On 11 April 2011 19:59, River Tarnell <river.tarn...@wikimedia.de> wrote:
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> Andrew Dunbar:
>> Hmm currently I'm using _vscprintf / vsprintf on Windows/MSVC inside
>> #ifdef _WIN32
>> and vasprintf Ubuntu/gcc inside #else
>
>> But my C-fu is to puny to figure out what #ifdefs to use to detect
>> what combination of C compiler and libc I'm compiling under.
>
>> Apparently it's important with snprintf since some implementations
>> return "an unspecified return value less than 1" with size=0.
>
> That's true, but no current Unix implementation should do that, since
> the return value of snprintf() is standardised in the current POSIX
> standard (IEEE 1003.1-2004) and in the C99 standard (ISO/IEC 9899:1999).
>
> The following code should handle Windows and both old and new Unix,
> although it's completely untested and comes with no warranty:
>
> /* Compile with "-D_XOPEN_SOURCE=600 -std=c99" */
> int
> my_vasprintf(char **res, char const *fmt, va_list args)
> {
> int sz, r;
> #ifdef _WIN32
> sz = _vscprintf(fmt, args);
> #else
> sz = snprintf(NULL, 0, fmt, args);
> #endif
>
> #if defined(_WIN32) || (defined(__STDC__) && __STDC__ >= 199901L) \
> || (defined(_XOPEN_VERSION) && (_XOPEN_VERSION >= 600))
> if (sz < 0)
> return sz;
> if (sz >= 0) {
> #else
> if (sz >= 1) {
> #endif
> if ((*res = malloc(sz + 1)) == NULL)
> return -1;
>
> if ((sz = sprintf(*res, fmt, args)) < 0) {
> free(*res);
> *res = NULL;
> }
>
> return sz;
> }
>
> #define MAXLN 65535
> *res = NULL;
> for (sz = 128; sz <= MAXLN; sz *= 2) {
> if ((*res = realloc(*res, sz)) == NULL)
> return -1;
> r = vsnprintf(*res, sz, fmt, args);
> if (r > 0 && r < sz)
> return r;
> }
>
> errno = ENOMEM;
>
> if (*res) {
> free(*res);
> *res = NULL;
> }
>
> return -1;
> }
>
> Obviously, this will be slower than the modern implementation, and you
> may want to tune the initial buffer size for the expected string length.
Thanks River, that's really helpful!
Andrew Dunbar (hippietrail)
> - river.
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