#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

int main(int argc, char *argv[]) {
  int numfiles;
  int colwidth;
  if ( argc < 2 ) {
    fprintf(stderr,"No arguments, printing nothing!\n");
    return EXIT_SUCCESS;
  }
  numfiles = argc-1;
  /* Everybody knows that all output devices have 80 columns ;-) */
  colwidth = 80/numfiles;
  /* NOTE: this use of dynamically sized arrays is a (very useful)
     GNU extension. It's not in C89 ! */
  FILE *files[numfiles];
  int i;
  for (i=0; i<numfiles; i++) {
    char *filename = argv[i+1];
    files[i] = fopen(filename,"r");
    if ( files[i] == NULL ) {
      /* This demonstrates a more helpful way to get error information
	 than just using perror(), which doesn't let you add vital
	 information such as the file you couldn't open! */
      fprintf(stderr,"Unable to open file %s: %s\n",
	filename,strerror(errno));
      return EXIT_FAILURE;
    }
  }
  char line[numfiles*colwidth+1]; /* 80 chars, plus null */
  int eof[numfiles];
  for (i=0;i<numfiles;i++) eof[i] = 0;
  int filesopen = numfiles;
  while ( filesopen > 0 ) {
    int col = 0; /* position in line */
    int c;
    for (i=0;i<numfiles;i++) {
      if ( !eof[i] ) {
	while ( 1 ) {
	  c = getc(files[i]);
	  if ( c == EOF ) {
	    eof[i] = 1;
	    filesopen--;
	    break;
	  } else if ( c == '\n' ) {
	    break;
	  } else if ( col < (i+1)*colwidth ) {
	    /* still room for another character */
	    line[col++] = c;
	  }
	}
      }
      /* pad the remainder of the column */
      while ( col < (i+1)*colwidth ) {
	line[col++] = ' ';
      }
    }
    line[col] = 0;
    printf("%s\n",line);
  }
  return EXIT_SUCCESS;
}