Kstat_open sets errno to "Not enough space"

Question

I wrote the following test program:
<pre>
int main()
kstat_ctl_t * kc;
perror("1");
kc = kstat_open();
perror("2");
if (!kc) printf("kstat_open failed\n");
</pre>
and got the following output:
1: Error 0
2: Not enough space
Reason I wrote the test was that in another program, I was calling kstat_open() and it was returning NULL with this error, and I wanted to know why. Seems the error is misleading. Any suggestions as to how else I might diagnose my problem, please?

Answer 1

Here are some general rules for error numbers:
<UL>
* For functions in section 2 of the manual, they return success OR they return failure and set errno. You should not check errno after a successful system call. These functions will clearly say in their man page that they set errno and what values it might get.
* For functions in section 3 of the manual, they just return success or failure. In either case, they may or may not change errno. The fact that errno did or did not change tells you nothing about whether they succeeded or why they failed. The man pages for these functions do not show error numbers, so you should not check errno or use perror.
</UL>
Your test shows that kstat_open can set errno to ENOMEM even when the open succeeds. This happens when kstat works to figure out how much buffer space it needs, and it is expected behavior.
When can kstat_open fail? It will fail if
* the open of /dev/kstat fails
* an ioctl(KSTAT_IOC_CHAIN_ID or KSTAT_IOC_READ) fails
* a call to calloc fails
If you use the truss command, you will get a change to see what system calls libkstat is doing. This might help shed some light on the problem.