zkUnistd.h is a portable wrapper for the Unix 98 header <unistd.h>. It is part of the libzk library because <sys/stat.h> doesn't exist in Standard C.

zkUnistd.h - Deprecated Functions from <unistd.h>
Don't Call This Instead, Call This Reason
chdir zkChdir Not in Standard C.
close zkClose Not in Standard C. Note that only file descriptors can be portably passed to this function, not socket descriptors.
ftruncate zkFtruncate Not in Standard C.
gethostname zkGethostname Not in Standard C.
getpid zkGetpid Not in Standard C.
lseek zkLseek Not in Standard C.
read zkRead Not in Standard C. Note that only file descriptors can be portably passed to this function, not socket descriptors.
sleep zkSleep Not in Standard C.
truncate zkTruncate Not in Standard C.
write zkWrite Not in Standard C. Note that only file descriptors can be portably passed to this function, not socket descriptors.