does perror() set errno?

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# 1  
Old 08-14-2009
Question does perror() set errno?

here the program gives a odd result:
#include <stdio.h>
int main(){
    return 0;


first: Success
next: Illegal seek

why? any resonable explanation? i found no information about this in man pages.
thanks in advance
# 2  
Old 08-14-2009
perror uses errno.

all it does is the equivalent of sprintf your phrase along with the result of strerror(errno).
You are experiencing undefined behavior - meaning do not call perror twice like that.

You idea may or may not work correctly - undefined means the run time library writer decides what if anything some aspect of C coding will do. In your case it probably trashed errno after the first call.

Write your own perror() if you don't want to use it conventionally.
# 3  
Old 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.

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