Strange warning on printf checks

NightStrike nightstrike@gmail.com
Sun May 31 18:32:33 GMT 2020


I'm curious if this is a gcc bug or not.  The warning I get is trying to
highlight a real problem, but it's referring to a string literal as a
directive, which I thought was just for the %XX printf commands.  Given the
following:

#include <stdio.h>

void f() {
    char x[4];
    char y[5];
    sprintf(x, "%s_%s", y, y);
}

$ gcc a.c -c -Wall
a.c: In function 'f':
a.c:6:16: warning: '_' directive writing 1 byte into a region of size
between 0 and 4 [-Wformat-overflow=]
    6 |  sprintf(x, "%s_%s", y, y);
      |                ^
a.c:6:2: note: 'sprintf' output between 2 and 10 bytes into a destination
of size 4
    6 |  sprintf(x, "%s_%s", y, y);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~


I hope the fixed width display shows this correctly.  The point is that the
^ points to the underscore, which is right, but the message calls the
underscore a printf directive.

Maybe I'm splitting hairs, but this confused me for a good half hour before
I realized what was wrong.


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