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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: More trouble with strtoul
Date: Fri, 20 Mar 2015 08:48:33 -0700
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jacob navia <jacob@jacob.remcomp.fr> writes:
> Le 20/03/2015 09:36, Robert Wessel a écrit :
>> does the GCC version set ERRNO correctly?
>
> YES! THAT's the thing to test.
>
> I copiled following program with gcc:
>
>   1 #include <stdio.h>
>   2 #include <math.h>
>   3 #include <stdlib.h>
>   4 #include <errno.h>
>   5 int main(int argc,char *argv[])
>   6 {
>   7     char *end;
>   8     char *s = "-2147483647";
>   9     if (argc > 1)
>  10         s = argv[1];
>  11     errno = 0;
>  12     unsigned long m = strtoul(s,&end,0);
>  13     printf("%lu %lld",m,end-s);
>  14     printf(" errno = %s\n",strerror(errno));
>  15 }
>
> and obtained
>
>   1 1 11 errno = Result too large

Is that the exact code you ran?  I get several warnings when I compile
it.  On line 13, "%lld" is the incorrect format for end-s (unless
ptrdiff_t happens to be long long). I suggest:

    printf("%lu %d", m, (int)end-s);

> Then, since I set errno to zero at line 11, that gcc is setting errno
> correctly meaning that this is an error, but not returning the error
> result, meaning it is negating something after the overflow.

On line 14, strerror is undeclared; you need #include <string.h>.  Also,
the printf call on line 13 is likely to clobber the value of errno, so
depending on the implementation the output from line 14 may be
meaningless.

> Is this a bug in gcc?

If it's a bug, it's not in gcc.  All gcc does is generate a call to the
strtoul function.  The implementation of the function is in the library.
What implementation are you using?  On most Linux systems, strtoul is
implemented by glic (GNU libc).  If you happen to be using Cygwin, the
library implementation is "newlib", if I recall correctly.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"