To call stat() and declare a struct stat you must include sys/types.h, sys/stat.h, and unistd.h. To use the variable errno you must include errno.h. Important: as discussed in the Reek text, some system calls and C standard library functions set errno to a nonzero value to indicate errors. However, these functions and system calls do not change or reset errno when they begin. This means that the caller (your code) must reset errno to zero before making any system calls that might set its value.