--- name: "shmget_secure" section: "2" platform: "linux" package: "libselinux-dev" version: "2002031409-4" release: "Debian 3.0 (Woody)" platforms: ["linux"] url: "https://new.carta.tech/shmget/2" --- # shmget\_secure(2) allocates a shared memory segment Part of [shmget(2)](/shmget/2.md). ## NAME shmget, shmget\_secure - allocates a shared memory segment ## SYNOPSIS **#include \** **#include \** **#include \ /\* for msgget\_secure \*/** **int shmget(key\_t** *key***, int** *size***, int** *shmflg***);** **int shmget\_secure(key\_t** *key***, int** *size***, int** *shmflg***, security\_id\_t** *sid);* ## DESCRIPTION **shmget()** returns the identifier of the shared memory segment associated to the value of the argument *key*. A new shared memory segment, with size equal to the round up of *size* to a multiple of **PAGE\_SIZE**, is created if *key* has value **IPC\_PRIVATE** or *key* isn't **IPC\_PRIVATE**, no shared memory segment is associated to *key*, and **IPC\_CREAT** is asserted in *shmflg* (i.e. *shmflg***&IPC\_CREAT** isn't zero). The presence in *shmflg* is composed of: - **IPC\_CREAT** to create a new segment. If this flag is not used, then **shmget()** will find the segment associated with *key*, check to see if the user has permission to receive the *shmid* associated with the segment, and ensure the segment is not marked for destruction. If [**shmget\_secure**(2)](/shmget_secure/2.md) is used the *sid* argument must match the value of the existing semaphore set in addition to the access permissions being checked. - **IPC\_EXCL** used with **IPC\_CREAT** to ensure failure if the segment exists. - **mode\_flags (lowest 9 bits)** specifying the permissions granted to the owner, group, and world. Presently, the execute permissions are not used by the system. If a new segment is created, the access permissions from *shmflg* are copied into the *shm\_perm* member of the *shmid\_ds* structure that defines the segment. The *shmid\_ds* structure: ``` struct shmid_ds { struct ipc_perm shm_perm; /* operation perms */ int shm_segsz; /* size of segment (bytes) */ time_t shm_atime; /* last attach time */ time_t shm_dtime; /* last detach time */ time_t shm_ctime; /* last change time */ unsigned short shm_cpid; /* pid of creator */ unsigned short shm_lpid; /* pid of last operator */ short shm_nattch; /* no. of current attaches */ }; ``` ``` struct ipc_perm { key_t key; ushort uid; /* owner euid and egid */ ushort gid; ushort cuid; /* creator euid and egid */ ushort cgid; ushort mode; /* lower 9 bits of shmflg */ ushort seq; /* sequence number */ }; ``` Furthermore, while creating, the system call initializes the system shared memory segment data structure **shmid\_ds** as follows: - **shm\_perm.cuid** and **shm\_perm.uid** are set to the effective user-ID of the calling process. - **shm\_perm.cgid** and **shm\_perm.gid** are set to the effective group-ID of the calling process. - The lowest order 9 bits of **shm\_perm.mode** are set to the lowest order 9 bit of *shmflg*. - **shm\_segsz** is set to the value of *size.* - **shm\_lpid**, **shm\_nattch**, **shm\_atime** and **shm\_dtime** are set to **0.** - **shm\_ctime** is set to the current time. If the shared memory segment already exists, the access permissions are verified, and a check is made to see if it is marked for destruction. ## SYSTEM CALLS - **fork()** After a **fork()** the child inherits the attached shared memory segments. - **exec()** After an **exec()** all attached shared memory segments are detached (not destroyed). - **exit()** Upon **exit()** all attached shared memory segments are detached (not destroyed). ## RETURN VALUE A valid segment identifier, *shmid*, is returned on success, -1 on error. ## ERRORS On failure, **errno** is set to one of the following: - **EINVAL** is returned if **SHMMIN** \> *size* or *size* \> **SHMMAX**, or *size* is greater than size of segment. - **EEXIST** is returned if **IPC\_CREAT \| IPC\_EXCL** was specified and the segment exists. - **EIDRM** is returned if the segment is marked as destroyed, or was removed. - **ENOSPC** is returned if all possible shared memory id's have been taken(**SHMMNI**) or if allocating a segment of the requested *size* would cause the system to exceed the system-wide limit on shared memory(**SHMALL**). - **ENOENT** is returned if no segment exists for the given *key*, and **IPC\_CREAT** was not specified. - **EACCES** is returned if the user does not have permission to access the shared memory segment or the calling process lacks permission to create a shared memory segment with the specified *sid.* - **ENOMEM** is returned if no memory could be allocated for segment overhead. ## NOTES **IPC\_PRIVATE** isn't a flag field but a **key\_t** type. If this special value is used for *key*, the system call ignores everything but the lowest order 9 bits of *shmflg* and creates a new shared memory segment (on success). The followings are limits on shared memory segment resources affecting a **shmget** call: - **SHMALL** System wide maximum of shared memory pages: policy dependent. - **SHMMAX** Maximum size in bytes for a shared memory segment: implementation dependent (currently 4M). - **SHMMIN** Minimum size in bytes for a shared memory segment: implementation dependent (currently 1 byte, though **PAGE\_SIZE** is the effective minimum size). - **SHMMNI** System wide maximum number of shared memory segments: implementation dependent (currently 4096). The implementation has no specific limits for the per process maximum number of shared memory segments (**SHMSEG**). ## BUGS Use of **IPC\_PRIVATE** doesn't inhibit to other processes the access to the allocated shared memory segment. As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a shared memory segment. Asserting both **IPC\_CREAT** and **IPC\_EXCL** in *shmflg* only ensures (on success) that a new shared memory segment will be created, it doesn't imply exclusive access to the segment. ## CONFORMING TO SVr4, SVID. SVr4 documents an additional error condition EEXIST. Neither SVr4 nor SVID documents an EIDRM condition. ## SEE ALSO [**ftok**(3)](/ftok/3.md), [**ipc**(5)](/ipc/5.md), [**shmctl**(2)](/shmctl/2.md), [**shmat**(2)](/shmat/2.md), [**shmdt**(2)](/shmdt/2.md), [**shmsid**(2)](/shmsid/2.md).