• When you are reading a file, how do you know when to stop?
• In the triangle example, the input file had a number specifying how many more lines were in the file.
• Although this strategy worked for that example, this is not a general solution.
• C provides several ways to know when you have reached the end of a file you are reading.

Detecting the end of file

When reading characters

• Unix uses a special character to simulate end of a file (control-D).
• stdin has a special integer constant named EOF whose value is -1.
• When using fscanf()
  • If you are reading input from a file, character by character, you know when you've hit the end-of-file when fscanf() returns EOF.
• When using fgetc()
  • Notice that the prototype for fgetc() is
    
    int fgetc (FILE *stream);
  • So even though fgetc() is designed to read one character at a time from the stream passed in as its argument, the return type is an int. Since chars are stored as ints of small value, that does not a present a problem. We can read a char in as an int and use it as a char in our code.
  • Warning : In order for EOF to be read properly from the file (it really is an int with the value -1), we MUST store the value it returns into a variable of type int, rather than char. See the fgetc() code example below.

When reading lines

• You can also detect EOF when reading lines.
• When EOF has been encountered, the pointer to the string returned is equal to NULL.
• There is also an feof( ) function that returns true when the end of file has been encountered. Many students have had problems with its use, and its behavior doesn't seem to be consistent, so we suggest testing the string for NULL instead.

Copying a file character by character using fscanf()

/*******************************************************
 * File: copyByChar.c
 * Author: Tim Finin
 * Date: 4/10/2005
 * Modified by: Sue Evans
 * Date: 9/26/05
 * Section: 01XX & 02XX
 * Email: bogar@cs.umbc.edu
 *
 * This program takes two command line arguments naming
 * files.  It copies the contents of the first file to the
 * second a character at a time.
 *
 ********************************************************/

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

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

   char c;
   FILE *ifp, *ofp;

   /*  check for correct number of command line arguments */
   if (argc != 3) {
      fprintf(stderr, "Usage: a.out inputfile outputfile\n"); 
      exit(-1);
   }

   /* open input and output files */
   ifp = fopen(argv[1], "r");
   if (ifp == NULL) {
      fprintf(stderr, "Can't open %s for reading.\n", argv[1]); 
      exit(-2);
   }

   ofp = fopen(argv[2], "w");
   if (ofp == NULL) {
      fprintf(stderr,"Can't open %s for writing\n", argv[2]); 
      exit(-3);
   }

   /* copy characters from input to output files until an EOF */
   while(fscanf(ifp, "%c", &c) != EOF){
      fprintf(ofp,"%c", c);
   }

   /*  close files and return */
   fclose(ifp);
   fclose(ofp);

   return(0);
}

Copying a file character by character using fgetc()

This program copies an input file into an output file character by character using fgetc().

/*******************************************************
 * File: eof.c
 * Author: Sue Evans
 * Date:11/25/05
 * Section: 01XX & 02XX
 * Email: bogar@cs.umbc.edu
 *
 * This program takes two command line arguments naming
 * files.  It copies the contents of the first file to the
 * second a character at a time.
 *
 ********************************************************/

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

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

   int c;
   FILE *ifp, *ofp;

   /*  check for correct number of command line arguments */
   if (argc != 3) {
      fprintf(stderr, "Usage: a.out inputfile outputfile\n");
      exit(-1);
   }

   /* open input and output files */
   ifp = fopen(argv[1], "r");
   if (ifp == NULL) {
      fprintf(stderr, "Can't open %s for reading.\n", argv[1]);
      exit(-2);
   }

   ofp = fopen(argv[2], "w");
   if (ofp == NULL) {
      fprintf(stderr,"Can't open %s for writing\n", argv[2]);
      exit(-3);
   }

   /* copy characters from input to output files until an EOF */
   while((c = fgetc(ifp)) != EOF){
      fprintf(ofp,"%c", c);
   }

   /*  close files and return */
   fclose(ifp);
   fclose(ofp);

   return 0;
}

Copying a file line by line

This program copies an input file into an output file line by line.

/*******************************************************
 * File: copyByLine.c
 * Author: Tim Finin
 * Date: 4/10/2005
 * Modified by: Sue Evans
 * Date: 9/26/05
 * Section: 01XX & 02XX
 * Email: bogar@cs.umbc.edu
 * 
 * This function takes two command line arguments naming
 * files.  It copies the contents of the first file to the
 * second a line at a time.
 *
 ********************************************************/

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

#define MAXLINELENGTH 999

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

   FILE *ifp, *ofp;
   char line[MAXLINELENGTH];

   /*  check for correct number of command line arguments */
   if (argc !=3) {
      fprintf(stderr, "Usage: a.out inputfile outputfile\n"); 
      exit(-1);
   }

   /* open input and output files */
   ifp = fopen(argv[1], "r");
   if (ifp == NULL) {
      fprintf(stderr, "Can't open %s for reading.\n", argv[1]); 
      exit(-2);
   }

   ofp = fopen(argv[2], "w");
   if (ofp == NULL) {
      fprintf(stderr,"Can't open %s for writing\n", argv[2]); 
      exit(-3);
   }

   /* copy lines from input to output files until an EOF */
   while (fgets(line, MAXLINELENGTH, ifp) != NULL ) {
      fputs(line, ofp);
   }

   /*  close files and return */
   fclose(ifp);
   fclose(ofp);

   return(0);
}