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Use ferror() rather than errno to check whether an error has occurred on a file stream (after a long string of stdio calls, for example). The ferror() function tests the error indicator for a specified stream and returns nonzero if and only if the error indicator is set for the stream.

Non-Compliant Coding Example

Many implementations of the stdio package adjust their behavior slightly if stdout is a terminal. To make the determination, these implementations perform some operation that happens to fail fails (with ENOTTY) if stdout is not a terminal. Although the output operation goes on to complete successfully, errno still contains ENOTTY. This behavior can be mildly confusing, but it is not strictly incorrect, because it is only meaningful for a program to inspect the contents of errno after an error has been reported. ( More precisely, errno is only meaningful after a library function that sets errno on error has returned an error code.)

errno = 0;
printf("This\n");
printf("is\n");
printf("a\n");
printf("test.\n");
if (errno != 0) {
  fprintf(stderr, "printf failed: %s\n", strerror(errno));
}

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