Streams, end-of-file, and error handling

The idea

A FILE pointer represents a C stream whose operations can fail independently of your program’s logic. fgetc returns the next unsigned-char value converted to int, or EOF. Store that result in int until checking EOF; narrowing to char first can lose the distinction between an ordinary byte and the status value. Do not use feof as a prediction that the next read will succeed.

After a read loop stops, inspect ferror when you need to distinguish a failure from ordinary end-of-file. Check creation and write results as well as reads. A function that owns a stream should close it on every relevant exit path, while a function borrowing a caller’s stream should follow the caller’s ownership contract. The demonstration uses a temporary stream and reports output only on successful I/O.

#include <stdio.h>
int main(void) {
  FILE *file = tmpfile();
  if (!file) return 1;
  if (fputs("AB", file) == EOF) { fclose(file); return 1; }
  rewind(file);
  int ch, count = 0;
  while ((ch = fgetc(file)) != EOF) ++count;
  if (ferror(file)) { fclose(file); return 1; }
  printf("%d\n", count);
  return fclose(file) == 0 ? 0 : 1;
}

2 on the successful temporary-stream path.

The loop counts the two bytes and stops on the read that reports EOF.