Error when function is in a separate C file - c

Question

I have a file API.c in which there are certain functions but no main function.
Inside one of the functions , I call shmat() and return :
#include<stdio.h>
#include<stdlib.h>
#include<sys/shm.h>
#include<unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/msg.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include<string.h>
#include<fcntl.h>
#include<errno.h>
#include<sys/stat.h>
#include<signal.h>
int array1[100]; // TO STORE THE SHMIDs
int array2[100]; // TO STORE THE RSHMIDs
char * array3[100]; //TO STORE THE SHMAT ADDRESSES
struct msgbuf
{
long type;
int key;
int rshmid;
int size;
void * addr;
int cmd;
int pid;
int shmid;
};
struct msgbuf my,rec;
int queue_id;
int rshmget(int key,int size)
{
queue_id = msgget(1234, IPC_CREAT | 0666);
printf("1\n");
my.type =1;
my.key=key;
my.size=size;
my.pid=getpid();
//SENDING REQUEST OF CLIENT TO SERVER
if (msgsnd (queue_id, &(my), sizeof (my), 0) == -1)
{
perror("msgsnd");
}
//SERVER CREATES SHARED MEMORY AND WRITES THE REPLY BACK
while(1)
{
if (msgrcv (queue_id, &(rec), sizeof (rec), getpid(), IPC_NOWAIT) != -1)
{
printf("GOT MSG\n");
break;
}
sleep(1);
}
//ARRAY 1 STORES THE ACTUAL SHMID OF SHARED MEMORY
//ARRAY 2 STORES RSHMID , WHICH IS THE ID RETURNED TO CLIENT
array1[key%100] = rec.shmid;
array2[key%100] = rec.rshmid;
return rec.rshmid;
}
void * rshmat(int rshmid, void* addr)
{
my.type =2;
my.rshmid = rshmid;
my.pid=getpid();
int i;
// IDENTIFYING THE SHMID CORRESPONDING TO THE RSHMIDS
for(i=0;i<100;i++)
{
if(array2[i]==rshmid)
break;
}
//ARRAY 3 STORES THE ADDRESSES CORRESPONDING TO EACH SHMAT
array3[i] = shmat(array1[i],NULL,0);
//INFORMING THE SERVER ABOUT THE ATTACHING
if (msgsnd (queue_id, &(my), sizeof (my), 0) == -1)
{
perror("msgsnd");
}
return array3[i];
}
In the file main.c , i am doing this:
int main()
{
queue_id = msgget(1234, IPC_CREAT | 0666);
int id1 = rshmget(521,200);
char * addr1 = (char *)rshmat(id1,NULL);
strcpy(addr1,"FIRST MESSAGE 123456");
}
When trying to executing the strcpy statement , segmentation fault occurs.
But if I keep the API functions and main in same file, strcpy executes successfully.
According to my understanding, shmat() will return a pointer in heap , so it should be accessible from main.
The fact that it works when code is in the same file might indicate that I am not linking the files properly , but other functions in the API.c file are able to return properly.
This is how I am compiling:
gcc -c API.c
gcc -c main.c
gcc main.o API.o -o client
I am using LINUX operating system.

Answer 1

In both files add prototypes for every function that you are using from other file with identifier extern in front of them.
e.g. in file main.c add extern void * rshmat(int shm_id);