Class 26 of 37 • System Programming (C) \t\t\t\t

Low-Level File I/O in Linux (C Programming)

In the Unix and Linux philosophy, "Everything is a file". Disk files, directories, terminal screens, pipes, hardware devices, and network sockets are all accessed through standard File Descriptors using low-level POSIX system calls.

Core Low-Level System Calls

     int open(const char *path, int flags, mode_t mode); : Opens existing file or creates a new one, returning an integer file descriptor.

     ssize_t read(int fd, void *buf, size_t count); : Reads up to count bytes from file descriptor into buffer.

     ssize_t write(int fd, const void *buf, size_t count); : Writes count bytes from buffer into file descriptor.

     off_t lseek(int fd, off_t offset, int whence); : Repositions the read/write file offset pointer.

     int close(int fd); : Closes the file descriptor and releases kernel resources.

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Complete C Program: Building a Custom "my_cp" (Copy File) Command

This complete, compilable C program demonstrates opening an input file in read-only mode, creating a destination file with 0644 permissions, and buffering data using read() and write():

/*
 * my_cp.c - Copies file1 to file2 using low-level Linux system calls
 * Compile: gcc my_cp.c -o my_cp
 * Run    : ./my_cp source.txt destination.txt
 */

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

#define BUFFER_SIZE 4096

int main(int argc, char *argv[]) {
    int src_fd, dest_fd;
    ssize_t bytes_read, bytes_written;
    char buffer[BUFFER_SIZE];

    if (argc != 3) {
        fprintf(stderr, "Usage: %s <source_file> <destination_file>\n", argv[0]);
        return 1;
    }

    // Open source file in Read-Only mode
    src_fd = open(argv[1], O_RDONLY);
    if (src_fd < 0) {
        perror("Error opening source file");
        return 2;
    }

    // Open/Create destination file: Write-only, Create if missing, Truncate if exists
    // Permissions: 0644 (Owner read/write, Group/Others read-only)
    dest_fd = open(argv[2], O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (dest_fd < 0) {
        perror("Error opening/creating destination file");
        close(src_fd);
        return 3;
    }

    // Loop: Read from source, Write to destination
    while ((bytes_read = read(src_fd, buffer, BUFFER_SIZE)) > 0) {
        bytes_written = write(dest_fd, buffer, bytes_read);
        if (bytes_written != bytes_read) {
            perror("Write error occurred");
            close(src_fd);
            close(dest_fd);
            return 4;
        }
    }

    if (bytes_read < 0) {
        perror("Read error occurred");
    } else {
        printf("SUCCESS: Copied '%s' to '%s' successfully.\n", argv[1], argv[2]);
    }

    // Close both file descriptors
    close(src_fd);
    close(dest_fd);

    return 0;
}
							

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Using lseek() for Random File Access

By default, file reading and writing occurs sequentially. The lseek() system call allows seeking to any byte offset:

// Seek to beginning of file
lseek(fd, 0, SEEK_SET);

// Seek 10 bytes forward from current position
lseek(fd, 10, SEEK_CUR);

// Seek to the end of file (find file size)
off_t file_size = lseek(fd, 0, SEEK_END);
printf("Total file size: %ld bytes\n", file_size);
							

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Frequently Asked Questions (FAQ)