Sharing Pointers Between Multiple Forked Processes

Question

If I want to share something like a char **keys array between fork()'d processes using shm_open and mmap can I just stick a pointer to keys into a shared memory segment or do I have to copy all the data in keys into the shared memory segment?

Answer 1

All data you want to share has to be in the shared segment. This means that both the pointers and the strings have to be in the shared memory.

Sharing something which includes pointers can be cumbersome. This is because mmap doesn't guarantee that a given mapping will end up in the required address.

You can still do this, by two methods. First, you can try your luck with mmap and hope that the dynamic linker doesn't load something at your preferred address.

Second method is to use relative pointers. Inside a pointer, instead of storing a pointer to a string, you store the difference between the address of the pointer and the address of the string. Like so:

char **keys= mmap(NULL, ...);
char *keydata= (char*) keys + npointers * sizeof(char*);
strcpy(keydata, firstring);
keys[0]= (char*) (keydata - (char*) &keys[0]);
keydata+= strlen(firststring)+1;

When you want to access the string from the other process, you do the reverse:

char **keys= mmap(NULL, ...);
char *str= (char*) (&keys[0]) + (ptrdiff_t) keys[0];

It's a little cumbersome but it works regardless of what mmap returns.