The information provided with the return code of I/O functions and with the perror() message associated with errno values may help you locate the source of errors and the reason for program failure. Because return codes and errno values do not exist for every possible system I/O failure, return codes and errno values are not useful for diagnosing all I/O errors. This topic discusses the use of the __amrc structure and the __amrc2 structure. For information on return codes from I/O functions see z/OS XL C/C++ Runtime Library Reference. For information on errno values and the associated perror() message see z/OS Language Environment Debugging Guide.