#include <stdio.h> void perror(const char *s)
#include <errno.h> int errno;
The perror() function produces a message on the standard error output (file descriptor 2) describing the last error encountered during a call to a system or library function. The argument string s is printed, followed by a colon and a blank, followed by the message and a NEWLINE character. If s is a null pointer or points to a null string, the colon is not printed. The argument string should include the name of the program that incurred the error. The error number is taken from the external variable errno, which is set when errors occur but not cleared when non-erroneous calls are made. See intro(2).
In the case of multithreaded applications, the -mt option must be specified on the command line at compilation time (see threads(5)). When the -mt option is specified, errno becomes a macro that enables each thread to have its own errno. This errno macro can be used on either side of the assignment as though it were a variable.
Messages printed from this function are in the native language specified by the LC_MESSAGES locale category. See setlocale(3C).
See attributes(5) for descriptions of the following attributes:
intro(2), fmtmsg(3C), gettext(3C), setlocale(3C), strerror(3C), attributes(5), standards(5), threads(5)