Question

I am implementing my own version of the (“cat”) command in Unix for practice. After i did that i became interested in implementing some flags like (-n) and (-b).

My Question: I am looking for a way to locate the blank and new lines while reading from my file. I can’t remember what library or function i should use.

Here is the source code I am working on:

#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;
}

Any ideas or suggestions concerning my question are greatly appreciated.
I would be even more grateful if you can type for me the complete function prototype that i shall be using as i am not an experienced programmer.

Thanks in advance for your help.

P.S: I am implementing the (-n) and (-b) flags. Thus, i am looking forward to write the line number at the beginning of each line in the file that i am reading.

Answer 1

While there is a function that does line-based file input in C (it’s called fgets), you can’t really use it for cat, because:

• There’s no way to know the maximum length of the line beforehand;
• You’ll lose portions of the input if it contains null bytes.

You’ll have to look for newline symbols in your buffer after you read it, and once you find any, print the prefix of the buffer, followed by newline, line number, and the rest of the buffer (with additional processing of remaining newlines, of course).

An easier solution would be to switch to processing input one byte at a time; you can use FILE* and fgetc to use CRT-provided buffering so that you don’t actually do a syscall for each read/write, or read file in blocks as you do now, and do byte processing inside the loop. Then it’s a matter of writing a state machine – if a previous read character was a newline, then output a line number, unless this character is a newline and -b option is used, etc.

This still results in a less efficient solution, so you may want to treat cat without arguments specially – i.e. switch to byte-per-byte processing only if you need it. In fact, this is exactly what at least one of actual cat implementations does.