[Bug libc/5468] Error in getline(3) manpage

["mtk.manpages at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=5468

--- Comment #3 from Michael Kerrisk <mtk.manpages at gmail dot com> ---
(In reply to Luke Hutchison from comment #0)
> The getline(3) manpage states:
> 
>        ssize_t getline(char **lineptr, size_t *n, FILE *stream);
>  [...]
>        If  *lineptr  is  NULL,  then getline() will allocate a buffer for
> storing the line, which should be
>        freed by the user program.  (The value in *n is ignored.)
> 
> Based on this description (particularly the part that says the value *n is
> ignored), you should be able to do something like:
> 
>     char *line = NULL;
>     getline(&line, NULL, stream);
> 
> However this always fails, because the code for getline does the following:
> 
>   if (!lineptr || !n || !stream)
>     {
>       errno = EINVAL;
>       return -1;
>     }
> 
>   if (!*lineptr)
>     {
>       *n = MIN_CHUNK;
>       *lineptr = malloc (*n);
>       if (!*lineptr)
> 	{
> 	  errno = ENOMEM;
> 	  return -1;
> 	}
>       *lineptr[0] = '\0';
>     }
> 
> (thus the !n check always causes -1 to be returned).
> 
> Probably the manpage should just be modified to say something like "(The
> value
> in *n is ignored on entry, but *n must still point to a valid int, and its
> value
> is updated on exit to reflect the size of the newly malloc'd buffer.)"
> 
> This is pretty confusing behavior otherwise -- I had to read the source for
> getline() to know what was going on.
> 
> Thanks!

getline(3) text changed to read:

       If  *lineptr  is  set  to NULL and *n is set 0 before the call,
       then getline() will allocate a buffer  for  storing  the  line,
       which should be freed by the user program.

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