C language provides function to work with files at both at low level and high level. We will be seeing following.

• Opening a file
• Character I/O from files
• Closing a file
• stdin,stdout and stderr
• Line I/O with files
• Formatted I/O with files
• Random access using fseek()

In order to open a file, use the function fopen(). Use it as:

fp = fopen(filename, mode);

where:

• Filename is a string that holds the name of the file on disk (including a path like /cs/course if necessary).
• Mode is a string representing how you want to open the file. Most often you'll open a file for reading ("r") or writing ("w").

Note that fopen() returns a FILE * that can then be used to access the file. When the file cannot be opened (e.g., we don't have permission or it doesn't exist when opening for reading), fopen() will return NULL.

Here are examples of opening files:

  1 FILE *ifp, *ofp;
  2 char *mode = "r";
  3 char outputFilename[] = "out.list";
  4 
  5 ifp = fopen("in.list", mode);
  6 
  7 if (ifp == NULL) {
  8   fprintf(stderr, "Can't open input file in.list!\n");
  9   exit(1);
 10 }
 11 
 12 ofp = fopen(outputFilename, "w");
 13 
 14 if (ofp == NULL) {
 15   fprintf(stderr, "Can't open output file %s!\n", outputFilename);
 16   exit(1);
 17 }

Note that the input file that we are opening for reading ("r") must already exist. In contrast, the output file we are opening for writing ("w") does not have to exist. If it doesn't, it will be created. If this output file does already exist, its previous contents will be thrown away (and will be lost).

Note: There are other modes you can use when opening a file, such as append ("a") to append something to the end of a file without losing its contents...or modes that allow you to both read and write. You can look up these other modes in a good C reference on stdio.h.

Once a file has been successfully opened, you can read from it using fscanf() or write to it using fprintf(). These functions work just like scanf() and printf(), except they require an extra first parameter, a FILE * for the file to be read/written.

Note: There are other functions in stdio.h that can be used to read or write files. Look them up in a good C reference.

Continuing our example from above, suppose the input file consists of lines with a username and an integer test score, e.g.:

 in.list
 ------
 foo 70
 bar 98
 biz A+
 ...

and that each username is no more than 8 characters long.

We might use the files we opened above by copying each username and score from the input file to the output file. In the process, we'll increase each score by 10 points for the output file:

  1 char username[9];  /* One extra for nul char. */
  2 int score;
  3 
  4 ...
  5 
  6 while (fscanf(ifp, "%s %d", username, &score)  ! = EOF) {
  7   fprintf(ofp, "%s %d\n", username, score+10);
  8 }
  9 
 10 ...

The function fscanf(), like scanf(), normally returns the number of values it was able to read in. However, when it hits the end of the file, it returns the special value EOF. So, testing the return value against EOF is one way to stop the loop.

The bad thing about testing against EOF is that if the file is not in the right format (e.g., a letter is found when a number is expected):

 in.list
 ------
 foo 70
 bar 98
 biz A+
 ...

then fscanf() will not be able to read that line (since there is no integer to read) and it won't advance to the next line in the file. For this error, fscanf() will not return EOF (it's not at the end of the file)....

Errors like that will at least mess up how the rest of the file is read. In some cases, they will cause an infinite loop.

One solution is to test against the number of values we expect to be read by fscanf() each time. Since our format is "%s %d", we expect it to read in 2 values, so our condition could be:

 1 while (fscanf(ifp, "%s %d", username, &score) == 2) {
 2    ...

Now, if we get 2 values, the loop continues. If we don't get 2 values, either because we are at the end of the file or some other problem occurred (e.g., it sees a letter when it is trying to read in a number with %d), then the loop will end.

Another way to test for end of file is with the library function feof(). It just takes a file pointer and returns a true/false value based on whether we are at the end of the file.

To use it in the above example, you would do:

 1 while ( ! feof(ifp)) {
 2   if (fscanf(ifp, "%s %d", username, &score)  ! = 2)
 3     break;
 4   fprintf(ofp, "%s %d", username, score+10);
 5 }

Note that, like testing != EOF, it might cause an infinite loop if the format of the input file was not as expected. However, we can add code to make sure it reads in 2 values (as we've done above).

Note: When you use fscanf(...) != EOF or feof(...), they will not detect the end of the file until they try to read past it. In other words, they won't report end-of-file on the last valid read, only on the one after it.

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Deepak Kumar Tala - All rights reserved

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