c format strings: printf argument containing format specifiers

Question

#include <stdio.h>

int main(void) {
    int b= 37;
    char a=10, str[32] = "deva%x %x", buf[32];
    snprintf(buf, sizeof(buf), str);
    printf("%s", buf);
    printf("\n %p", &b);
    return 0;
}

Ouput: deva80482b9 40020930 0xbfb80aac

Wondering how printf is working here & what is the value it's printing. Does it have any significance ??

unwind · Accepted Answer · 2017-03-14 10:58:38Z

2

This is undefined behavior, since the arguments passed to snprintf() don't match the format string.

No further analysis of what happens in general is very interesting, since what happens is undefined and might change from one compilation of the program to another.

The most likely explanation is that printf() proceeds as if the arguments where there, reading data from foo-knows-where.

answered Mar 14, 2017 at 10:58

unwind

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6 Comments

Deva Over a year ago

I always assumed the first argument in the printf statement is the one that should contain format specifiers, is my understanding is not correct.

unwind Over a year ago

Sure, but this is a call to snprintf(), which is not the same function as printf(). This formats to a size-limited string buffer (which is what the prefix sn typically means).

Deva Over a year ago

Yup, got confused. Wondering will it try to read data in stack ??

unwind Over a year ago

@Deva That is undefined. Nobody knows, except maybe the people who implemented your particular combination of compiler and standard library. You can of course disassemble and/or debug the program to find out what it does, but that doesn't tell you much in general.

JeremyP Over a year ago

@Deva You can make a pretty good guess as to where it reads the arguments from if you know the calling conventions. Most likely they come from the fourth and fifth registers or from the stack somewhere below the return address.

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Sourav Ghosh · Accepted Answer · 2017-03-14 11:00:46Z

0

In your statement

 snprintf(buf, sizeof(buf), str);

the format string contains conversion specifiers (%x), but corresponding arguments are missing. So, this invokes undefined behavior.

Quoting C11, chapter §7.21.6.1/p2,

[....] If there are insufficient arguments for the format, the behavior is undefined. [...]

You need to write

snprintf(buf, sizeof(buf), str, <list of arguments matching the conversion specifier>);

Check the man page for more details.

answered Mar 14, 2017 at 11:00

Sourav Ghosh

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Comments

Ramkrushna · Accepted Answer · 2017-03-14 11:09:07Z

0

undefined behavior, as because you have passed wong arguments snprintf() don't match the format string.

answered Mar 14, 2017 at 11:09

Ramkrushna

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