Pass pointer to integer on an execv call in C

Question

I am writing a rudimentary shell program in C which uses a parent process to handle shell events and fork() to create child processes that call execv on another executable (also C).

I am trying to keep a process counter on the parent process. And as such I thought of the possibility of creating a pointer to a variable that keeps track of how many processes are running.

However, that seems to be impossible since the arguments execv (and the program executed by it) takes are of type char * const argv[].

I have tried to keep track of the amount of processes using mmap for shared memory between processes, but couldn't get that to work since after the execv call the process simply dies and doesn't let me update the process counter.

In summary, my question is: Is there a way for me to pass a pointer to an integer on an execv call to another program?

Thank you in advance.

rici · Accepted Answer · 2018-09-29 18:26:50Z

You cannot meaningfully pass a pointer from one process to another because the pointer is meaningless in the other process. Each process has its own memory, and the address is relative to that memory space. In other words, the virtual memory manager lets every process pretend it has the entire machine's memory; other processes are simply invisible.

However, you do have a few options for setting up communications between related processes. The most obvious one is a pipe, which you've presumably already encountered. That's more work, though, because you need to make sure that some process is always listening for pipe communications.

Another simple possibility is to just leave a file descriptor open when you fork and exec (see the close-on-exec flag to see how to accomplish the latter); although mmap is not preserved by exec, you can remap the memory to the open fd in the child process. If you don't want to pass the fd, you can mmap the memory to a temporary file, and use an environment variable to record the name of the temporary file.

Another possibility is Posix shared memory. Again, you might want to communicate the shm name through an environment variable, rather than hard-coding it in to the application.

Note that neither shared mmaps nor shared memory are atomic. If you're incrementing a counter, you'll need to use some locking mechanism to avoid race conditions.

For possibly a lot more information than you really wanted, you can read ESR's overview of interprocess communication techniques in Chapter 7 of The Art of Unix Programming.

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