Forks and Shared Memory in C (Linux OS)

[cwarren13]

So, I have a server process that forks for every client that connects. I keep a list of connected clients that I want each fork process to have. Of course, when the process forks the child only has what it had when the fork occurred. For the solution to this, I decided to use shared memory. For simplicities sake (and because this is my main issue I'm dealing with right now) I'll illustrate how I've attempted to pass an integer from the parent to child processes.

Parent process:

key_t key_numentries;
int shmid_numentries;
char *shm_numentries, *shm1;

char buf[];

//Key for numentries is 2345

key_numentries = 2345;

//Create the shared memory segment

if((shmid_numentries = shmget(key_numentries, 8, IPC_CREAT | 0666)) < 0) {
	perror("shmget");
	exit(1);
}

//Attach shared memory to dataspace

if((shm1 = shmat(shmid_numentries, NULL, 0)) == (char *) -1) {
	perror("shmat");
	exit(1);
}

//did this because I wasn't sure how to send an int over the shm
//numentries does contain the right value, I've tested it.

sprintf (buf,"%d",numentries);

//not sure if this would work, but a printf(%s) with &shm1[0] printed 
//the correct value
shm1 = buf;

//pretty sure the code below fork is irrelevant

if(!fork()){
		close(socket_in);
		recievecmds(new_fd);
		close(new_fd);
		exit(0);}

Child process:

int shmid_numentries;
key_t key_numentries;
char *shm_numentries, *shm1;

//set key to 2345

key_numentries = 2345;

//locate the segment

if ((shmid_numentries = shmget(key_numentries, 8, 0666)) < 0) {
	perror("shmget");
	exit(1);
}

//attach the segment to our data space
	
if ((shm1 = shmat(shmid_numentries, NULL, 0)) == (char *) -1) {
	perror("shmat");
	exit(1);
}

// I have no idea what to do here... here are some of the things I've tried


			//send # of entries to client
			//char c = (char)&shm1;
			//printf("got this from the parent too %s\n",c);
			//printf("in SHM buf: %s\n",&shm1[0]);
			numentries = (int)&shm1[0];
			//numentries = strtol(&shm1[0],NULL,0);
			
			for (; shm1 != NULL; shm1++)
        			putchar(*shm1);
    			putchar('\n');

heres where I'm not sure what to do... how do I get the data from the pointer? Everything I've tried is either nothing or it's a bunch of random gibberish that obviously I didn't store in the pointer... so I'm afraid the memory space isn't actually being shared. Can anyone point me in the right direction here? I am BRAND NEW to shared memory to be honest. Haven't dealt with forks that much either.

Thanks in advance! Please let me know if I need to clarify anything.

[venomxxl]

I had once held a small project for Linux but that was a few months ago, and I hardly remember anything since it was my last project which I wrote for Linux. I mostly program for Windows now, but I'm not going to write my autobiography here!

Here's a little snippet I can provide, not sure if it works though:

#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>

typedef struct{
     // Variables
}SharedMemory;

static int FileDescriptor;

static SharedMemory * Memory;

void SharedMemoryMap(){
     FileDescriptor=open("/dev/zero", O_RDWR);

     if(FileDescriptor==-1){
         // Error
     }

     Memory=(SharedMemory *)mmap(0, sizeof(SharedMemory), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

     if(Memory==(SharedMemory *)-1){
          // Error
     }
}

Change it to suit your needs, hope it helps.