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void perror(const char *s);
const char *sys_errlist;
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sys_errlist, sys_nerr: _BSD_SOURCE
The routine perror() produces a message on the standard error output,
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 the message and a new-line.
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 num-
ber 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 per-
ror(), the value of errno should be saved.
The function perror() and the external errno (see errno(3)) conform to
C89, C99, 4.3BSD, POSIX.1-2001. The externals sys_nerr and sys_errlist
conform to BSD.
The externals sys_nerr and sys_errlist are defined by glibc, but in