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Generally, when you have a program attempt to open a file, you should also
have that program check to see whether its attempt was successful. To
arrange for such a check, you can take advantage of the fact that
fopen returns a pointer to zero, also known as the null
pointer, when it fails to open a file:
FILE* trade_source;
trade_source = fopen("test.data", "r");
/* Test for successful opening */
while (! trade_source) {
printf ("The attempt to open the test.data file failed!\n");
...
}
Note that when you work on a pointer with the negation operation, the
result is 0, of type int, if the pointer is the null pointer;
otherwise, the result is 1, of type int.