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FILE *fptr = NULL;
int c = 'a';
while (c <= 'z') {
  putc(c++, fptr ? fptr : (fptr = fopen(file_name, "w"));
}

It is possible for the putc macro to instantiate its file descriptor argument 2 or more times. This might still seem safe, as the ?: operator ostensibly prevents multiple calls to fopen, there is no guarentee that these would happen in distinct sequence points. So this could would also violate EXP30-C. Do not depend on order of evaluation between sequence points.

Compliant Solution: putc()

In the compliant solution, c++ is no longer an the file descriptor argument to putc() no longer has side effects.

FILE *fptr = fopen(file_name, "w");
if (fptr == NULL) {
  /* Handle Error */
}

int c = 'a';

while (c <= 'z') {
  putc(c++, fptr);
}

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