Comment by rcxdude

5 days ago

most calls to system() that I've seen could be replaced with exec without much difficulty. There's relatively few that actually need the shell functionality.

17 comments

rcxdude

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oguz-ismail  5 days ago

system() involves fork()ing, setting up signal handlers, exec()ing and wait()ing. You won't be replacing it with exec, most of the time you'll be reimplementing it for absolutely no reason.

  • kragen  5 days ago

    Python has os.spawnl, os.spawnv, etc., which fork()s, wait4()s, etc., without involving a shell. This is much better; this is the library function you should be using instead of system() most of the time. Unfortunately I don't think glibc has an equivalent!

        strace -o tmp.spawnlp -ff python3 -c 'import os; os.spawnlp(os.P_WAIT, "true", "true")'

    In parent:

        clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fdc03233310) = 225954
    wait4(225954, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 225954
    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=225954, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---

    In child:

        set_robust_list(0x7fdc03233320, 24)     = 0
    gettid()                                = 225954
    clock_gettime(CLOCK_MONOTONIC, {tv_sec=2458614, tv_nsec=322829153}) = 0
    clock_gettime(CLOCK_MONOTONIC, {tv_sec=2458614, tv_nsec=323030718}) = 0
    execve("/usr/local/bin/true", ["true"], 0x7ffdc5008458 /* 44 vars */) = -1 ENOENT (No such file or directory)
    execve("/usr/bin/true", ["true"], 0x7ffdc5008458 /* 44 vars */) = 0

    Here, I think strace shows clone() rather than fork() because glibc's fork() is a library function that invokes clone(), rather than a real system call.

  • panzi  4 days ago

    There is posix_spawn(). Some operating systems even implement that as a system call (not Linux). Implementing that as a system call has the advantage that spawning a new process from a process that has huge memory mapping is fast, because the memory mappings don't need to be copied (yes, I know the memory is copy on write, but the mappings themselves have to be correctly copied with the information needed for copy on write).