为什么Valgrind抱怨printf而不是我的未初始化的计数器？

Question

I just started using Valgrind, and I'm not sure the error messages I get are what they should be. For instance, Valgrind just gave me a very long chain of warnings about printf() making jumps based on uninitialized memory. It was obviously that the issue wasn't with printf() . Rather, my program was giving printf() tainted memory.

I managed to produce the following MCVE:

#include <stdio.h>

int main(void)
{
    int d;
    d++;
    printf("%d\n", d);
    return 0;
}

It's really obvious that the problem here lies in d++ . However, Valgrind only detects and warns me of uninitialized memory usage in the next line, with 6 messages in the form

==12178== Conditional jump or move depends on uninitialised value(s)
==12178== at 0x4E7F79D: vfprintf (vfprintf.c:1636)
==12178== by 0x4E871F8: printf (printf.c:33)
==12178== by 0x1086D1: main (mcve.c:7)

I compiled this with

gcc mcve.c -g -O0

And I ran Valgrind with

valgrind --leak-check=yes ./a.out

Then I discovered there's --track-origins=yes . It attempts to help, but it gets lost easily with pointers. For instance, it doesn't work for the next MCVE:

#include <stdio.h>

int f2(int *p)
{
    (*p)++;
    return *p;
}

int f1(void)
{
    int d;
    return f2(&d);
}

int main(void)
{
    printf("%d\n", f1());
    return 0;
}

Here it says that the error is in the stack frame of f1() . It's kind of helpful, but considering the performance penalty involved, maybe it's not worth it.

What can I do to make best use of Valgrind?

Answer 1

A deliberate decision in Valgrind is that it does not complain just because some uninitialised memory is copied or similar. It complaints only when the use of uninitialised memory changes the behaviour of your program. This deliberate choice avoids a lot of false-positive errors.

See http://www.valgrind.org/docs/manual/mc-manual.html#mc-manual.uninitvals for more information.