[Bug c/102337] New: possibly wrong warning about truncation

[rootkit85 at yahoo dot it]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102337

            Bug ID: 102337
           Summary: possibly wrong warning about truncation
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rootkit85 at yahoo dot it
  Target Milestone: ---

The following code:

#include <stdio.h>

// build with: -O2 -Werror=format-truncation

void poke_kprobe_events(int add)
{
        char cmd[192], probename[128], probefunc[128];

        snprintf(probename, sizeof(probename), "a");

        snprintf(probefunc, sizeof(probefunc), "b");

        snprintf(cmd, sizeof(cmd), "%s %s", probename, probefunc);
}

Generates the following warning:

$ gcc -O2 -c test.c -Werror=format-truncation
test.c: In function ‘poke_kprobe_events’:
test.c:11:40: error: ‘%s’ directive output may be truncated writing up to 127
bytes into a region of size between 64 and 191 [-Werror=format-truncation=]
   11 |         snprintf(cmd, sizeof(cmd), "%s %s", probename, probefunc);
      |                                        ^~              ~~~~~~~~~
test.c:11:9: note: ‘snprintf’ output between 2 and 256 bytes into a destination
of size 192
   11 |         snprintf(cmd, sizeof(cmd), "%s %s", probename, probefunc);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

I'm using the Fedora 34 packages:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array
--with-isl=/builddir/build/BUILD/gcc-11.2.1-20210728/obj-x86_64-redhat-linux/isl-install
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-gnu-indirect-function --enable-cet --with-tune=generic
--with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.1 20210728 (Red Hat 11.2.1-1) (GCC)