c-F_GETFL和F_SETFL的用法

Question

在尝试fcntl()与command F_GETFL和结合使用时F_SETFL，我遇到了一些问题：

1. 为什么fcntl(fd, F_GETFL)打开文件时返回的标志仅包含我设置的位的子集？它仅显示可修改的内容吗？

2. 使用时fcntl(fd, F_SETFL, flag)，我应该如何传递标志参数，我是否需要先通过标志读取标志fcntl(fd, F_GETFL)，然后对其进行修改并传递它？还是在内部它只是&对新的参数做了一点操作？

3. 在哪里可以找到打开文件标志的32位（或更少）位的完整列表？

代码-[dup_fd_share.c]：

// prove duplicated fd shared file offset and open file status,
// TLPI exercise 5.5

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>

#define BUF_SIZE 100

void fd_share() {
    char *fp = "/tmp/fd_share.txt";
    char *buf = "abc\n";
    int write_size = 4;
    int fd, fd2;
    off_t cur, cur2;
    int open_flag, open_flag2;

    // open
    int flag = O_RDWR | O_CREAT | O_TRUNC | O_APPEND;
    printf("file flag param: %x\n", flag);
    fd = open(fp, flag, 0644);

    // dup
    fd2 = dup(fd);

    // initial offset
    cur = lseek(fd, 0, SEEK_CUR);
    printf("fd[%d] offset: %ld\n", fd, cur);
    cur2= lseek(fd2, 0, SEEK_CUR);
    printf("fd[%d] offset: %ld\n", fd2, cur2);

    // write, offset change,
    write(fd, buf, 4);
    printf("write %d chars\n", write_size);

    // new offset
    cur = lseek(fd, 0, SEEK_CUR);
    printf("fd[%d] offset: %ld\n", fd, cur);
    cur2= lseek(fd2, 0, SEEK_CUR);
    printf("fd[%d] offset: %ld\n", fd2, cur2);

    // get original open file flag,
    open_flag = fcntl(fd, F_GETFL);
    printf("fd[%d] open flag: %x\n", fd, open_flag);
    open_flag2 = fcntl(fd2, F_GETFL);
    printf("fd[%d] open flag: %x\n", fd2, open_flag2);

    // change open file flag,
    open_flag &= ~O_APPEND;
    if(fcntl(fd, F_SETFL, open_flag) == -1) {
        printf("failed to set flag\n");
        return;
    }
    printf("change open file flag, remove %s\n", "O_APPEND");

    open_flag = fcntl(fd, F_GETFL);
    printf("fd[%d] open flag: %x\n", fd, open_flag);
    open_flag2 = fcntl(fd2, F_GETFL);
    printf("fd[%d] open flag: %x\n", fd2, open_flag2);

    close(fd);
}

int main(int argc, char *argv[]) {
    fd_share();
    return 0;
}

输出：

file flag param: 642

fd[3] offset: 0
fd[4] offset: 0
write 4 chars
fd[3] offset: 4
fd[4] offset: 4

fd[3] open flag: 402
fd[4] open flag: 402
change open file flag, remove O_APPEND
fd[3] open flag: 2
fd[4] open flag: 2

Answer 1

您问：

为什么fcntl（fd，F_GETFL）返回的标志仅包含我在打开文件时设置的位的子集？它仅显示可修改的内容吗？

没有; 它仅显示系统“记住”的内容，例如O_RDWR。这些实际上可以称为“标志”。该oflag参数中ORed的其他一些位更像是open系统调用的“命令性指令” ：例如，O_CREAT说“请创建该文件（如果不存在）”和O_TRUNC“请截断它”，但这两个都不是“标志”。在创建时被截断的文件与在创建时未被截断的文件是没有区别的：它们都只是“文件”。因此，在open完成创建或截断文件后，不必“记住”该历史记录。它只会“记住”重要的事情，

编辑添加：这些不同种类的标志具有半官方名称。O_RDWR是“访问模式”（记住的，不可修改的）； O_APPEND是一种“操作模式”（记住，通常可以修改）；O_TRUNC是一个“开放时间标志”（与open操作本身有关，与文件描述符无关；因此不被记住）。请注意，“访问模式”不可修改-您不能使用fcntl将只读fd转换为只写fd。

使用时fcntl(fd, F_SETFL, flag)，我应该如何传递标志参数，我是否需要先通过标志读取标志fcntl(fd, F_GETFL)，然后对其进行修改并传递它？还是在内部它只是&对新的参数做了一点操作？

F_SETFL用您传入的内容完全覆盖这些标志（尽管它将忽略您为设置位（实际上不是）标志而做的微不足道的尝试，例如O_TRUNC）。如果要设置特定标志，而其他标志保持原样，则必须先F_GETFL插入旧标志，再|插入新标志，然后再返回F_SETFL结果。这必须作为两个单独的系统调用来完成；据我所知，没有原子或线程安全的方法来完成它。

在哪里可以找到打开文件标志的32位（或更少）位的完整列表？

在fcntl.h或其文档（man fcntl）中。例如，在我的MacBook上，手册页显示：

The flags for the F_GETFL and F_SETFL commands are as follows:

      O_NONBLOCK   Non-blocking I/O; if no data is available to a read call, or if a write operation would block, the read or write
                   call returns -1 with the error EAGAIN.

      O_APPEND     Force each write to append at the end of file; corresponds to the O_APPEND flag of open(2).

      O_ASYNC      Enable the SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be
                   read.

换句话说，您可以在OS X上完全设置（或取消设置）三个位。而在Linux上，手册页显示如下：

On Linux this command can change only the O_APPEND, O_ASYNC,
O_DIRECT, O_NOATIME, and O_NONBLOCK flags.

顺便说一下，某些Linux文件系统在文件系统级别具有“仅附加文件”的概念。如果打开这些文件之一，然后尝试清除生成的描述符的O_APPEND标志，则会收到EPERM错误消息。可以使用chattr实用程序在文件系统级别上控制文件的“仅追加” 功能。

这是测试程序的更系统的版本。它可能对您不感兴趣，但是我通过编写来学习了一些东西，所以我将其留在这里。:)

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int main() {
    int fd = open("/tmp/fd_share.txt", O_RDWR | O_CREAT | O_TRUNC | O_APPEND, 0644);

    // append to empty file
    write(fd, "aaaaaaaaaa", 10);

    off_t cur = lseek(fd, 1, SEEK_SET);
    printf("offset after being set to 1: %ld\n", (long)cur);

    // append
    write(fd, "bbbbbbbb", 8);

    cur = lseek(fd, 0, SEEK_CUR);
    printf("offset after appending bbbbbbbb: %ld\n", (long)cur);

    cur = lseek(fd, 2, SEEK_SET);
    printf("offset after being set to 2: %ld\n", (long)cur);

    // now toggle "append mode" to FALSE
    int open_flag = fcntl(fd, F_GETFL);
    if (fcntl(fd, F_SETFL, open_flag & ~O_APPEND) == -1) {
        printf("failed to set flag\n");
        return 0;
    }

    cur = lseek(fd, 0, SEEK_CUR);
    printf("offset after unsetting O_APPEND: %ld\n", (long)cur);

    cur = lseek(fd, 3, SEEK_SET);
    printf("offset after being set to 3: %ld\n", (long)cur);

    // write without appending
    write(fd, "cccc", 4);

    cur = lseek(fd, 0, SEEK_CUR);
    printf("offset after writing cccc: %ld\n", (long)cur);

    // now toggle "append mode" to TRUE
    open_flag = fcntl(fd, F_GETFL);
    if (fcntl(fd, F_SETFL, open_flag | O_APPEND) == -1) {
        printf("failed to set flag\n");
        return 0;
    }

    cur = lseek(fd, 0, SEEK_CUR);
    printf("offset after unsetting O_APPEND: %ld\n", (long)cur);

    // append
    write(fd, "dd", 2);

    cur = lseek(fd, 0, SEEK_CUR);
    printf("offset after appending dd: %ld\n", (long)cur);

    close(fd);
}

该程序在我的MacBook上的输出（应在任何POSIX系统AFAIK上输出）是：

offset after being set to 1: 1
offset after appending bbbbbbbb: 18
offset after being set to 2: 2
offset after unsetting O_APPEND: 2
offset after being set to 3: 3
offset after writing cccc: 7
offset after unsetting O_APPEND: 7
offset after appending dd: 20

Answer 2

1) fcnl 的返回是描述函数是否成功以及如何执行的代码：

返回值

   For a successful call, the return value depends on the operation:

   F_DUPFD  The new descriptor.

   F_GETFD  Value of file descriptor flags.

   F_GETFL  Value of file status flags.

   F_GETLEASE
            Type of lease held on file descriptor.

   F_GETOWN Value of descriptor owner.

   F_GETSIG Value of signal sent when read or write becomes possible, or
            zero for traditional SIGIO behavior.

   F_GETPIPE_SZ, F_SETPIPE_SZ
            The pipe capacity.

   F_GET_SEALS
            A bit mask identifying the seals that have been set for the
            inode referred to by fd.

   All other commands
            Zero.

   On error, -1 is returned, and errno is set appropriately.

错误

   EACCES or EAGAIN
          Operation is prohibited by locks held by other processes.

   EAGAIN The operation is prohibited because the file has been memory-
          mapped by another process.

   EBADF  fd is not an open file descriptor

   EBADF  cmd is F_SETLK or F_SETLKW and the file descriptor open mode
          doesn't match with the type of lock requested.

   EBUSY  cmd is F_SETPIPE_SZ and the new pipe capacity specified in arg
          is smaller than the amount of buffer space currently used to
          store data in the pipe.

   EBUSY  cmd is F_ADD_SEALS, arg includes F_SEAL_WRITE, and there
          exists a writable, shared mapping on the file referred to by
          fd.

   EDEADLK
          It was detected that the specified F_SETLKW command would
          cause a deadlock.

   EFAULT lock is outside your accessible address space.

   EINTR  cmd is F_SETLKW or F_OFD_SETLKW and the operation was
          interrupted by a signal; see signal(7).

   EINTR  cmd is F_GETLK, F_SETLK, F_OFD_GETLK, or F_OFD_SETLK, and the
          operation was interrupted by a signal before the lock was
          checked or acquired.  Most likely when locking a remote file
          (e.g., locking over NFS), but can sometimes happen locally.

   EINVAL The value specified in cmd is not recognized by this kernel.

   EINVAL cmd is F_ADD_SEALS and arg includes an unrecognized sealing
          bit.

   EINVAL cmd is F_ADD_SEALS or F_GET_SEALS and the filesystem
          containing the inode referred to by fd does not support
          sealing.

   EINVAL cmd is F_DUPFD and arg is negative or is greater than the
          maximum allowable value (see the discussion of RLIMIT_NOFILE
          in getrlimit(2)).

   EINVAL cmd is F_SETSIG and arg is not an allowable signal number.

   EINVAL cmd is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and l_pid
          was not specified as zero.

   EMFILE cmd is F_DUPFD and the process already has the maximum number
          of file descriptors open.

   ENOLCK Too many segment locks open, lock table is full, or a remote
          locking protocol failed (e.g., locking over NFS).

   ENOTDIR
          F_NOTIFY was specified in cmd, but fd does not refer to a
          directory.

   EPERM  Attempted to clear the O_APPEND flag on a file that has the
          append-only attribute set.

   EPERM  cmd was F_ADD_SEALS, but fd was not open for writing or the
          current set of seals on the file already includes F_SEAL_SEAL.

2）要设置的标志是您的选择：：

F_SETFL (int)

   Set the file status flags to the value specified by arg.  File
   access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation
   flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are
   ignored.  On Linux this command can change only the O_APPEND,
   O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.  It is not
   possible to change the O_DSYNC and O_SYNC flags; see BUGS,
   below.

3）在这里你有一个完整的描述。