Next: Running UNIX Commands from C Up: UNIX and C Previous: errno . A process is basically a single running program. ... When UNIX runs a process it gives each process a unique number - a process ID, pid . The UNIX command ps will list all current processes running on your machine and will list the pid. The C function int getpid() will return the pid of process that called this function. ... Running UNIX Commands from C . ... Piping in a C program . ... Interrupts and Signals . ...