https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93646
Bug ID: 93646
Summary: confusing -Wstringop-truncation on strncat where
-Wstringop-overflow is expected
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
The -Wstringop-overflow warning in the code below is expected because a call to
strncat with a bound equal to the length of the source might overflow (the
bound should be equal to the size of the space remaining in the destination).
But -Wstringop-truncation is not expected because strncat a) never copies the
terminating nul from the source and b) always appends a terminating nul to the
destination.
$ cat z.c && gcc -O2 -S -Wall z.c
void f (char *d, const char *s)
{
__builtin_strncat (d, s, __builtin_strlen (s));
}
char a[4];
void g (void)
{
f (a, "123");
}
z.c: In function ‘f’:
z.c:3:3: warning: ‘__builtin_strncat’ output truncated before terminating nul
copying as many bytes from a string as its length [-Wstringop-truncation]
3 | __builtin_strncat (d, s, __builtin_strlen (s));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘f’,
inlined from ‘g’ at z.c:10:3:
z.c:3:3: warning: ‘__builtin_strncat’ specified bound 3 equals source length
[-Wstringop-overflow=]
3 | __builtin_strncat (d, s, __builtin_strlen (s));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
