在尝试将fcntl()与命令F_GETFL和F_SETFL一起使用时，我遇到了一些问题：
为什么fcntl(fd, F_GETFL)返回的标志只包含我在打开文件时设置的位的子集？它只显示可修改的那些吗？
当使用fcntl(fd, F_SETFL, flag)时，我应该如何传递标志参数，我是否需要首先通过fcntl(fd, F_GETFL)读取标志，然后修改它并传递它？或者在内部它只是用新的参数进行&操作？
在哪里可以找到32位（或更少）位的打开文件标志的完整列表？
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
答案 0 :(得分：4)
1）fcnl的返回是一个代码，描述了该函数是否已成功以及如何：
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.
<强>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）HERE您有完整的说明。
答案 1 :(得分：4)
你问：
为什么fcntl（fd，F_GETFL）返回的标志只包含我在打开文件时设置的位的子集？它只显示可修改的那些吗？
没有;它只显示了记住的那些＆＃34;由系统，例如O_RDWR。这些可以真正称为＆＃34; flags＆＃34;。 OR oflag参数中的一些其他位更像是＆＃34;命令性指令＆＃34;到open系统调用：例如，O_CREAT说＆＃34;如果文件不存在，请创建此文件＆＃34;并且O_TRUNC说＆＃34;请截断它＆＃34;，它们都不是＆＃34;标志＆＃34;。在创建时被截断的文件与在创建时不被截断的文件无法区分：它们只是＆＃34;文件＆＃34;。因此，在open完成创建或截断文件后，它不会打扰＆＃34;记住＆＃34;那段历史。它只记得＆＃34;重要的事情，比如文件是否可以阅读或写作。
编辑添加：这些不同类型的旗帜有半官方名称。 O_RDWR是＆＃34;访问模式＆＃34; （记住，不可修改）; O_APPEND是一种＆＃34;运营模式＆＃34; （记住，通常可以修改）; O_TRUNC是一个＆＃34;开放时间标志＆＃34; （属于open操作本身，而不属于文件描述符;因此不记得）。请注意＆＃34;访问模式＆＃34;不可修改 - 您无法使用fcntl将只读fd转换为只写fd。
当使用
fcntl(fd, F_SETFL, flag)时，我应该如何传递标志参数，我是否需要首先通过fcntl(fd, F_GETFL)读取标志，然后修改它并传递它？或者在内部它只是用新的参数进行&操作？
F_SETFL用完全覆盖你所传入的内容（尽管它会忽略你设置的那些微不足道的真正标志，例如O_TRUNC）。如果要设置特定标志并保持其他标志不变，则必须F_GETFL旧标志，|新标志，然后F_SETFL结果。此必须作为两个单独的系统调用完成;据我所知，没有原子或线程安全的方法来实现它。
在哪里可以找到32位（或更少）位的打开文件标志的完整列表？
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，在OS X上可以设置（或取消设置）正好三位。而在Linux上，手册页显示this：
On Linux this command can change only the O_APPEND, O_ASYNC,
O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
顺便提一下，一些Linux文件系统具有＆＃34;仅附加文件的概念＆＃34;在文件系统级别;如果您打开其中一个文件，然后尝试清除生成的描述符的O_APPEND标记，则会出现EPERM错误。可以使用chattr实用程序在文件系统级别控制文件的＆＃34;仅附加＆＃34; -
这是您的测试程序的更系统化版本。你可能不感兴趣，但我通过写它来学到了东西，所以我把它留在这里。 ：）
#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);
}
我的MacBook上的这个程序的输出（因为它应该在任何POSIX系统AFAIK上）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