shmflg

shmflg is the integer argument used by System V shared memory interfaces in Unix-like systems to specify how a shared memory segment should be created or accessed. It is passed to shmget to determine creation and access permissions, and to shmat to indicate how the segment should be attached to a process’s address space.

In the context of shmget, shmflg combines permission bits and creation flags. The permission bits follow the standard file permission model (for example, S_IRUSR, S_IWUSR, S_IRGRP, S_IWGRP, S_IROTH, S_IWOTH) and are commonly written as octal values such as 0600, 0666, or 0644. Creation flags include IPC_CREAT to create the segment if it does not exist, and IPC_EXCL to fail if it already exists. A typical usage is shmget(key, size, IPC_CREAT | 0666), which creates a segment with read/write permissions for user, group, and others.

For shmat, the shmflg parameter controls how the segment is attached to the process’s address space. The flag SHM_RDONLY can be supplied to map the segment as read-only; if this flag is not set, the mapping is read/write. The same shmflg name is used, but its interpretation differs by call.

Some systems may include additional, non-portable flags (for example, for specialized memory handling) that can be ORed into shmflg, though such flags are uncommon and not standardized.

Security and portability notes: permissive permission bits can allow other processes to access the segment, so use the minimum necessary permissions and creation flags. shmflg and related semantics come from the System V IPC interface and may differ or be unavailable on non–System V systems, where POSIX shared memory is often used instead.

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