My C program ends with segfault - valgrind

Let’s have a basic failing program:

#include <iostream>

void fail() {
    int *p1;
    int *p2(NULL);
    int *p3 = p1;
    if (p3) {
        std::cout << *p3 << std::endl;
    } 
}

int main() { 
    fail();
}

Build it (add -g to include debug info):

g++ -g -o main main.cpp

Run:

$ ./main
Segmentation fault (core dumped)
$

Let’s debug it with valgrind:

$ valgrind ./main
==8515== Memcheck, a memory error detector
==8515== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==8515== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==8515== Command: ./main
==8515==
==8515== Conditional jump or move depends on uninitialised value(s)
==8515==    at 0x400813: fail() (main.cpp:7)
==8515==    by 0x40083F: main (main.cpp:13)
==8515==
==8515== Invalid read of size 4
==8515==    at 0x400819: fail() (main.cpp:8)
==8515==    by 0x40083F: main (main.cpp:13)
==8515==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==8515==
==8515==
==8515== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==8515==  Access not within mapped region at address 0x0
==8515==    at 0x400819: fail() (main.cpp:8)
==8515==    by 0x40083F: main (main.cpp:13)
==8515==  If you believe this happened as a result of a stack
==8515==  overflow in your program's main thread (unlikely but
==8515==  possible), you can try to increase the size of the
==8515==  main thread stack using the --main-stacksize= flag.
==8515==  The main thread stack size used in this run was 8388608.
==8515==
==8515== HEAP SUMMARY:
==8515==     in use at exit: 72,704 bytes in 1 blocks
==8515==   total heap usage: 1 allocs, 0 frees, 72,704 bytes allocated
==8515==
==8515== LEAK SUMMARY:
==8515==    definitely lost: 0 bytes in 0 blocks
==8515==    indirectly lost: 0 bytes in 0 blocks
==8515==      possibly lost: 0 bytes in 0 blocks
==8515==    still reachable: 72,704 bytes in 1 blocks
==8515==         suppressed: 0 bytes in 0 blocks
==8515== Rerun with --leak-check=full to see details of leaked memory
==8515==
==8515== For counts of detected and suppressed errors, rerun with: -v
==8515== Use --track-origins=yes to see where uninitialised values come from
==8515== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)
$

First we focus on this block:

==8515== Invalid read of size 4
==8515==    at 0x400819: fail() (main.cpp:8)
==8515==    by 0x40083F: main (main.cpp:13)
==8515==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

The first line tells us that segfault is caused by reading 4 bytes. The second and the third lines are call stack. It means that the invalid read is performed at the fail() function, line 8 of main.cpp, which is called by main, line 13 of main.cpp.

Looking at line 8 of main.cpp we see

std::cout << *p3 << std::endl;

But we check the pointer first, so what’s wrong? Lets check the other block:

==8515== Conditional jump or move depends on uninitialised value(s)
==8515==    at 0x400813: fail() (main.cpp:7)
==8515==    by 0x40083F: main (main.cpp:13)

It tells us that there is an unitialized variable at line 7 and we read it:

if (p3) {

Which points us to the line where we check p3 instead of p2. But how is it possible that p3 is uninitialized? We initialize it by:

int *p3 = p1;

Valgrind advices us to rerun with --track-origins=yes, let’s do it:

valgrind --track-origins=yes ./main

The argument for valgrind is just after valgrind. If we put it after our program, it would be passed to our program.

The output is almost the same, there is only one difference:

==8517== Conditional jump or move depends on uninitialised value(s)
==8517==    at 0x400813: fail() (main.cpp:7)
==8517==    by 0x40083F: main (main.cpp:13)
==8517==  Uninitialised value was created by a stack allocation
==8517==    at 0x4007F6: fail() (main.cpp:3)

Which tells us that the uninitialized value we used at line 7 was created at line 3:

int *p1;

which guides us to our uninitialized pointer.

Found a mistake? Have a question or improvement idea? Let me know.

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