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paulf38 • yesterday at 3:36 PM • 1 reply • view on HN

> Valgrind is a mock of standard library/OS functions and I think its existence is a good thing.

That is mostly wrong.

Valgrind wraps syscalls. For the most part it just checks the arguments and records any reads or writes to memory. For a small number of syscalls it replaces the syscall rather than wrapping it (for instance calls like getcontext where it needs to get the context from the VEX synthetic CPU rather than the real CPU).

Depending on the tool it can also wrap or replace libc and libpthread functions. memcheck will replace all allocation functions. DRD and Helgrind wrap all pthread functions.

Replies

1718627440 • yesterday at 4:14 PM

    $ cat test.c
    void main (void) {
      malloc (1000);
    }
    
    $ make test
    cc     test.c   -o test
    
    $ valgrind --leak-check=full --show-leak-kinds=all -s ./test
    Memcheck, a memory error detector
    Command: ./test
    
    HEAP SUMMARY:
        in use at exit: 1,000 bytes in 1 blocks
      total heap usage: 1 allocs, 0 frees, 1,000 bytes allocated
    
    1,000 bytes in 1 blocks are still reachable in loss record 1 of 1
       at 0x483877F: malloc (vg_replace_malloc.c:307)
       by 0x109142: main (in test.c:2)
    
    LEAK SUMMARY:
       definitely lost: 0 bytes in 0 blocks
       indirectly lost: 0 bytes in 0 blocks
         possibly lost: 0 bytes in 0 blocks
       still reachable: 1,000 bytes in 1 blocks
            suppressed: 0 bytes in 0 blocks
> vg_replace_malloc.c:307

What do you think that is? Valgrind tracks allocations by providing other implementations for malloc/free/... .