Checking the Return Value of a Function

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Many C and C++ runtime library functions have a return value associated with them for error-checking purposes. For example:

The _Rfeov() function returns 1 if the file has moved from one volume to the next.
The fopen() function returns NULL if a file is not opened successfully.

For information about the ILE C/C++ function return values, see ILE C/C++ Runtime Library Functions.

To verify that each runtime library function has completed successfully, a program should check the function return values.

Example:

The following figure shows how to check the return value of the fopen() function.

Figure 1. ILE C Source to Check for the Return Value of fopen()

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
    FILE *fp;
    if (( fp = fopen ( "MYLIB/QCSRC(TEST)", "ab" )) == NULL )
    {
        printf ("Cannot open file QCSRC(TEST)\n");
        exit (99);
    }
}