ferror() rather than
errno to check whether an error has occurred on a file stream (for example, after a long chain of
stdio calls). 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.
Noncompliant Code 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 fails (with
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 meaningful for a program to inspect the contents of
errno only after an error has been reported. More precisely,
errno is meaningful only after a library function that sets
errno on error has returned an error code.
This compliant solution uses
ferror() to detect an error. In addition, if an early call to
printf() fails, later calls may modify
errno, whether they fail or not, so the program cannot rely on being able to detect the root cause of the original failure if it waits until after a sequence of library calls to check.
errno after multiple calls to library functions can lead to spurious error reporting, possibly resulting in incorrect program operation.
|LDRA tool suite
|The error indicator 'errno' shall not be used
|SEI CERT C++ Coding Standard
|VOID ERR01-CPP. Use ferror() rather than errno to check for FILE stream errors