https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85598
Bug ID: 85598
Summary: Incorrect warning only at -O2 and -O3
Product: gcc
Version: 7.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: thomas.mercier.jr at gmail dot com
Target Milestone: ---
Created attachment 44049
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44049&action=edit
Preprocessed file of program that reproduces the behavior
$ gcc -v
Using built-in specs.
COLLECT_GCC=x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/brazil-pkg-cache/packages/GCC/GCC-7.x.200154.0/AL2012/DEV.STD.PTHREAD/build/gcc-7.3.0/bin/../libexec/gcc/x86_64-pc-linux-gnu/7.3.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:
/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/private/src/gcc-7.3.0/configure
--prefix=/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/gcc-7.3.0
--enable-languages=c,c++,fortran --disable-plugin
--with-build-config=bootstrap-lto
--with-gmp=/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/private/stage2
--with-mpfr=/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/private/stage2
--with-mpc=/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/private/stage2
--with-fpmath=sse --with-cpu-64=core2 --disable-multilib --disable-bootstrap
--with-glibc-version=2.12
--with-cloog=/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/private/stage2
--with-ppl=/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/private/stage2
--with-isl=/local/p4clients/pkgbuild-Kfsa6/workspace/src/GCC/build/private/stage2
--enable-libsanitizer --disable-stage1-checking
Thread model: posix
gcc version 7.3.0 (GCC)
When compiled with -O2 or -O3 optimization enabled, the following warning is
printed:
$ gcc -Wall -O2 /tmp/test.c
/tmp/test.c: In function ‘main’:
/tmp/test.c:9:31: warning: ‘snprintf’ output may be truncated before the last
format character [-Wformat-truncation=]
snprintf(str, 4, "%%%02X", x);
^
/tmp/test.c:9:7: note: ‘snprintf’ output between 4 and 5 bytes into a
destination of size 4
snprintf(str, 4, "%%%02X", x);
~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
With lesser levels of optimization, no warning is printed:
$ gcc -Wall -O1 /tmp/test.c
$
This is the entire program (preprocessed attached):
#include <stdio.h>
int main()
{
char temp[100];
unsigned int x;
char *str = temp;
for(x=0; x<256; ++x) {
snprintf(str, 4, "%%%02X", x);
}
}
Calling snprintf with NULL and 0 for the first two arguments returns 3 for all
values of the loop. I believe the correct behavior from GCC is no warning at
any optimization level.
