GCC's Signed Overflow Trapping With -ftrapv Silently Doesn't Work

2016-05-25 - By Robert Elder

It Doesn't Work In GCC Releases From 2004 to 2014

Whenever the signed/unsigned debates comes up, I usually see someone recommend using the '-ftrapv' flag to catch run-time undefined behaviour, and I thought it was worth writing a short post to point out that it silently doesn't work in many gcc versions that are in common use.

I'm on Ubuntu 14.04 and if I do

sudo apt-get update && sudo apt-get install gcc

and then put this into main.c

#include <limits.h>
#include <stdio.h>
int main(void){
        int i = INT_MAX;
        return printf("%d\n",i + 1);
}

then run

gcc -ftrapv main.c && ./a.out

the result is

-2147483648

but it does seem to work in clang

clang -ftrapv main.c && ./a.out

gives

Illegal instruction (core dumped)

Broke For Years

It has been documented as though it worked in the man page for some time, but it looks like it has been broken for years:

• Filed in 2004, Resolved in 2014

It Works In Ubuntu 16's GCC

Looking at the bug report above, I saw that the bug was closed in 2014 with a fix, so I launched an EC2 instance to see if I could verify it working on a newer version.  I tried launching an Ubuntu 15 instance, but it never came up, but the Ubuntu 16 instance did, and the overflow gets correctly trapped there:

Aborted (core dumped)

Having said this, it looks like this functionality is still flaky: Why does ftrapv not work with optimization?

Seeing that the bug was fixed in 2014 might make you think this is a non-issue, but if you're on Ubuntu, chances are you're not already on the latest LTS which was only released a month ago, so your version likely has this bug.