When using the -O3 optimizing option (-O2 and -O1 as well), a sprintf call, in
which the target string and the source string are the same, will no longer show
the same behavior. Not sure, whether this is actually a Bug. Here is an
example:
test.c:
#include <stdlib.h>
#include <stdio.h>
char mystring[30];
int main(int argc, char *argv[]){
sprintf(mystring,"hello");
printf("%s\n",mystring);
sprintf(mystring,"%s world",mystring);
printf("%s\n",mystring);
return 0;
}
terminal:
m...@here ~ $ gcc -o test test.c
m...@here ~ $ ./test
hello
hello world
m...@here ~ $ gcc -O3 -o test test.c
m...@here ~ $ ./test
hello
world
m...@here ~ $ gcc --version
gcc (Gentoo 4.3.4 p1.0, pie-10.1.5) 4.3.4
I'm fully aware that the line
sprintf(mystring,"%s world",mystring);
is kind of dangerous, as it will fail as soon as %s is not the first object
inside the string or mystring has reached full length, and should be replaced
with a call to strcat or similar. However, under normal conditions it will work
fine, and it's quite intuitive that the optimizing option changes this
behavior.
Might be related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40479
--
Summary: O3 option changes sprintf behavior
Product: gcc
Version: 4.3.4
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: schoth at itp dot physik dot tu-berlin dot de
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42173
