https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89990
Andrew D'Addesio <modchipv12 at gmail dot com> changed:
What |Removed |Added
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CC| |modchipv12 at gmail dot com
--- Comment #5 from Andrew D'Addesio <modchipv12 at gmail dot com> ---
GCC already warns about this at compile time, but the warning sometimes doesn't
appear (due to PR88058, as Andrew Pinski just mentioned). Plus, the warning is
a bit confusing and could be reworded.
For example, create the following files:
foo.c:
int foo(const unsigned char *buf)
{
(void)buf; /* unused parameter */
return 1;
}
test.c:
int foo(const unsigned char *buf);
struct mytype {
char c;
};
static struct mytype d = { 42 };
int test(int x)
{
const unsigned char buf[32];
const struct mytype *ptr = &d;
if (x != 0)
ptr = &(const struct mytype){ 43 };
foo(buf);
#ifdef CALL_FOO_TWICE
foo(buf);
#endif
return ptr->c;
}
int main()
{
return test(1); /* returns 43 on GCC8, 0 on GCC9+ */
}
Compiling with one foo() call gives us a warning:
$ gcc -std=c99 -Wall -Wextra -pedantic -O1 -o test test.c foo.c
test.c: In function ‘main’:
test.c:26:15: warning: ‘<U2cf0>.c’ is used uninitialized in this function
[-Wuninitialized]
26 | return ptr->c;
| ~~~^~~
$ ./test
$ echo $?
0
However, compiling with two foo() calls makes the warning disappear, for some
reason:
$ gcc -DCALL_FOO_TWICE -std=c99 -Wall -Wextra -pedantic -O1 -o test test.c
foo.c
$ ./test
$ echo $?
0
My GCC version is 9.3.1 20200317 (Red Hat 9.3.1-1) on Fedora 31 x86-64.
