08-22-2009
What Jim means is that
errno is undefined after a successful library call. You should only call
perror() immediately after a failing call.
As a matter of fact, the glibc implementation of
perror() calls some other functions such as
dup(),
fdopen() that modify
errno, even if
perror() itself always succeed.
I would personally expect that clean library code doesn't expose their mess to outside. But we cannot complain, as POSIX/SUS allows such behavior.
Cheers,
Loïc
--
My Blog:
Loïc OnStage
“UNIX is simple. It just takes a genius to understand its simplicity.” - Dennis Ritchie
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LEARN ABOUT OPENSOLARIS
errno
perror(3C) Standard C Library Functions perror(3C)
NAME
perror, errno - print system error messages
SYNOPSIS
#include <stdio.h>
void perror(const char *s)
#include <errno.h>
int errno;
DESCRIPTION
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 NEW-
LINE 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.
USAGE
Messages printed from this function are in the native language specified by the LC_MESSAGES locale category. See setlocale(3C).
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
|MT-Level |MT-Safe |
+-----------------------------+-----------------------------+
SEE ALSO
Intro(2), fmtmsg(3C), gettext(3C), setlocale(3C), strerror(3C), attributes(5), standards(5), threads(5)
SunOS 5.11 12 Jul 2007 perror(3C)