What are File Pointers ?




C communicates with files using a new datatype called a file pointer. This type is defined within stdio.h, and written as FILE *. A file pointer called output_file is declared in a statement like
FILE *output_file;

Opening a file pointer using fopen

Your program must open a file before it can access it. This is done using the fopen function, which returns the required file pointer. If the file cannot be opened for any reason then the value NULL will be returned. You will usually use fopen as follows

if ((output_file = fopen("output_file", "w")) == NULL) 

    fprintf(stderr, "Cannot open %s\n", "output_file");


 fopen takes two arguments, both are strings, the first is the name of the file to be opened, the second is an access character, which is usually one of:
  • "r"   Open file for reading
  • "w"   Open file for writing
  • "a"   Open file for appending
Closing a file using fclose

The fclose command can be used to disconnect a file pointer from a file. This is usually done so that the pointer can be used to access a different file. Systems have a limit on the number of files which can be open simultaneously, so it is a good idea to close a file when you have finished using it. This would be done using a statement like

fclose(output_file);

If files are still open when a program exits, the system will close them for you. However it is usually better to close the files properly.

Input and Output using file pointers

Having opened a file pointer, you will wish to use it for either input or output. C supplies a set of functions to allow you to do this. All are very similar to input and output functions that you have already met.

Character Input and Output with Files

This is done using equivalents of getchar and putchar which are called getc and putc. Each  takes an extra argument, which identifies the file pointer to be used for input or output.

putc(c, fp) 

getc (fp)



File Handling in C


Formatted Input Output with File Pointers

Similarly there are equivalents to the functions printf and scanf which read or write data to files. These are called fprintf and fscanf.

The functions are used in the same way, except that the fprintf and fscanf take the file pointer as an additional first argument.


fprintf ( output_file, "This is a listing file\n" ) ;
fscanf ( input_file, "%d", &counter ) ;

Formatted Input Output with Strings

These are the third set of the printf and scanf  families. They are called sprintf and sscanf.
sprintf
puts formatted data into a string which must have sufficient space allocated to hold it. This can be done by declaring it as an array of char. The data is formatted according to a control string of the same form as that for p rintf.

sscanf
takes data from a string and stores it in other variables as specified by the control string. This is done in the same way that scanf reads input data into variables. sscanf is very useful for converting strings into numeric v values.

Whole Line Input and Output using File Pointers

Predictably, equivalents to gets and puts exist called fgets and  fputs. The programmer should be careful in using them, since they are incompatible with gets and puts. gets requires the programmer to specify the maximum  number of characters to be read. fgets and fputs retain the trailing  newline character on the line they read or write, wheras  gets and puts discard the newline.
When transferring data from files to standard input / output channels, the simplest way to avoid incompatibility with the newline is to use fgets  and fputs for files and standard channels too.
For Example, read a line from the keyboard using

  fgets(data_string, 80, stdin);

and write a line to the screen using

  fputs(data_string, stdout);

EOF, The End of File Marker

EOF  is a character which indicates the end of a file. It is returned by read commands of the getc and scanf families when they try to read beyond the end of a file.

When you are reading from a file it is useful to be able to check whether or not you have reached the end, without just failing to read. The function feof can be used to find out if you have hit the end :

if ( feof (input_file ) ) printf ( "BANG!\n" ) ;


This function returns the value 0 if the end of the file has not been reached, and another value if it has. It takes a FILE pointer as a parameter. 


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