Availability

  • Windows: only clang has them
  • Linux: both gcc and clang have them
  • macOS: only Apple clang has them

Usage

A sanitizer is a tool built into gcc/clang that helps you find memory leaks, out-of-bounds and negative indices, integer overflow, and the rest of the family of things that cause runtime errors. There are two relevant ones: AddressSanitizer and UndefinedBehaviorSanitizer. Just add -g -fsanitize=address,undefined when you invoke clang. For example:

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#include<bits/stdc++.h>
using namespace std;
int main(){
    int a[5];
    for(int i=0;i<5;i++)cin>>a[i];
    for(int i=-1;i<=5;i++)printf("%d ",a[i]);
    a[1]=(a[1]+100)*INT_MAX;
    return 0;
}

With the input 1 2 3 4 5, a build without sanitizers prints:

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67812992 1 2 3 4 5 -2068840411

Garbage, and not a word of warning.

With sanitizers, it prints:

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test.cpp:6:40: runtime error: index -1 out of bounds for type 'int [5]'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test.cpp:6:40 in
test.cpp:6:40: runtime error: load of address 0x00016dc3319c with insufficient space for an object of type 'int'
=================================================================
==73013==ERROR: AddressSanitizer: stack-buffer-underflow on address 0x00016dc3319c at pc 0x0001021cfb2c bp 0x00016dc33170 sp 0x00016dc33168
READ of size 4 at 0x00016dc3319c thread T0
    #0 0x1021cfb28 in main test.cpp:6
    #1 0x102249088 in start+0x204 (dyld:arm64e+0x5088)

Address 0x00016dc3319c is located in stack of thread T0 at offset 28 in frame
    #0 0x1021cf664 in main test.cpp:3

  This frame has 1 object(s):
    [32, 52) 'a' (line 4) <== Memory access at offset 28 underflows this variable
SUMMARY: AddressSanitizer: stack-buffer-underflow test.cpp:6 in main
[...]
==73013==ABORTING

The first four lines already point at both bugs, at line 6 column 40. The pile of addresses that follows can be ignored — you only need the location and the message to pin the bug down exactly. It is a fantastic tool.