *s);
const char * const sys_errlist;
int errno; /* Not really declared this way; see errno(3). */
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
To be of most use, the argument string should include the name of the function that incurred the error. The error number is taken from the external variable errno, which is set when errors occur but not cleared when successful calls are made.
The global error list sys_errlist 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.
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 library 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.