Using system calls to implement the unix cat command

Question

For my OS class I have the assignment of implementing Unix's cat command with system calls (no scanf or printf). Here's what I got so far:

(Edited thanks to responses)

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



main(void)
{


   int fd1; 
   int fd2;

   char *buffer1;
   buffer1 = (char *) calloc(100, sizeof(char));


   char *buffer2;
   buffer2 = (char *)calloc(100, sizeof(char));

   fd1 = open("input.in", O_RDONLY);    
   fd2 = open("input2.in", O_RDONLY);


   while(eof1){ //<-lseek condition to add here
   read (fd1, buffer1, /*how much to read here?*/ );
   write(1, buffer1, sizeof(buffer1)-1);     
   }


   while (eof2){ 

    read (fd2,buffer2, /*how much to read here?*/);  
    write(1, buffer2, sizeof(buffer2)-1);

    }

}

The examples I have seen only show read with a known number of bytes. I don't know how much bytes each of the read files will have, so how do I specify read's last paramether?

Answer 1

• Before you can read into a buffer, you have to allocate one. Either on the stack (easiest) or with mmap.
• perror is a complicated library function, not a system call.
• exit is not a system call on Linux. But _exit is.
• Don't write more bytes than you have read before.
• Or, in general: Read the documentation on all these system calls.

Edit: Here is my code, using only system calls. The error handling is somewhat limited, since I didn't want to re-implement perror.

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

static int
cat_fd(int fd) {
  char buf[4096];
  ssize_t nread;

  while ((nread = read(fd, buf, sizeof buf)) > 0) {
    ssize_t ntotalwritten = 0;
    while (ntotalwritten < nread) {
      ssize_t nwritten = write(STDOUT_FILENO, buf + ntotalwritten, nread - ntotalwritten);
      if (nwritten < 1)
        return -1;
      ntotalwritten += nwritten;
    }
  }

  return nread == 0 ? 0 : -1;
}

static int
cat(const char *fname) {
  int fd, success;

  if ((fd = open(fname, O_RDONLY)) == -1)
    return -1;

  success = cat_fd(fd);

  if (close(fd) != 0)
    return -1;

  return success;
}


int
main(int argc, char **argv) {
  int i;

  if (argc == 1) {
    if (cat_fd(STDIN_FILENO) != 0)
      goto error;
  } else {
    for (i = 1; i < argc; i++) {
      if (cat(argv[i]) != 0)
        goto error;
    }
  }
  return 0;

error:
  write(STDOUT_FILENO, "error\n", 6);
  return 1;
}