musl/arch/x86_64/bits/shm.h

#define SHMLBA 4096

#define SHM_RDONLY 010000
#define SHM_RND    020000
#define SHM_REMAP  040000
#define SHM_EXEC   0100000

/* linux extensions */
#define SHM_LOCK        11
#define SHM_UNLOCK      12

struct shmid_ds
{
	struct ipc_perm shm_perm;
	size_t shm_segsz;
	time_t shm_atime;
	time_t shm_dtime;
	time_t shm_ctime;
	pid_t shm_cpid;
	pid_t shm_lpid;
	unsigned long shm_nattch;
	unsigned long __pad1;
	unsigned long __pad2;
};