Today, I wrote a test code for fork/execvp/waitpid. In the parent process, it fork 100 child processes which only execute "date" to print the current datetime. When any child process die, the parent process will receive a SIGCHLD signal. Then, the parent process will re-fork-execvp the child process again.

I find the system memory(NOT includes the buffers and caches) will go higher and higher and even I shutdown all of the parent process and the child processes, it will not go down. Why?

My environment is Linux 2.6.9 and g++ 3.4.2

The following is my code:

//////////////////////////////////////////////////////////////////////////////////
// fork.cpp
// g++ -Wall fork.cpp -o fork

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>

char* argv[] = { "data", (char*)0 };

void run()
{
// Child process
if (fork() == 0)
if (execvp("date", argv) < 0)
exit(0);
}

// SIGCHLD signal handler
void sig_child(int sig)
{
pid_t pid;
int stat;
while ((pid = waitpid(-1, &stat, WNOHANG)) > 0)
run();
}

int main()
{
// Setup signal handler
struct sigaction act;
memset(&act, 0, sizeof(act));
act.sa_handler = sig_child;
sigemptyset(&act.sa_mask);
if (sigaction(SIGCHLD, &act, 0) < 0)
return -1;

// Run 100 child processes
for (int n = 0; n < 100; n++)
run();

// Zzz...
for (;
pause();
return 0;
}

//////////////////////////////////////////////////////////////////////////////////

Thanks,
Richard

http://man.he.net/man2/waitpid
[EDIT] I'm wrong, forget it.

[EDIT 2] This is a fun one. Are you sure you want the WNOHANG flag? I think that indicates the function should return even if the found process is not finished.

Thanks.

From "man waitpid"

WNOHANG which means to return immediately if no child has exited.

If I don't use WNOHANG, "while ((pid = waitpid(-1, &stat, 0)) > 0)" will run for ever except -1 return.

From lots of examples and books, it looks "while ((pid = waitpid(-1, &stat, WNOHANG)) > 0)" is OK. For example, Richard Stevens's UNIX Netowrk Programming's example.

Also, I tested without WNOHANG, the memory still go higher.

What processes are still running?
Are the processes from your execvp still running when you're looking at your memory?

No child processes are running because it is just "date" which only prints the current datetime and exit immediately.

=============================================
char* argv[] = { "date", (char*)0 };

void run()
{
// Child process
if (fork() == 0)
if (execvp("date", argv) < 0)
exit(0);
}

=============================================

Did you check that no processes are running with a ps -Al ?

because execvp replaces the running process I was wondering if any zombies are left hanging about

Yes, I already check. There is not any child processes and no zombies ( I already call waitpid in a loop).

When I kill parent process, even there are some child processes which are not "waited" by parent process, they will be "waited" by init daemon. So I don't think the memory "disappeared" are used by both parent process or child processes.

After check /proc/meminfo, I found the memory gone is LowFree. I think this means kernel memory leak, right?

When I test on other linux with version 2.6.9-34, everything is fine. The leaking version is 2.6.9-1.

Thanks guys.

Never trust a .1 release