linux c 的 open(文件路径，O_WRONLY | O_CREAT) 里面的与运算为什么可以实现打不开就创建
发布网友
发布时间：2022-04-23 09:02
共5个回答
热心网友
时间：2023-10-08 23:56
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
返回值:成功返回新分配的文件描述符,出错返回-1并设置errno
在Man Page中open 函数有两种形式,一种带两个参数,一种带三个参数,其实在C代码
中open 函数是这样声明的:
int open(const char *pathname, int flags, ...);
最后的可变参数可以是0个或1个,由flags 参数中的标志位决定,见下面的详细说明。
pathname 参数是要打开或创建的文件名,和fopen 一样,pathname 既可以是相对路径也可以是绝
对路径。flags 参数有一系列常数值可供选择,可以同时选择多个常数用按位或运算符连接起
来,所以这些常数的宏定义都以O_开头,表示or。
必选项:以下三个常数中必须指定一个,且仅允许指定一个。
O_RDONLY 只读打开
O_WRONLY 只写打开
O_RDWR 可读可写打开
以下可选项可以同时指定0个或多个,和必选项按位或起来作为flags 参数。可选项有很多,这
里只介绍一部分,其它选项可参考open(2)的Man Page:
O_APPEND 表示追加。如果文件已有内容,这次打开文件所写的数据附加到文件的末尾而不
覆盖原来的内容。
O_CREAT 若此文件不存在则创建它。使用此选项时需要提供第三个参数mode ,表示该文件
的访问权限。
O_EXCL 如果同时指定了O_CREAT,并且文件已存在,则出错返回。
O_TRUNC 如果文件已存在,并且以只写或可读可写方式打开,则将其长度截断
(Truncate)为0字节。
O_NONBLOCK 对于设备文件,以O_NONBLOCK 方式打开可以做非阻塞I/O(Nonblock I/O).
热心网友
时间：2023-10-08 23:56
O_CREAT：如果打不开就创建
O_WRONLY | O_CREAT中间使用“|”，所以支持打不开就创建
热心网友
时间：2023-10-08 23:57
热心网友
时间：2023-10-08 23:57
如：二进制的 010 | 001 结果是 011，而 010 & 001 结果就是0了。
O_WRONLY 和 O_CREAT 的关系就相当于上面的 010 和 001。他们位或的值不是0，位与的值就是0了。0表示什么都不做。用了位或后，就在一个整型的值上设置了不同的标志位，open函数会检测对应的标志位，如果该标志位设置为1了，就执行对应的操作。
O_CREAT的意思就是创建的意思，在这里就是将 创建文件 的标志位设置为1，这样open函数无法写这个文件的时候就会创建他。
热心网友
时间：2023-10-08 23:58
int open(const char *pathname, int oflag, ... /*
mode_t mode */ );
We show the third argument as ..., which is the ISO C way to specify that the number and types of the remaining arguments may vary. For this function, the third argument is used only when a new file is being created, as we describe later. We show this argument as a comment in the prototype.
The pathname is the name of the file to open or create. This function has a multitude of options, which are specified by the oflag argument. This argument is formed by ORing together one or more of the following constants from the <fcntl.h> header:
O_RDONLY
Open for reading only.
O_WRONLY
Open for writing only.
O_RDWR
Open for reading and writing.
Most implementations define O_RDONLY as 0, O_WRONLY as 1, and O_RDWR as 2, for compatibility with older programs.
One and only one of these three constants must be specified. The following constants are optional:
O_APPEND
Append to the end of file on each write. We describe this option in detail in Section 3.11.
O_CREAT
Create the file if it doesn't exist. This option requires a third argument to the open function, the mode, which specifies the access permission bits of the new file. (When we describe a file's access permission bits in Section 4.5, we'll see how to specify the mode and how it can be modified by the umask value of a process.)
O_EXCL
Generate an error if O_CREAT is also specified and the file already exists. This test for whether the file already exists and the creation of the file if it doesn't exist is an atomic operation. We describe atomic operations in more detail in Section 3.11.
O_TRUNC
If the file exists and if it is successfully opened for either write-only or readwrite, truncate its length to 0.
O_NOCTTY
If the pathname refers to a terminal device, do not allocate the device as the controlling terminal for this process. We talk about controlling terminals in Section 9.6.
O_NONBLOCK
If the pathname refers to a FIFO, a block special file, or a character special file, this option sets the nonblocking mode for both the opening of the file and subsequent I/O. We describe this mode in Section 14.2.
