Using errno for application / library error reporting

Question

I'm writing a C library for a software project. I need to do some error reporting, but I'm a little bit too lazy to implement my own complex set of error-codes, variables and functions. Is it acceptable to use the errno facility provided by the libc for custom error reporting? All my errors fit into the categories given by the E... macros.

For instance, let's say my code includes a function that reads a SHA256 hash in hexdecimal notation and converts it into some sort of internal format. I want to use errnoto report errors:

#include <errno.h> int hash_fromstr(hash_t *out, const char *in) { /* ... */ if (strlen(in) != 65) { errno = EINVAL; return -1; } /* ... */ }

Of course this example is ridiculously simplified, in reality much more errors may happen in other functions.

As long as your library can emit the standard error codes without breaking the proper functioning of errno in other parts of a users application that links against your library I don't see a problem with it. Perhaps you might use the errno variable but not alter it and return a copy of it? — Chimera
– Chimera, Commented May 14, 2012 at 21:00
@Jim I want to use errno for errors that occur inside the library itself - not necessarily in the function of the libc called by the library. — fuz
– fuz, Commented May 14, 2012 at 21:07
So are you asking if it's ok to set the value of errno to report errors to the caller of your library? If so you might want to check out stackoverflow.com/questions/9856822/should-i-set-errno — Chimera
– Chimera, Commented May 14, 2012 at 21:13

Community · Accepted Answer · 2017-05-23 10:30:55Z

You can modify the value of errno as you please, just be sure that your library code checks that errno isn't set before doing so to ensure your library code can still properly detect internal standard failures that cause errno to be set. You might also check out "should I set errno" for more information.

Felipe Lavratti · Accepted Answer · 2014-10-28 17:12:59Z

1

Yes, you can modify it and it has thread scope, which is very desirable in that kind of error handling.

Using errno error family (E...) and maybe extending it can be a very powerful yet simple error handling pattern. It may be considered bad tasted approach by others, but IMHO it produces cleaner code and a standardized pattern for error handling.

answered Oct 28, 2014 at 17:12

Felipe Lavratti

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3 Comments

fuz Over a year ago

How can you expand errno? As far as I know there is no portable way to find out which errno values are unallocated and even if you did, you get a conflict with other libraries that try to do the same.

Felipe Lavratti Over a year ago

@FUZxxl It is true that is not possible to know which values are unallocated, but the int type range is big enough to accommodate safe margin. About the conflicts, I see no conflicts happening, considering that the errno value has only the scope of a single function call, so, depending the library, the error shall be processed against the library error list only, avoiding conflicts. Do you agree?

fuz Over a year ago

@fani I disagree, because using errno for custom things breaks things like strerror—the custom errno value remains after a failed library call and potentially messes up outside processing.

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