shmwrite - write SysV shared memory
Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
detaching from it. When reading, VAR must be a variable which will
hold the data read. When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes. Return TRUE if successful, or FALSE if there is an error.