C++ memory leak while using std::string in child process

Question

I'm having problem with this piece of code, valgrind detects memory leaks in std::basic_string and I don't know what am I doing wrong. The leak is only when std::string is used in child process. Could you please tell me, where the problem is? I've never used fork() in C++ before so I don't have much exprience.

#include <iostream> #include <string> #include <cerrno> #include <cstdio> #include <cstdlib> #include <unistd.h> #include <sys/wait.h> int main(int argc, char *argv[]) { pid_t childPid = fork(); if (childPid == -1) { perror("fork"); return EXIT_FAILURE; } else if (childPid == 0) { std::cout << "Child PID: " << getpid() << std::endl; std::string str("something"); //valgrind detects mem leak here _Exit(EXIT_SUCCESS); } else { //std::string str("something"); //but not here } waitpid(-1, 0, 0); return EXIT_SUCCESS; }

Instead of calling _Exit try returning like you do at the end of the function. — Jonathan Potter
– Jonathan Potter, Commented Mar 27, 2013 at 22:39
Does _Exit prevent the stack from unwinding? That might prevent the std::string destructor from running. — Ben
– Ben, Commented Mar 27, 2013 at 22:40

GManNickG · Accepted Answer · 2013-03-27 22:49:10Z

_Exit will not run any destructors or atexit functions, it simply ends immediately.

Obviously this punches a giant hole through RAII, so Don't Do That.

One way of implementing Don't Do That while maintaining the same exit strategy might be:

template <typename Func, typename... Args> int wrap_cpp_code(Func&& func, Args&&.. args) { try { return std::forward<Func>(func)(std::forward<Args>(args)...); } catch (...) { return EXIT_FAILURE; } }

Which won't cough up an return value until all destructors under its scope are run, giving:

int child_main(int argc, char *argv[]) { std::cout << "Child PID: " << getpid() << std::endl; std::string str("something"); return EXIT_SUCCESS; } int main(int argc, char *argv[]) { pid_t childPid = fork(); if (childPid == -1) { perror("fork"); return EXIT_FAILURE; } else if (childPid == 0) { int ret = wrap_cpp_code(child_main, argc, argv); _Exit(ret); } else { /* int ret = wrap_cpp_code(xyz, argc, argv); */ } waitpid(-1, 0, 0); return EXIT_SUCCESS; }

But this still fails to account for atexit functions or global destructors. So still Avoid Doing That.

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