A Minimal File Copier with POSIX System Calls

This short C program copies bytes from one file to another using the POSIX system calls open, read, write, and creat. It is a compact example of moving data through a fixed-size buffer without loading the entire file into memory.

It is also deliberately minimal rather than robust: write() may write fewer bytes than requested, but this version does not retry the remainder. It treats a read error like end-of-file until the final check, and it does not report system error details. Keep it as a first systems-programming exercise, not as a general-purpose copy utility.

#include <sys/types.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[]);

#define BUF_SIZE 4096
#define OUTPUT_MODE 0700

int main(int argc, char *argv[]) {

  int in_fd, out_fd, byte_letti, byte_scritti;
  char buffer[BUF_SIZE];

  if (argc != 3) exit(1);
  in_fd = open(argv[1], O_RDONLY);
  if(in_fd < 0) exit(2);
  out_fd = creat(argv[2], OUTPUT_MODE);
  if(out_fd < 0) exit(3);

  while(1) {
    byte_letti = read(in_fd, buffer, BUF_SIZE);
    if(byte_letti <= 0) break;
    byte_scritti = write(out_fd, buffer, byte_letti);
    if (byte_scritti <= 0) exit(4);
  }

  close(in_fd); close(out_fd);
  if (byte_letti == 0) exit(0);
  else exit(5);
}