Hi, Carsten Kunze wrote on Sun, Aug 19, 2018 at 02:32:49PM +0200:
> Many system calls (e.g. close(2), > http://pubs.opengroup.org/onlinepubs/9699919799/functions/close.html) > return 0 on success and -1 on error and set errno. > > My understanding is that errno is only reliably set when the system > call returns -1, not for < -1 nor for > 0, What usually matters more than knowing whether errno has been set or not is detecting failure - even though close(2) is the most typical example of a system call where checking the return value / checking for failure is almost never useful, and simply ignoring both the return value and errno is what you should do in most cases. > so I do only check the error condition with "if (close(fd) == -1)" > ignoring all other values. That is best practice. You should assume the system behaves as specified and test for the exact values that can be returned according to the specification. If the system violates the specification, well, then you are in unspecified territory anyway and your program cannot run reliably either way. > Some are testing for "< 0" but would it not be consequent > for them to check for "!= 0"? Neither would be best practice, but if the system behaves according to specification, it doesn't make a difference in practice - it merely looks confusing to a code auditor. > So when can I rely on errno to be set, for -1, for "< 0" or for "!= 0"? You can rely on the function to return -1 on failure and 0 on success, and you can rely on errno being set on failure. Other return values can never happen unless there is a severe bug in the kernel or in the C library. > In case of library functions (e.g. fclose(3), > http://pubs.opengroup.org/onlinepubs/9699919799/functions/fclose.html) > we have a similar situation with the return values 0 and EOF. > > I did test for errors with "if (fclose(fh) == EOF)", That is best practice. > others are testing for "!= 0". See above; not best practice, but no real difference in practice. > When can I rely here for errno to be set, only in case of EOF > or als for "!= 0"? Values other than 0 or EOF cannot happen, and EOF is specified to be < 0, so != 0 is correct, too, just slightly confusing. Finally, note that * bugs with errno (not being set even though it should, being set to the wrong code, or being clobbered even though the function succeeded) are relatively common in C libraries, so the only purposes errno should be used for are - reporting the (probable) cause of failure to the user - (trying to) distinguish different causes of failure for taking appropriate action - detecting failure for those severely ill-designed functions where the return value is unhelpful, most notably strtol(3) - needs very careful coding, though * not all functions adhere to the 0/-1 scheme to report success or failure, allways check the spec. Behaviour of some is implementation-dependent in some respects, most notably in the printf(3) family. Yours, Ingo
