Can you explain what the system call lseek is used for in an operating system?

The lseek system call is used to change the current read/write position in an open file descriptor. This allows moving around the file, setting the pointer to a specific position from which subsequent read or write operations will be performed.

Example of using lseek in C:

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

int main() {
    int fd = open("file.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    // Move the pointer 10 bytes from the beginning of the file
    off_t offset = lseek(fd, 10, SEEK_SET);
    if (offset == -1) {
        perror("lseek");
        close(fd);
        return 1;
    }

    char buffer[20];
    ssize_t bytesRead = read(fd, buffer, sizeof(buffer));
    if (bytesRead == -1) {
        perror("read");
    } else {
        write(STDOUT_FILENO, buffer, bytesRead);
    }

    close(fd);
    return 0;
}

Thus, lseek allows arbitrary movement within a file, which is useful for implementing random access to data.