Myujth

gcc 7.1 has introduced a new warning that tells you if you use functions such as snprintf and your arguments would result in output truncation.

The documentation implies that it is only raised if you don't check and act upon the return value:

Level 1 of -Wformat-truncation enabled by -Wformat employs a
conservative approach that warns only about calls to bounded functions
whose return value is unused and that will most likely result in
output truncation.

Here's a sample compilation unit, compiled with version 7.3.0 that illustrates the issue.

#include <stdio.h>
#include <stdlib.h>

int main() 

 char w;

 int size = snprintf(&w, 1, "%s", "hello world");
 if(size<0) 
 abort();
 

 char *buffer = malloc(size+1);
 snprintf(buffer, size+1, "%s", "hello world");

 printf("Wrote %d characters: %sn", size, buffer);
 return 0;

Compiled like this:

$ gcc -Wformat-truncation=1 test.c
test.c: In function ‘main’:
test.c:8:31: warning: ‘%s’ directive output truncated writing 11 bytes into a region of size 1 [-Wformat-truncation=]
 int size = snprintf(&w, 1, "%s", "hello world");
 ^~ ~~~~~~~~~~~~~
test.c:8:7: note: ‘snprintf’ output 12 bytes into a destination of size 1
 int size = snprintf(&w, 1, "%s", "hello world");

Am I misinterpreting the documentation? I can't see how I could check the return value more than I am doing already.

Reference: previous SO question that implies the warning should not be raised. I really don't like disabling warnings and usually compile with -Wall -Werror so I'd appreciate some guidance here.