https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102875
Bug ID: 102875
Summary: __builtin_strncpy output may be truncated copying
bytes from a string of length
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: johnnymarler at gmail dot com
Target Milestone: ---
GCC Version: 9.2.0
System: NixOS
$ gcc -v
Using built-in specs.
COLLECT_GCC=/nix/store/b3zsk4ihlpiimv3vff86bb5bxghgdzb9-gcc-9.2.0/bin/gcc
COLLECT_LTO_WRAPPER=/nix/store/b3zsk4ihlpiimv3vff86bb5bxghgdzb9-gcc-9.2.0/libexec/gcc/x86_64-unknown-linux-gnu/9.2.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with:
Thread model: posix
gcc version 9.2.0 (GCC)
How to reproduce:
wtf.c
------------------------------
#include <string.h>
// value of 2 works???
//#define COPY_COUNT 2
// values >3 don't work
#define COPY_COUNT 3
typedef struct {
// NOTE: these padding values don't fix it
//int something_to_pad_between_elements_1;
char array[100];
//int something_to_pad_between_elements_2;
} StringBuf;
int main()
{
StringBuf a[COPY_COUNT];
StringBuf b[COPY_COUNT];
for (unsigned i = 0; i < COPY_COUNT; i++) {
strncpy(a[i].array, b[i].array, 100);
}
}
------------------------------
$ gcc -Wall wtf.c
In file included from
/nix/store/fwpn2f7a4iqszyydw7ag61zlnp6xk5d3-glibc-2.30-dev/include/string.h:494,
from wtf.c:1:
In function ‘strncpy’,
inlined from ‘main’ at wtf.c:21:5:
/nix/store/fwpn2f7a4iqszyydw7ag61zlnp6xk5d3-glibc-2.30-dev/include/bits/string_fortified.h:106:10:
warning: ‘__builtin_strncpy’ output may be truncated copying 100 bytes from a
string of length 299 [-Wstringop-truncation]
106 | return __builtin___strncpy_chk (__dest, __src, __len, __bos
(__dest));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Expected Results:
This example is expected to compile with no issues.
Actual Results:
The compiler erroneously warns about a truncated strcpy when no such truncation
occurs.
