c - Linux System V IPC: Unable to initialize semaphore value -

i have created simple semaphore initialization example demo purpose:

#include <stdio.h> #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> #include <unistd.h> #include <stdlib.h>  int init_semaphore(int semid, int semnum, int initval) {     union semun pack;     pack.val = initval;     return semctl(semid,semnum,setval,pack); } 

but getting error:

error: aggregate ‘init_semaphore(int, int, int)::semun pack’ has incomplete type , cannot defined 

i not able understand why compiler throwing error. headers included properly.

you have explicitly declare union semun yourself.

per posix standard semctl():

the semctl() function provides variety of semaphore control operations specified cmd. fourth argument optional , depends upon operation requested. if required, of type union semun, which application shall explicitly declare:

union semun {     int val;     struct semid_ds *buf;     unsigned short  *array; } arg; 

per linux man page:

this function has 3 or 4 arguments, depending on cmd. when there four, fourth has type union semun. calling program must define union follows:

   union semun {        int              val;    /* value setval */        struct semid_ds *buf;    /* buffer ipc_stat, ipc_set */        unsigned short  *array;  /* array getall, setall */        struct seminfo  *__buf;  /* buffer ipc_info                                    (linux-specific) */    };