c-F_GETFL和F_SETFL的用法
在尝试将c-F_GETFL和F_SETFL的用法,c,file-io,fcntl,C,File Io,Fcntl,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）返回的标志只包含打开文件时设置的位的子集？它是否只显示可修改的内容 使用fcntl（fd，F_SETFL，flag）时，我应该如何传递flag参数，我是否需要先通过fcntl（fd，F_GETFL）读取flag，然后修改并传递它？或者在内部，它只是对新参数执行一点&操作 在哪里可以找到32位（或更少）打开文件标志的完整列表 代码-[dup\u fd\u share.c
fcntl（）F_GETFLF_SETFLfcntl（fd，F_GETFL）fcntl（fd，F_SETFL，flag）fcntl（fd，F_GETFL）&// 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;
}
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
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.
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.
O_RDWRoflag打开的系统调用的“命令指令”：例如，O_create
表示“如果不存在请创建此文件”，而O_TRUNC
表示“请截断它”，两者都不是“标志”。创建时被截断的文件与创建时未被截断的文件无法区分：它们都只是“文件”。因此，在open
创建或截断文件后，它就不必“记住”该历史。它只“记住”重要的事情，比如文件是否打开以供读取或写入
编辑添加：这些不同种类的旗帜有半官方名称O_RDWR
是“访问模式”（记住，不可修改）O_APPEND
是一种“操作模式”（记住，通常可修改）O_TRUNC
是一个“打开时间标志”（属于open
操作本身，不属于文件描述符；因此不被记住）。请注意，“访问模式”不可修改-不能使用fcntl
将只读fd转换为只读fd
使用fcntl（fd，F_SETFL，flag）
时，我应该如何传递flag参数，我是否需要先通过fcntl（fd，F_GETFL）
读取flag，然后修改并传递它？或者在内部，它只是对新参数执行一点&
操作
F_SETFL
用您传入的内容覆盖标志（尽管它会忽略您设置非真正标志位的微小尝试，例如O_TRUNC
）。如果要设置一个特定的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。这必须作为两个独立的系统调用来完成；据我所知，没有原子或线程安全的方法来实现它
在哪里可以找到32位（或更少）打开文件标志的完整列表
在fcntl.h
或其文档中（manfcntl
）。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）的位正好有三位。而在Linux上，手册页上说：
顺便说一句，一些Linux文件系统在文件系统级别具有“仅附加文件”的概念；如果打开其中一个文件，然后尝试清除结果描述符的O_APPEND
标志，则会出现EPERM
错误。可以使用该实用程序在文件系统级别控制文件的“仅附加”属性
这是您的测试程序的一个更系统的版本。你可能对它不感兴趣，但我通过写作学到了一些东西，所以我把它留在这里。：）
O_create
和O_TRUNC
是控制open
调用本身行为的修饰符；它们不是应用于打开文件的模式。
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.
On Linux this command can change only the O_APPEND, O_ASYNC,
O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
#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);
}
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