On Mon, Jun 23, 2008 at 12:37:50PM +0200, Michael Kerrisk wrote:
[...]
> NOTES
> The GNU C library supports a non-standard extension that causes
> the library to dynamically allocate a string of sufficient size
> for input strings for the %s and %a[range] conversion
> specifiers. To make use of this feature, specify a as a length
> modifier (thus %as or %a[range]). The caller must free(3) the
> returned string, as in the following example:
>
> char *p;
>
> scanf("%a[a-zA-Z]", &p);
You need to check the return value of scanf (could be 0 or EOF,
caused by the absence of letters in the input, or an EOF or read
error or ENOMEM.
> printf("%s0, p);
typo? printf("%s\n", p)
> free(p);
That might free something unallocated.
> This feature is not available if the program is compiled with
> cc -std=cc99 or cc -D_ISOC99_SOURCE (unless _GNU_SOURCE is also
> specified), in which case the a is interpreted as a specifier
> for floating point numbers (see above).
Maybe "gcc" instead of "cc" as I don't expect all the cc
implementations to have such a -std option.
> Since version 2.7, glibc also provides the m modifier for the
> same purpose as the a modifier. The m modifier has the follow-
> ing advantages:
>
> * It may also be applied to %c conversion specifiers (e.g.,
> %3mc).
>
> * It avoids ambiguity with respect to the %a floating-point
> conversion specifier (and is unaffected by cc -std=99 etc.)
>
> * It is specified in the upcoming revision of the POSIX.1 stan-
> dard.
Thanks for that, I hadn't thought of checking the draft.
I can see the draft makes it clear that you don't need to free
the buffer if scanf fails. I think it should be mentionned in
the man page as well as it seems to also be the case for %a.
So:
errno = 0;
n = scanf(..., &p);
if (n == 1) {
printf("OK: %s\n", p);
free(p);
} else if (errno != 0) {
perror("scanf");
}
else {
fprintf(stderr, "expected letters, not \"%s\"\n", ...);
}
--
Stéphane
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