shmem_create (nr 508)

Create a shared memory object and return a file descriptor.

Signature

shmem_create(size: u64, flags: u32) → fd or -errno

Arguments

Arg	Register	Description
size	rdi	Size of the shared memory object in bytes (must be > 0)
flags	rsi	Flags: `SHM_CLOEXEC` (0x01) sets close-on-exec on the fd

Return value

On success, returns a file descriptor for the shared memory object.

Errors

Error	Condition
EINVAL	`size` is 0, or unknown flags are set
ENOMEM	Not enough physical memory to allocate the backing frames
EMFILE	Process fd table is full

Description

Allocates a shared memory object backed by eagerly-allocated, zeroed physical frames. Returns a file descriptor referring to it.

The fd can be inherited by child processes (via clone + execve, unless SHM_CLOEXEC is set) or transferred via IPC fd-passing (ipc_send / ipc_recv). Both sides can then call mmap(MAP_SHARED, fd) to map the same physical pages into their address spaces.

Physical frames are reference-counted. A frame is freed only when all mappings are removed and the last fd referring to the shared memory object is closed.

Flags

Flag	Value	Description
`SHM_CLOEXEC`	0x01	Set close-on-exec on the returned fd (analogous to Linux’s `MFD_CLOEXEC`)

Userspace usage (C)

#define SYS_SHMEM_CREATE 508
#define SHM_CLOEXEC      0x01

static long shmem_create(unsigned long size, unsigned int flags) {
    return syscall(SYS_SHMEM_CREATE, size, flags);
}

/* Create 4 KiB shared memory, mmap it */
int fd = shmem_create(4096, 0);
void *ptr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

Implementation

osl/src/syscalls/shmem.rs — sys_shmem_create

Backing struct: libkernel/src/shmem.rs — SharedMemInner

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