Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

Chapter 8 ■ Arrays
If in is NULL, the program prints a message and stops. If in is not NULL, the program proceeds
as before.
The predefined function exit is used to terminate execution of a program and return control
to the operating system. It is conventional to use exit(0) to indicate normal termination; other
arguments are used to indicate some sort of error.
To use exit, we must write the directive
#include <stdlib.h>
at the head of our program, since exit is defined in the “standard library,” stdlib.h. Among other
things, this library contains functions for working with random numbers, functions for searching,
and functions for sorting.
As usual, we can assign a value to in and test it for NULL, using the following:
FILE * in;
if ((in = fopen("passage.txt", "r")) == NULL) {
printf("File cannot be found\n");
exit(1);
}
Note that we cannot use FILE * in in the if condition, since a declaration is not
permitted there.
Similarly, when we write
FILE * out = fopen("output.txt", "w");
we are assuming that the file output.txt exists or can be created. If it does not exist and cannot
be created (the disk may be write protected or full, for instance), fopen will return NULL. We can
test for this as follows:
FILE * out;
if ((out = fopen("output.txt", "w")) == NULL) {
printf("File cannot be found or created\n");
exit(1);
}
So far, we have written the name of our file in the fopen statement. To use a different file,
we would have to change the name in the statement, and we would have to re-compile the
program. Our program would be more flexible if we let the user tell us the name of the file when
the program is run.
We can declare dataFile (say) to hold the name of the file with
char dataFile[40];
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