fcntl.md

fcntl.h - File Control Options

The fcntl.h header defines several macros and functions for performing control operations on file descriptors.

Facilities Overview

Facility Category	Key Symbols	Description
File Opening	open, creat	Opening and creating files or devices.
Descriptor Control	fcntl	Performing various operations on file descriptors.
File Status Flags	O_RDONLY, O_WRONLY, O_RDWR	Access modes for open.
Operation Flags	O_CREAT, O_APPEND, O_TRUNC	Creation and behavior flags for open.

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Types & Variables

Access Modes

• O_RDONLY: Open for reading only.
• O_WRONLY: Open for writing only.
• O_RDWR: Open for reading and writing.

Creation Flags

• O_CREAT: Create file if it does not exist.
• O_EXCL: Error if O_CREAT and file exists.
• O_TRUNC: Truncate length to 0 if file exists and is regular.

Status Flags

• O_APPEND: The file is opened in append mode.
• O_NONBLOCK: Do not block on open or subsequent I/O.
• O_SYNC: Write operations are performed synchronously.

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Functions

File Opening

open

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

Opens the file specified by pathname. If O_CREAT is specified in flags, a third argument mode_t mode must be provided to specify permissions.

Example:

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

int main(void) {
    int fd = open("test.txt", O_CREAT | O_WRONLY | O_TRUNC, 0644);
    if (fd != -1) {
        write(fd, "Hello Linux!\n", 13);
        close(fd);
    }
    return 0;
}

Descriptor Control

fcntl

int fcntl(int fd, int cmd, ...)

Performs one of various operations on the open file descriptor fd.

Common Commands:

• F_GETFL: Get file status flags.
• F_SETFL: Set file status flags.
• F_DUPFD: Duplicate file descriptor.

Example:

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main(void) {
    int fd = open("log.txt", O_RDONLY);
    if (fd == -1) return 1;
    int flags = fcntl(fd, F_GETFL, 0);
    
    if (flags & O_APPEND) {
        printf("File is in append mode\n");
    }
    close(fd);
    return 0;
}