Noticed this in wait.2, though I imagine other occurences abound. I believe non-null is clearer when refering to a pointer than non-zero, which is a bit 80s, and less likely to be mistaken for the value pointed to. This same page also refers to non-zero values, fwiw.
Index: wait.2 =================================================================== RCS file: /home/cvs/src/lib/libc/sys/wait.2,v retrieving revision 1.30 diff -u -p -r1.30 wait.2 --- wait.2 1 May 2017 00:08:31 -0000 1.30 +++ wait.2 31 Jan 2020 10:28:42 -0000 @@ -70,7 +70,7 @@ On return from a successful .Fn wait call, the .Fa status -area, if non-zero, is filled in with termination information about the +area, if non-null, is filled in with termination information about the process that exited (see below). .Pp The @@ -137,7 +137,7 @@ signal also have their status reported. .Pp If .Fa rusage -is non-zero, a summary of the resources used by the terminated +is non-null, a summary of the resources used by the terminated process and all its children is returned (this information is currently not available for stopped processes).
