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.