ERR01-A. Use ferror() to check for any accumulated error

Use ferror() to check for any accumulated errors, for example, after a long string of stdio calls.

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 which happens to fail (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));
}

Compliant Solution

errno = 0;
printf("This\n");
printf("is\n");
printf("a\n");
printf("test.\n");
ferror();

Risk Assessment

Passing values to character handling functions that cannot be represented as an unsigned char may result in unintended program behavior.

Related Vulnerabilities

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

References

[[Horton 90]] Section 14 p. 254
[[ISO/IEC 9899-1999]] Section 6.3.1.1, "Boolean, characters, and integers", Section 7.1.4, Section 7.9.10.3
[[Koenig 89]] Section 5.4 p. 73

ERR00-A. Detect errors by checking return values      13. Error Handling (ERR)       ERR02-A. Return an errno-style value to indicate an error