UNIX Advanced Environment Programming Chapter 3 File IO

Time:2019-8-26

3.3 Functions open and openat

open

int open(const char *pathname, int flags);

int open(const char *pathname, int flags, mode_t mode);

pathnameOpen or create file names

flags :

Required options: One of the three constants must be specified, and only one must be allowed.
O_RDONLY: Read-only Open
O_WRONLY: Write Only Open
O_RDWR: Readable Writable Open
Options:
O_APPEND denotes additions. If the file already has content, the data written in the open file is appended to the end of the file without overwriting the original content.
O_CREAT Creates this file if it does not exist. When using this option, you need to provide a third parameter, mode, to indicate access to the file.
If O_EXCL specifies O_CREAT at the same time and the file already exists, an error returns.
O_TRUNC truncates a file to 0 bytes if it already exists and opens in a write-only or read-write manner.
For device files, O_NONBLOCK can be opened in O_NONBLOCK mode to do non-blocking I/O (Nonblock I/O).

Mode: Specify file permissions
It can be expressed in octal numbers, such as 0644 for - RW - R - r--, or in bits or in combination with macro definitions such as S_IRUSR and S_IWUSR.
It should be noted that file permissions are determined by both the open mode parameter and the umash mask of the current process.

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