C - Returning negative value as unsigned sets errno?

Question

Good day,

I'm reading through some old code I've been asked to maintain, and I see a number of functions that are like so:

uint32_t myFunc(int* pI);

In the case of error conditions within the body of the function, the function exits early by returning a negative errno value, ie: return (-EINVAL);

Is this value standard (ie: C99, ANSI), C? I've read in the comments that apparently doing this sets the errno to EINVAL, but I can't find any documentation to support this. Wouldn't it be better to just declare the function as a signed int (ie: int32_t myFunc(int* pI)) and treat negative values as error codes, rather than attempt to set errno in this manner?

I don't think any error will be returned anyway... I think your negative integer will be converted to unsigned (The same binary representation of the negative integer will be used for the unsigned you assign the value) — SomethingSomething, Aug 25 '14 at 15:45
i think so. use positive number this place is weird. as least it go against linux convention. — Jason Hu, Aug 25 '14 at 15:45
For this _interesting_ function, certainly `myFunc()` returns values 0 ... 0xFFFFFFFF-4096 are "normal" return values and 0xFFFFFFFF-4096+1 ... 0xFFFFFFFF are "error values. — chux - Reinstate Monica, Aug 25 '14 at 16:03

score 4 · Accepted Answer · answered Aug 25 '14 at 15:58

Returning a negative value does not set errno. This is misinformation you picked up (or more likely, which the original author picked up) out-of-context: the mechanism by which the Linux kernel system calls report errors is returning a negative value in the range -4095 to -1, which the userspace code making the syscall then uses (in most cases) to fill in the value of errno. If you want to set errno yourself, however, just assign a value to it.

ryyker · Answer 2 · 2014-08-25T17:08:14.343

Addressing your question: Is this value standard (ie: C99, ANSI), C?

For C99

    "errno which expands to a modifiable lvalue that has type int, the
    value of which is set to a positive error number by several library
    functions." -- N1256 7.5p2

For POSIX

    "The <errno.h> header shall define the following macros which shall
    expand to integer constant expressions with type int, distinct
    positive values" -- errno.h DSCRIPTION

    "Values for errno are now required to be distinct positive values
    rather than non-zero values. This change is for alignment with the
    ISO/IEC 9899:1999 standard." -- errno.h CHANGE HISTORY Issue 6

A good discussion (and the source of these quotes) is found HERE (search "when to return EINVAL")

C - Returning negative value as unsigned sets errno?

2 Answers2