how to use vasprintf() with special characters (%)?

Question

I have the following sample code.

#include <stdio.h>
#include <unistd.h>
#include <stdarg.h>

int test(const char *fmt,...)
{
        va_list args;
        char *vacmd=NULL;

        va_start(args,fmt);
        vasprintf(&vacmd, fmt, args);
        printf("vacmd is %s\n", vacmd);

        return 0;
}

int main(void)
{
        int ret = 0;
        char *cmd="@wilso%nqw";
        ret = test(cmd);
}

Output is :

vacmd is @wilsoqw

It removed the %n from the string. So my question is does vasprintf() works with specials characters or not? or am I missing something?

you need `"%%"`, since `"%n"` itself is a conversion specifier. — The Paramagnetic Croissant, Jun 18 '15 at 13:57
You don't want to use `vasprintf`, or you did not read its documentation on [vasprintf(3)](http://man7.org/linux/man-pages/man3/vasprintf.3.html) — Basile Starynkevitch, Jun 18 '15 at 13:58
I wish we had a close reason on RTFM and/or common sense. Seriously, just think about it. If all conversion specifiers start with `%`, then how **possibly** could poor `printf()` magically read your mind and deduce that you didn't *intend* to use one particular `%` character as a formatting command? — The Paramagnetic Croissant, Jun 18 '15 at 14:00
@TheParamagneticCroissant Also, If I'm not wrong, the present code exhibits UB, due to missing argument to `%n`, right? — Sourav Ghosh, Jun 18 '15 at 14:27
@TheParamagneticCroissant Note: poor `printf()` still is unable to print `'\0'` from a format string - - certainly not much of a loss though. C _could_ have specified `"%\0"` to print `'\0'`, but _that_ would run into trouble with the definition of a format _string_. IAC, poor `printf()` has many issues and its overworked and under appreciated. Would not surprise me if it went on strike. — chux - Reinstate Monica, Jun 18 '15 at 15:35
"You don't want to use vasprintf" Why not? What would you suggest instead? — Daniel Ryan, Sep 08 '16 at 23:44

Sourav Ghosh · Answer 1 · 2015-06-18T14:27:11.753

For printf() and family functions,

Each conversion specification is introduced by the character %.

So, the % in a format string has a special meaning when used with printf()/scanf() family. You can use %% to discard the special meaning.

To quote the standard in this regard, from fprintf() function specification

%

A % character is written. No argument is converted. The complete conversion specification shall be %%.

^{FWIW, your current code exhibits undefined behaviour, as "If there are insufficient arguments for the format, the behavior is undefined." As per your code, there is no argument supplied for %n format specifier.}

how to use vasprintf() with special characters (%)?

1 Answers1