[SOLVED] Implementing the cp command using read/write system calls

Issue

I am trying to implement the cp command only using read/write system calls.
Here is my code:

/**
 * cp file1 file 2
 */

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

int main(int argc, char *argv[])
{
    int errsv;
    char contents[1024];
    int fd_read, fd_write;
    fd_read = open(argv[1], O_RDONLY);
    if (fd_read == -1)
    {
        errsv = errno;
        printf("Error occured: %d\n", errsv);
    }
    read(fd_read, contents, sizeof(contents));
    fd_write = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0744);
    if (fd_write == -1)
    {
        errsv = errno;
        printf("Error occured: %d\n", errsv);
    }
    write(fd_write, contents, sizeof(contents));
    close(fd_read);
    close(fd_write);
    return 0;
}

I tested the code using the commands:

cc test.c 
./a.out file1 file2

Here is my file1:

dummy text
dummy text

After running the code, although file2 contains the text from file1, it also has some gibberish characters. [not keeping this here.]
Why is this so?

Solution

You need to call read() and write() in a loop to copy the entire file. read() returns 0 when you reach EOF, or a negative result if there's an error, then you can end the loop.

read() returns the number of bytes that were read, which may be less than the size of the buffer. You need to use that number when calling write(), otherwise you'll write extra characters to the output file. These will be unitialized characters on the first iteration, and on other iterations they'll be left over characters from previous iterations.

int main(int argc, char *argv[])
{
    char contents[1024];
    int fd_read, fd_write;
    fd_read = open(argv[1], O_RDONLY);

    if (fd_read == -1)
    {
        perror("open input file");
        exit(1);
    }
    fd_write = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0744);
    if (fd_write == -1)
    {
        perror("open output file");
        exit(1)
    }

    int n_read;
    while ((n_read = read(fd_read, contents, sizeof(contents))) > 0) {
        write(fd_write, contents, n_read);
    }

    close(fd_read);
    close(fd_write);
    return 0;
}

Answered By - Barmar

Answer Checked By - Terry (WPSolving Volunteer)