https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81483
Bug ID: 81483
Summary: spurious -Wformat-overflow warning for limited types
Product: gcc
Version: 7.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: arnd at linaro dot org
Target Milestone: ---
After looking in more detail at -Wformat-overflow warnings in the linux
kernels, I managed to reduce one warning to this test case:
#include <stdio.h>
void alienware_zone_init(unsigned char num_zones)
{
unsigned char zone;
char buffer[10];
for (zone = 0; zone < num_zones; zone++)
sprintf(buffer, "zone%02X", zone);
}
I get this warning:
$ x86_64-linux-gcc-7.1.1 -Wall -S /tmp/test4.c -O2
/tmp/test4.c: In function 'alienware_zone_init':
/tmp/test4.c:9:24: warning: '%02X' directive writing between 2 and 8 bytes into
a region of size 6 [-Wformat-overflow=]
sprintf(buffer, "zone%02X", zone);
^~~~
/tmp/test4.c:9:19: note: directive argument in the range [0, 2147483647]
sprintf(buffer, "zone%02X", zone);
^~~~~~~~~~
/tmp/test4.c:9:3: note: 'sprintf' output between 7 and 13 bytes into a
destination of size 10
sprintf(buffer, "zone%02X", zone);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
which is surprising because the range "[0, 2147483647]" is larger
than an 'unsigned char' can hold. In similar test cases that use
an 'unsigned char' without the loop, we do not get the warning.
