Lernen File Operations: open, read, write, close, lseek

Understanding how Linux manages files at a low level is crucial for any systems programmer. In Linux, when you interact with files using system calls, you work with file descriptors. A file descriptor is a non-negative integer that uniquely identifies an open file within a process. The most fundamental system calls for file operations are open, read, write, close, and lseek. Each plays a specific role:

open: opens a file and returns a file descriptor;
read: reads data from a file descriptor into a buffer;
write: writes data from a buffer to a file descriptor;
close: closes an open file descriptor, releasing resources;
lseek: repositions the offset of the file descriptor for random access.

These calls provide the foundation for all file manipulation in Linux, allowing you to perform tasks such as copying files, reading logs, or updating configuration files directly from C programs.

file_ops_example.c


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

#define BUFFER_SIZE 128

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

    // Open source file for reading
    src_fd = open("input.txt", O_RDONLY);
    if (src_fd == -1) {
        perror("open source");
        return 1;
    }

    // Open destination file for writing (create if doesn't exist, truncate if does)
    dest_fd = open("output.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (dest_fd == -1) {
        perror("open destination");
        close(src_fd);
        return 1;
    }

    // Read from source and 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");
            close(src_fd);
            close(dest_fd);
            return 1;
        }
    }
    if (bytes_read == -1) {
        perror("read");
    }

    // Use lseek to move to the beginning of the destination file
    if (lseek(dest_fd, 0, SEEK_SET) == -1) {
        perror("lseek");
    } else {
        // Read first few bytes from destination file as a check
        bytes_read = read(dest_fd, buffer, 16);
        if (bytes_read > 0) {
            buffer[bytes_read] = '\0';
            printf("First 16 bytes of output.txt: %s\n", buffer);
        }
    }

    // Close both files
    close(src_fd);
    close(dest_fd);

    return 0;
}

This program demonstrates how file descriptors are managed and manipulated using system calls. When you call open, the kernel returns a file descriptor that you use for subsequent operations. Error handling is essential: after each system call, the return value is checked, and if an error occurs (such as failing to open a file), the program prints an error message using perror and exits or cleans up as needed. The lseek system call is used here to reposition the file offset to the start of the destination file (SEEK_SET), enabling random access. This allows the program to read from any position within the file, not just sequentially, which is useful for scenarios like updating specific records or skipping headers.

Which statement best describes the role of file descriptors and the purpose of the open, read, write, close, and lseek system calls in Linux file operations?

Select the correct answer

File descriptors are unique integers for opened files; open returns a file descriptor, read reads data from it, write writes data to it, close releases it, and lseek repositions the file offset for random access

File descriptors are not needed in C; open, read, write, close, and lseek are replaced by standard library functions

File descriptors are only used for network sockets; open creates files, read deletes files, write moves files, close has no effect, and lseek is obsolete

File descriptors are file names; open reads data, write closes files, read writes data, lseek deletes files, and close opens files

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open: opens a file and returns a file descriptor;
read: reads data from a file descriptor into a buffer;
write: writes data from a buffer to a file descriptor;
close: closes an open file descriptor, releasing resources;
lseek: repositions the offset of the file descriptor for random access.