关于fcntl(fd, F_SETFD, FD_CLOEXEC)设置exec时close的属性

关于fcntl(fd, F_SETFD, FD_CLOEXEC)设置exec时close的属性

snd_ctl_hw_open
#define SNDRV_FILE_CONTROL    ALSA_DEVICE_DIRECTORY "controlC%i"
sprintf(filename, SNDRV_FILE_CONTROL, card); // 路径/dev/snd/controlC0
fd = snd_open_device(filename, fmode);
fcntl(fd, F_SETFD, FD_CLOEXEC); // 这里设置为FD_CLOEXEC表示当程序执行exec函数时本fd将被系统自动关闭,表示不传递给exec创建的新进程, 如果设置为fcntl(fd, F_SETFD, 0);那么本fd将保持打开状态复制到exec创建的新进程中[luther.gliethttp].
进入内核系统调用
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看到的对FD_CLOEXEC解释

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.