如何讓子進程不繼承父進程的資源

關於fcntl(fd, F_SETFD, FD_CLOEXEC)設定exec時close的屬性

snd_ctl_hw_open
#define SNDRV_FILE_CONTROL    ALSA_DEVICE_DIRECTORY "controlC%i"
sprintf(filename, SNDRV_FILE_CONTROL, card); // 路徑/dev/snd/controlC0
fd = snd_open_device(filename, fmode);
fcntl(fd,
F_SETFD, FD_CLOEXEC); //
這裡設定為FD_CLOEXEC表示當程式執行exec函數時本fd將被系統自動關閉,表示不傳遞給exec建立的新進程,
如果設定為fcntl(fd, F_SETFD, 0);那麼本fd將保持開啟狀態複製到exec建立的新進程中[luther.gliethttp].
進入核心系統調用
sys_fcntl
do_fcntl
    case F_SETFD:
        err = 0;
        set_close_on_exec(fd, arg & FD_CLOEXEC);

void fastcall set_close_on_exec(unsigned int fd, int flag)
{
    struct files_struct *files = current->files;
    struct fdtable *fdt;
    spin_lock(&files->file_lock);
    fdt = files_fdtable(files);
    if (flag)
        FD_SET(fd, fdt->close_on_exec);
    else
        FD_CLR(fd, fdt->close_on_exec);
    spin_unlock(&files->file_lock);
}
下面是man fcntl看到的對FD_CLOEXEC解釋

File descriptor flags
   The  following  commands manipulate the flags associated with a file descriptor.  Currently, only one such flag is
   defined: FD_CLOEXEC, the close-on-exec flag.  If the FD_CLOEXEC bit is 0, the file  descriptor  will  remain  open
   across an execve(2), otherwise it will be closed.

   F_GETFD (void)
          Read the file descriptor flags; arg is ignored.
   F_SETFD (long)
          Set the file descriptor flags to the value specified by arg.