Hello! snprintf() function in mingw-w64 is broken. Seems that it just calls MSVC's _snprintf() function which is not same as snprintf(). MSVC until ucrt does not provide snprintf() at all, just _snprintf().
Differences are that MSVC's _snrpintf() does not ensure that filled
buffer would be null-termed and when overflow occurs it returns -1
instead of length which is needed to fully format string.
See following example:
#include <stdio.h>
int main() {
char buf[5] = "XXXX";
int ret = snprintf(buf, 3, "%d", 123456789);
printf("buf=%s ret=%d\n", buf, ret);
return 0;
}
It prints:
buf=123X ret=-1
So for MSVC's _snprintf() it is needed to write wrapper, e.g. following:
#define snprintf(str, size, format, ...) ( (((size_t)(size) > 0) ?
(_snprintf((str), (size), (format), __VA_ARGS__), ((char
*)str)[(size_t)(size)-1] = 0, 0) : 0), _scprintf((format), __VA_ARGS__) )
It is simple inline-able macro so it does not increase executable code
size too much.
MSVC's _scprintf() function returns length of formatted string, so
exactly what snprintf() return value should be.
With that snprintf() define, output of above example is:
buf=12 ret=9
Which is correct now.
Would you consider fixing snprintf() in mingw-w64?
Similarly also vsnprintf() can be fixed:
#define vsnprintf(str, size, format, ap) ( (((size_t)(size) > 0) ?
(_vsnprintf((str), (size), (format), (ap)), ((char *)str)[(size_t)(size)-1] =
0, 0) : 0), _vscprintf((format), (ap)) )
--
Pali Rohár
pali.ro...@gmail.com
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