Jawaharlal Nehru Technological University Hyderabad B.Tech IV Year REAL TIME SYSTEMS Previous Question Paper (February/March - 2022 - R18)

The read() and write() system calls are fundamental primitives in Unix-like operating systems for performing I/O operations on file descriptors.

• read() System Call: Used to read data from a file descriptor into a buffer.
ssize_t read(int fd, void *buf, size_t count);
• Parameters:
  • fd: The file descriptor associated with the file to read from.
  • buf: A pointer to the memory buffer where the read data will be stored.
  • count: The maximum number of bytes to read.
• Return Value: Returns the number of bytes read on success, 0 on end-of-file, or -1 on error.
• write() System Call: Used to write data from a buffer to a file descriptor.
ssize_t write(int fd, const void *buf, size_t count);
• Parameters:
  • fd: The file descriptor associated with the file to write to.
  • buf: A pointer to the buffer containing the data to be written.
  • count: The number of bytes to write from the buffer.
• Return Value: Returns the number of bytes written on success, or -1 on error.

The following C program demonstrates file copying using the open(), read(), write(), and close() system calls.

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

int main() {
    int fd_src, fd_dest;
    char buffer[1024];
    ssize_t n;

    fd_src = open("source.txt", O_RDONLY);
    if (fd_src < 0) { perror("Error opening source"); return 1; }

    fd_dest = open("dest.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd_dest < 0) { perror("Error opening destination"); return 1; }

    while ((n = read(fd_src, buffer, sizeof(buffer))) > 0) {
        write(fd_dest, buffer, n);
    }

    close(fd_src);
    close(fd_dest);
    return 0;
}

Note: The O_CREAT flag creates the destination file if it does not exist, and O_TRUNC clears existing content.