When is an error not an err?

If you step off the golden path of trying to read a file – you can quickly end up in in trouble and the diagnostics do not help.

I had some simple code

FILE * hFile = fopen(...); 
recSize = fread(pBuffer ,1,bSize,hFile); 
if (recSize == 0)
{
  // into the bog!
 if (feof(hFile))printf("end of file\n");
 else if (ferror(hFile)) printf("ferror(hFile) occurred\n");
 else printf("Cannot occur condition\n");
 
}

When running a unit test of the error path of passing a bad file handle, I got the “cannot occur condition because the ferror() returned “OK – no problem “

The ferror() description is

General description: Tests for an error in reading from or writing to the specified stream. If an error occurs, the error indicator for the stream remains set until you close the stream, call rewind(), or call clearerr().
If a non-valid parameter is given to an I/O function, z/OS XL C/C++ does not turn the error flag on. This case differs from one where parameters are not valid in context with one another.

This gave me 0, so it was not able to detect my error. ( So what is the point of ferror()?)

If I looked at errno and used perror() I got

errno 113
EDC5113I Bad file descriptor. (errno2=0xC0220001)

You may think that I need to ignore ferror() and check errno != 0 instead. Good guess, but it may not be that simple.

The __errno2 (or errnojr – errno junior)) description is

General description: The __errno2() function can be used when diagnosing application problems. This function enables z/OS XL C/C++ application programs to access additional diagnostic information, errno2 (errnojr), associated with errno. The errno2 may be set by the z/OS XL C/C++ runtime library, z/OS UNIX callable services or other callable services. The errno2 is intended for diagnostic display purposes only and it is not a programming interface. The __errno2() function is not portable.
Note: Not all functions set errno2 when errno is set. In the cases where errno2 is not set, the __errno2() function may return a residual value. You may use the __err2ad() function to clear errno2 to reduce the possibility of a residual value being

If you are going to use __errno2 you should clear it using __err2ad() before invoking a function that may set it.

I could not find if errno is clean or if it may return a residual value, so to be sure to set it before every use of a C run time library function.

Having got your errno value what do you do with it?

There are #define constants in errono.h such as

#define EIO 122 /* Input/output error */

You case use if ( errno == EIO ) …

Like many products there is no mapping of 122 to “EIO”, but you can use strerror(errno) to map the errno to the error string like EDC5113I Bad file descriptor. (errno2=0xC0220001). This also provides the errno2 string value.

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