An fork with an invalid command causes a memory leak in valgrind

Question

I have the following code that executes an invalid command inside fork. The following code returns a memory leak in valgrind.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdarg.h>
#include <errno.h>
#include <unistd.h>


int external_cmd(char **argv)
{
    int pid;

    if ((pid = fork()) == -1)
        return -1;

    if (pid == 0) {
        /* child */
        execvp(argv[0], argv);
        exit(0);

    } else if (pid < 0)
        return -1;

    int status;
    while (wait(&status) != pid);

    return 0;
}

int main ()
{
    char *argv[8] = {0};
    argv[0] = "tawtaw"; //<--------- invalid command
    argv[1] = "-a";

    char *mem = strdup("anychar");

    /* fork call */
    external_cmd(argv);

    free(mem);

   return(0);
}

executing the above code with valgrind return:

$ valgrind --leak-check=full --show-leak-kinds=all ./test
==11573== Memcheck, a memory error detector
==11573== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==11573== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==11573== Command: ./test
==11573== 
==11574== 
==11574== HEAP SUMMARY:
==11574==     in use at exit: 8 bytes in 1 blocks
==11574==   total heap usage: 1 allocs, 0 frees, 8 bytes allocated
==11574== 
==11574== 8 bytes in 1 blocks are still reachable in loss record 1 of 1
==11574==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11574==    by 0x4EBF729: strdup (strdup.c:42)
==11574==    by 0x400747: main (in /home/mohamed/Desktop/tech/test/test)
==11574== 
==11574== LEAK SUMMARY:
==11574==    definitely lost: 0 bytes in 0 blocks
==11574==    indirectly lost: 0 bytes in 0 blocks
==11574==      possibly lost: 0 bytes in 0 blocks
==11574==    still reachable: 8 bytes in 1 blocks
==11574==         suppressed: 0 bytes in 0 blocks
==11574== 
==11574== For counts of detected and suppressed errors, rerun with: -v
==11574== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
==11573== 
==11573== HEAP SUMMARY:
==11573==     in use at exit: 0 bytes in 0 blocks
==11573==   total heap usage: 1 allocs, 1 frees, 8 bytes allocated
==11573== 
==11573== All heap blocks were freed -- no leaks are possible
==11573== 
==11573== For counts of detected and suppressed errors, rerun with: -v
==11573== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

NOTE. If I execute the code with a valid ls instead of tawtaw, then valgring will not return a memory leak.

What am I missing?

Answer 1

It is expected. When execve()it cannot execute the command, it returns control to your code, and then you exit and never free memory from strdup().

execve , , , strdup().

Answer 2

"", , . , . .

- , exit(0) return -1;. exit, , . ++ RAII , . , RAII:

#include <cstdlib>
#include <cstdarg>
#include <cerrno>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <stdexcept>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

// This function should be standard in some kind of POSIX C++ library
void execvp(const std::string& program, const std::vector<std::string>& args)
{
    std::vector<const char*> real_argv(args.size()+1);
    for (const std::string& s : args)
        real_argv.push_back(s.c_str());
    real_argv.push_back(nullptr);
    // have to use const_cast because of the broken const model of C
    execvp(program.c_str(), const_cast<char**>(real_argv.data()));
    throw std::runtime_error((std::string("Could not execvp ") + args[0]).c_str());
}

int external_cmd(const std::string& program, const std::vector<std::string>& args)
{
    int pid;

    if ((pid = fork()) == -1)
        return -1;

    if (pid == 0) {
        /* child */
        execvp(program, args);

    } else if (pid < 0)
        return -1;

    int status;
    while (wait(&status) != pid);

    return 0;
}

int main ()
{
    try {
        std::vector<std::string> args;
        args.push_back("tawtaw");
        args.push_back("-a");

        std::string s("12345678"); // RAII

        /* fork call */
        external_cmd(args[0], args);

        return EXIT_SUCCESS;
    } catch (std::exception& e) {
        std::cerr << e.what() << ". Exiting.\n";
        return EXIT_FAILURE;
    } catch ( ... ) {
        std::cerr << "Unexpected error. OS message is: " << strerror(errno) << ". Exiting.\n";
        return EXIT_FAILURE;
    }
}