Shared Memory in C (sysV)

Question

struct shared_memory_t { int value1; int value2; char* buffer; }; shmid = shmget(key, sizeof(shared_memory_t) + segsize, 0666|IPC_CREAT); shared_memory_t* mem = (shared_memory_t*) shmat(*shmid, NULL, 0);

So I was trying to map the shared memory to a custom structure. Now I do not know how large the segsize is until user starts program and inputs a value. I wanted buffer to be pointer to beginning of the memory space after the int values. Now if I do this I get memory faults. I can attach it and get the starting memory space with:

void* mem = shmat(shmid, NULL, 0);

Any hints on how I can get it in a state where I can do mem->value1 and access the data buffer for raw data bytes?

Have you read in the manual that size needs to be a multiple of a memory page size? Probably a multiple of 4096. — Cheatah
– Cheatah, Commented Nov 6, 2021 at 2:09
Also always check the return value of shmget, don't assume that it will just work. — Cheatah
– Cheatah, Commented Nov 6, 2021 at 2:10
@Cheatah: size does not have to be a multiple of the page size; the system will automatically round it up. — Nate Eldredge
– Nate Eldredge, Commented Nov 6, 2021 at 2:19

Nate Eldredge · Accepted Answer · 2021-11-06 02:17:36Z

Use a flexible array member:

struct shared_memory_t { int value1; int value2; char buffer[]; // flexible array member }; shmid = shmget(key, sizeof(shared_memory_t) + segsize, 0666|IPC_CREAT); shared_memory_t* mem = (shared_memory_t*) shmat(shmid, NULL, 0); memset(mem.buffer, 42, segsize); // all valid

It really was that simple, after a couple hours reading through the manuals. Thank you!

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