pclose（）不会给我进程退出代码

Question

I've got a strange problem here.

I wrote some C++ code with uses popen(3) , read(3) and pclose(3) to run various shell commands, parse their output, and do some other stuff. The program seems to work well.

Then I extracted some of the code into a shared library and tried to use it from a C# program running under Mono. Now the C++ code doesn't work at all.

Running the code in GDB, it seems that pclose(3) always unconditionally returns -1 rather than the real exit code of the child process.

Watching strace , it seems that wait4(2) fails with ECHILD . And, as best as I can tell, it does that because SIGCHLD is being delivered to one of Mono's other threads, which promptly does several wait4(2) calls, and then carries on with its day.

Doing some Google searching, it appears the kernel chooses a thread at random to send SIGCHLD to. (In other words, in a multithreaded program it is guaranteed to always choose the "wrong" thread.)

Obviously I have no control over what the Mono runtime decides to do. So how can I invoke external processes and reliably get their exit codes?

Answer 1

pclose() never returns the real exit code from the child process.

pclose rather returns the value of the stat_loc parameter to the wait() call.

In cases, when waitpid() with the process id of the child returns -1 , pclose() returns -1 as well.

This may happen if you have broken code in other threads that waits for arbitrary child processes instead of only waiting for self created child processes.