warning: '__builtin_snprintf' output may be truncated before the last format character [-Wformat-truncation=]

Question

warning: ‘__builtin_snprintf’ output may be truncated before the last format character [-Wformat-truncation=]
  "%s", evspan->text);
     ^

len = strlen(evspan->text);
evspan->ent->content = malloc(len+1);

snprintf(evspan->ent->content, len,
"%s", evspan->text);

I saw this warning on gcc 8. How do I prevent this without using -Wformat-truncation option?

Whats the point of using `snprintf` in this code? `strcpy` has the same result. — tkausl, Feb 07 '22 at 01:40
Don't use `snprintf()` when `strcpy()` or `memmove()` (or `memcpy()`) would be safe and sensible. I assume you do check the return value from `malloc()` to protect against memory shortages. — Jonathan Leffler, Feb 07 '22 at 01:40
[Can't reproduce](https://godbolt.org/z/ooEx8Psdz) - please make a [mre] — Ted Lyngmo, Feb 07 '22 at 01:45

score 0 · Accepted Answer · answered Feb 07 '22 at 01:55

The second argument to snprintf is the length of the buffer, not the maximum string length.

The call should be:

snprintf(evspan->ent->content, len + 1, "%s", evspan->text);

This matches the length passed to malloc.

The comments given on snprintf not being the best choice do apply, if your full code is as simple as what's shown here.

warning: '__builtin_snprintf' output may be truncated before the last format character [-Wformat-truncation=]

1 Answers1