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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 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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printf("This\n");
printf("is\n");
printf("a\n");
printf("test.\n");
if (ferror(stdout)) {
  fprintf(stderr, "printf failed\n");
}

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

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CERT C++ Secure Coding Standard: ERR01-CPP. Use ferror() rather than errno to check for FILE stream errors

ISO/IEC 9899:1999 Section 2011 Section 6.3.1.1, "Boolean, characters, and integers," Section 7.1.4, "Use of library functions," and Section 7.921.10.3, "The ferror function"

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