fcntl函数
读法:file control函数
作用:复制文件描述符、设置/获取文件的状态标志
cpp
int fcntl(int fd, int cmd, ... /* arg */ )
fd是文件描述符,cmd是命令(定义的宏),... 表示可变的参数(可有可不有)
cpp
man 2 fcntl
DESCRIPTION:
fcntl() performs one of the operations described below on the open file descriptor fd . The operation is determined by cmd .
1、Duplicating a file descriptor 复制文件描述符
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F_DUPFD (int)
Duplicate the file descriptor fd using the lowest-numbered available file descriptor greater than or equal to arg. This is different from dup2(2), which uses exactly the file descriptor specified. (dup2复制指定的fd,而F_DUPFD用的是lowerst)On success, the new file descriptor is returned.
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F_DUPFD_CLOEXEC (int; since Linux 2.6.24)
2、 File descriptor flags 文件描述标志
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 set, the file descriptor will automatically be closed during a successful execve(2). (If the execve(2) fails, the file descriptor is left open.) If the FD_CLOEXEC bit is not set, the file descriptor will remain open across an execve(2).
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F_GETFD (void)
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F_SETFD (int)
3、 File status flags 文件状态标志
Each open file description has certain associated status flags , initialized by open(2) and possibly modified by fcntl(). Duplicated file descriptors (made with dup(2), fcntl(F_DUPFD), fork(2), etc.) refer to the same open file description, and thus share the same file status flags.
- F_GETFL (void) 获取 指定的文件描述符文件状态flag
Return (as the function result) the file access mode and the file status flags; arg is ignored.
- F_SETFL (int) 设置 文件描述符文件状态flag
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.
4、...
总结:
cpp
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ...);
参数:
fd : 表示需要操作的文件描述符
cmd: 表示对文件描述符进行如何操作
- F_DUPFD : 复制文件描述符,复制的是第一个参数fd,得到一个新的文件描述符(返回值)
int ret = fcntl(fd, F_DUPFD);
- F_GETFL : 获取指定的文件描述符文件状态flag
获取的flag和我们通过open函数传递的flag是一个东西。
- F_SETFL : 设置文件描述符文件状态flag
必选项:O_RDONLY, O_WRONLY, O_RDWR 不可以被修改
可选性:O_APPEND, O_NONBLOCK
O_APPEND 表示追加数据
O_NONBLOK 设置成非阻塞
阻塞和非阻塞:描述的是函数调用的行为。
阻塞函数:调用add函数,进程被发起,并且得到结果之后才会返回。比如终端等待你输入的状态
非阻塞函数:调用函数时立马就能返回,不会阻塞当前的进程。比如输入命令后终端立马执行。
测试:
cpp
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
int main() {
// 1.复制文件描述符
// int fd = open("1.txt", O_RDONLY);
// int ret = fcntl(fd, F_DUPFD); //复制描述符
// 2.修改或者获取文件状态flag
int fd = open("1.txt", O_RDWR); // 这里是RDWR,不能是只有读权限RDONLY,O_APPEND只是往后面追加
if(fd == -1) {
perror("open");
return -1;
}
// 获取文件描述符状态flag
int flag = fcntl(fd, F_GETFL);
if(flag == -1) {
perror("fcntl");
return -1;
}
flag |= O_APPEND; // flag = flag | O_APPEND 在原先flag基础上加入O_APPEND
// 修改文件描述符状态的flag,给flag加入O_APPEND这个标记
int ret = fcntl(fd, F_SETFL, flag); //flag不能直接写O_APPEND,会直接覆盖掉用来的O_RDWR
if(ret == -1) {
perror("fcntl");
return -1;
}
char * str = "nihao";
write(fd, str, strlen(str));
close(fd);
return 0;
}
运行前后:
cpp
1
1nihao