how to use shared memory to communicate between two processes

The other two answers told you what's wrong, but I want to give you a runnable code. You can modify it to pass anything, the principle is you need to save the length of every element you passed to the other side.

//write.c

#include <stdio.h> #include <string.h> #include <sys/shm.h> #include <sys/stat.h> int main () { key_t shm_key = 6166529; const int shm_size = 1024; int shm_id; char* shmaddr, *ptr; int next[2]; printf ("writer started.\n"); /* Allocate a shared memory segment. */ shm_id = shmget (shm_key, shm_size, IPC_CREAT | S_IRUSR | S_IWUSR); /* Attach the shared memory segment. */ shmaddr = (char*) shmat (shm_id, 0, 0); printf ("shared memory attached at address %p\n", shmaddr); /* Start to write data. */ ptr = shmaddr + sizeof (next); next[0] = sprintf (ptr, "mandy") + 1; ptr += next[0]; next[1] = sprintf (ptr, "73453916") + 1; ptr += next[1]; sprintf (ptr, "amarica"); memcpy(shmaddr, &next, sizeof (next)); printf ("writer ended.\n"); /*calling the other process*/ system("./read"); /* Detach the shared memory segment. */ shmdt (shmaddr); /* Deallocate the shared memory segment.*/ shmctl (shm_id, IPC_RMID, 0); return 0; } 

//read.c

#include <stdio.h> #include <sys/shm.h> #include <sys/stat.h> int main () { key_t shm_key = 6166529; const int shm_size = 1024; int shm_id; char* shmaddr, *ptr; char* shared_memory[3]; int *p; /* Allocate a shared memory segment. */ shm_id = shmget (shm_key, shm_size, IPC_CREAT | S_IRUSR | S_IWUSR); /* Attach the shared memory segment. */ shmaddr = (char*) shmat (shm_id, 0, 0); printf ("shared memory attached at address %p\n", shmaddr); /* Start to read data. */ p = (int *)shmaddr; ptr = shmaddr + sizeof (int) * 2; shared_memory[0] = ptr; ptr += *p++; shared_memory[1] = ptr; ptr += *p; shared_memory[2] = ptr; printf ("0=%s\n", shared_memory[0]); printf ("1=%s\n", shared_memory[1]); printf ("2=%s\n", shared_memory[2]); /* Detach the shared memory segment. */ shmdt (shmaddr); return 0; } 

//Result of run:

> [lex:shm]$ ./write > writer started. > shared memory attached at address 0x7fa20103b000 > writer ended. > shared memory attached at address0x7fd85e2eb000 > 0=mandy > 1=73453916 > 2=amarica 

char* shared_memory[3]; ... shared_memory[3] = (char*) shmat (segment_id, 0, 0); 

You declare shared_memory as an array capable of holding three pointers to char, but what you actually do with it is to write a pointer one place behind the end of the array. Since there is no telling what the memory there is otherwise used for, what happens next is generally unpredictable.

Things go conclusively bad afterwards when you try to make use of the pointers in shared_memory[0] through shared_memory[2], because those pointers have never been initialized. They are filled with meaningless garbage from the stack -- thus the segmentation fault.

It seems, in general, that you're failing to distinguish between the array and its elements. You should go and make yourself a lot more comfortable with arrays and pointers in sequential code before you try your hand at shared-memory IPC.

Note that shared memory is one of the more easy-to-get-wrong ways of doing IPC out there. Unless you have rigid efficiency constraints and are going to exchange a lot of data, it's much easier to work with pipes, named pipes, or sockets.

You should be reserving enough shared memory to exchange the data. Processes aren't supposed to access each others memory, even if using shared pointers. Keep in mind only the raw data you write during runtime is shared, there's no type checking or any other metadata passed. You can use a common structure, if your data allows it using fixed-size arrays, to access the data more easily. Otherwise, you'll have to manually marshal the data between the processes.

#include <stdio.h> #include <string.h> #include <unistd.h> #include <sys/ipc.h> #include <sys/shm.h> #define SIZE 1 #define KEY 1234 int main() { int id = shmget(KEY, SIZE, 0666 | IPC_CREAT); if (id == -1) char* ptr = (char*) shmat(id, NULL, 0); if (ptr == (char*)(-1)); while (1) { strcpy(ptr, "1234"); strcpy(ptr, "4321"); } shmctl(id, IPC_RMID, NULL); }