"Thankfully that case will never happen because you can't be a child if there was an error. :)"
Uh, no. If there is an error, there will be no child process but the parent process will think it is the child.
From the fork man page, emphasis mine: "On success, the PID of the child process is returned in the parent, and 0 is returned in the child."
"Also the check as presumably written would never miss an error, it would just potentially assume valid return values were also errors."
That was already discussed as a possibility; I was addressing the other. In the case you describe the software would never work at all, even when fork successfully forks, because the child will always think there was an error and presumably fall over rather than getting things done. That's probably the better case, in terms of development progress, because it would be spotted and fixed right away. But hopefully fixed correctly, and not converted to the broken-but-working-when-fork-succeeds other variant that also uses "<= 0".
