Question

I know many questions have been asked previously about error handling in C but this is specifically about errno stuff.

I want to ask whether we should use the errno/perror functionality to handle errors gracefully at runtime.I am asking this because MSVC uses it and Win32 api also uses it heavily.I don’t know anything about gcc or ‘linux api’.Today both gcc and MSVC say that errno/perror can be used safely in a multithreaded environment.So what’s your view?

thanks.

Answer 1

Note that using errno alone is a bad idea: standard library functions invoke other standard library functions to do their work. If one of the called functions fails, errno will be set to indicate the cause of the error, and the library function might still succeed, if it has been programmed in a manner that it can fall back to other mechanisms.

Consider malloc(3) — it might be programmed to try mmap(.., MAP_PRIVATE|MAP_ANONYMOUS) as a first attempt, and if that fails fall back to sbrk(2) to allocate memory. Or consider execvp(3) — it may probe a dozen directories when attempting to execute a program, and many of them might fail first. The ‘local failure’ doesn’t mean a larger failure. And the function you called won’t set errno back to 0 before returning to you — it might have a legitimate but irrelevant value left over from earlier.

You cannot simply check the value of errno to see if you have encountered an error. errno only makes sense if the standard library function involved also returned an error return. (Such as NULL from getcwd(3) or -1 from read(2), or “a negative value” from printf(3).)

But in the cases when standard library functions do fail, errno is the only way to discover why they failed. When other library functions (not supplied by the standard libraries) fail, they might use errno or they might provide similar but different tools (see e.g. ERR_print_errors(3ssl) or gai_strerror(3).) You’ll have to check the documentation of the libraries you’re using for full details.