Wformat-overflow warning with two-dimensional array

[Felix Windsheimer (Fujitsu)]

Hi,

The following code:

#include <stdio.h>
#include <string.h>

int main(void) {
        char names[1000][20];
        memset(names, 0, sizeof(names));

        for (unsigned int i = 0; i < 5; i++) {
                char s[40];
                sprintf(s, "/dev/%s", names[i]);
        }
}

Should - in our opinion - be correct as names[i] is guaranteed to be null-terminated due to the memset.
However, GCC 15.2 produces a warning:

~ gcc-15 -Wall -O1 bug.c
bug.c: In function 'main':
bug.c:10:34: warning: '%s' directive writing up to 19999 bytes into a region of size 35 [-Wformat-overflow=]
   10 |                 sprintf(s, "/dev/%s", names[i]);
      |                                  ^~
bug.c:10:17: note: 'sprintf' output between 6 and 20005 bytes into a destination of size 40
   10 |                 sprintf(s, "/dev/%s", names[i]);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To us it seems like GCC misunderstands the size of a two-dimensional array, or rather, the size of a single element in this array.
Note that changing the sprintf to use a constant instead of `i` produces no warning:

        for (unsigned int i = 0; i < 5; i++) {
                char s[40];
                sprintf(s, "/dev/%s", names[0]); // any constant [0, 999] works
        }

Is this simply a compiler bug or are we misunderstanding something here?
Thanks!

Kind regards
Felix

~ gcc-15 -v
Using built-in specs.
COLLECT_GCC=gcc-15
COLLECT_LTO_WRAPPER=/usr/lib64/gcc/x86_64-suse-linux/15/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-suse-linux
Configured with: ../configure CFLAGS=' -fmessage-length=0 -grecord-gcc-switches -O2 -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -g' CXXFLAGS=' -fmessage-length=0 -grecord-gcc-switches -O2 -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -g' XCFLAGS=' -fmessage-length=0 -grecord-gcc-switches -O2 -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -g' TCFLAGS=' -fmessage-length=0 -grecord-gcc-switches -O2 -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -g' GDCFLAGS=' -fmessage-length=0 -grecord-gcc-switches -O2 -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -g' --prefix=/usr --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64 --enable-languages=c,c++,objc,fortran,obj-c++,ada,go,d,m2 --enable-host-pie --enable-offload-targets=nvptx-none, --enable-offload-defaulted --without-cuda-driver --enable-checking=release --disable-werror --with-gxx-include-dir=/usr/include/c++/15 --with-libstdcxx-zoneinfo=/usr/share/zoneinfo --enable-ssp --disable-libssp --disable-libvtv --enable-cet=auto --disable-libcc1 --disable-plugin --with-bugurl=https://bugs.opensuse.org/ --with-pkgversion='SUSE Linux' --with-slibdir=/
lib64 --with-system-zlib --enable-libstdcxx-allocator=new --disable-libstdcxx-pch --enable-libphobos --enable-version-specific-runtime-libs --with-gcc-major-version-only --enable-linker-build-id --enable-linux-futex --enable-gnu-indirect-function --program-suffix=-15 --without-system-libunwind --enable-multilib --with-arch-32=x86-64 -with-tune=generic --enable-link-serialization --build=x86_64-suse-linux --host=x86_64-suse-linux
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.2.0 (SUSE Linux)