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 NUL) 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
non-erroneous 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
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.