[groff] [off-topic] Reliable use of errno

[Carsten Kunze]

Hello,

sorry for misusing the list for something completely off-topic (while it still 
refers to the documentation of the UNIX system), but since there are many UNIX 
experts on the list maybe someone has a definite answer.

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, so I do only check the error condition 
with "if (close(fd) == -1)" ignoring all other values. Some are testing for "< 
0" but would it not be consequent for them to check for "!= 0"? So when can I 
rely on errno to be set, for -1, for "< 0" or for "!= 0"?

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)", others are testing for "!= 
0". When can I rely here for errno to be set, only in case of EOF or als for 
"!= 0"?

Thank you and sorry for the noice.

--Carsten