C61. C File Pointers

File Pointers

Accessing a stream in a C program is done through a file pointer, which has type
FILE * (the FILE type is declared in ). Certain streams are repre­sented by file pointers with standard names; we can declare additional tile pointers as needed. For example, if a program needs two streams in addition to the standard ones, it might contain the following declaration:

FILE *fpl, *fp2;

A program may declare any number of FILE * variables, although operating sys­tems usually limit the number of streams that can be open at one time.

Standard Streams and Redirection

provides three standard streams . These streams are ready to use we don't declare them, and we don't open or close them.

File Pointer	Stream	Default Meaning
stdin stdout stderr	Standard input Standard output Standard error	Keyboard Screen Screen

The functions that we've used in previous chapters printf, scanf, putchar. getchar. puts, and gets obtain input from stdin and send out­put to stdout. By default, stdin represents the keyboard; stdout and stderr represent the screen. However, many operating systems allow these default meanings to be changed via a mechanism known as redirection.

Typically, we can force a program to obtain its input from a file instead of from the keyboard by putting the name of the file on the command line, preceded by the < character:

This technique, known as input redirection. essentially makes the stdin stream represent a file (in. dat. in this case) instead of the keyboard. The beauty of redi­rection is that the demo program doesn't realize that it's reading from in. dat; as far as it knows, any data it obtains from stdin is being entered at the keyboard.

Output redirection is similar. Redirecting the stdout stream is usually done by putting a file name on the command line, preceded by the > character:

demo >out.dat

All data written to stdout will now go into the out. dat file instead of appear-

ing on the screen. Incidentally, we can combine output redirection with input redi­rection:

One problem with output redirection is that everything written to stdout is put into a file. If the program goes off the rails and begins writing error messages, we won't see them until we look at the file. This is where stderr comes in. By writing error messages to stderr instead of stdout, we can guarantee that those messages will appear on the screen even when stdout has been redirected. (Operating systems often allow stderr itself to be redirected, though.)