I am working on implementing a database server in C that will handle requests from multiple clients. I am using fork() to handle connections for individual clients.
The server stores data in the heap, which consists of a root pointer to hash tables of dynamically allocated records. The records are structs that have pointers to various data-types. I would like for the processes to be able to share this data so that, when a client makes a change to the heap, the changes will be visible for the other clients.
I have learned that fork() uses COW (Copy On Write), and my understanding is that it copies the heap (and stack) memory of the parent process when the child tries to modify the data in memory.
I have found out that I can use the shm library to share memory.
Would the code below be a valid way to share heap memory (in shared_string)? If a child were to use similar code (i.e. starting from //start), would other children be able to read/write to it while the child is running and after it's dead?
key_t key;
int shmid;
key = ftok("/tmp",'R');
shmid = shmget(key, 1024, 0644 | IPC_CREAT);
//start
char * string;
string = malloc(sizeof(char) * 10);
strcpy(string, "a string");
char * shared_string;
shared_string = shmat(shmid, string, 0);
strcpy(shared_string, string);
Here are some of my thoughts/concerns regarding this:
I'm thinking about sharing the root pointer of the database. I'm not sure if that would work or if I have to mark all allocated memory as shared.
I'm not sure if the parent / other children are able to access memory allocated by a child.
I'm not sure if a child's allocated memory stays on the heap after it is killed, or if that memory is released.