https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119148
Bug ID: 119148
Summary: Inconsistent -Wstringop-truncation warning when using
strncpy
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: paul.f.fee at gmail dot com
Target Milestone: ---
I'm seeing an unexpected warning with GCC relating to strncpy(). First some
minimal code to illustrate the issue:
1 #include <string.h>
2 #include <assert.h>
3
4 int main()
5 {
6 const char *src = "123456";
7 char arr[6] = {1,2,3,4,5,6};
8 size_t l = 5;
9 [[maybe_unused]] char *ptr = arr;
10
11 #define dst arr
12 // #define dst ptr
13
14 strncpy(dst, src, l);
15 dst[l] = '\0';
16 assert(strlen(dst) == l);
17 }
Compiled with: cc truncation.c -Wall -O1 && ./a.out
When the destination is an array, lines 14 and 15 in combination are safe,
there's no warning. However, when we swap macros on lines 11 and 12, we get a
warning, though the assert still passes.
truncation.c:14:5: warning: ‘strncpy’ output truncated copying 5 bytes from a
string of length 6 [-Wstringop-truncation]
Why do we get a warning when the destination is a pointer rather than an array?
