Semaphores are one of three System V IPC mechanisms. This series focuses on semaphores.

All three share the same key infrastructure: ftok() for key generation, IPC_CREAT/IPC_EXCL flags, and ipc_perm for ownership. The ipcs command lists all SysV IPC objects on your system.

POSIX semaphores are single-valued objects. System V semaphores are grouped in sets — one kernel identifier controls a whole array of semaphore counters. This is useful for atomically managing access to multiple resource types simultaneously (e.g., semop() can modify multiple semaphores in a single atomic operation).

Learn how to create a new semaphore set or open an existing one, understand the key mechanism, and handle all error conditions.

Master all semctl() operations with complete working code examples and a reusable helper library.

Understand the kernel metadata structure behind every semaphore set and how to read, interpret, and change it.

# List all System V semaphore sets on the system
ipcs -s

# Detailed view with times and permissions
ipcs -s -t
ipcs -s -c
ipcs -s -l   # show system limits

# Remove a semaphore set by semid
ipcrm -s <semid>

# Remove by key (hex)
ipcrm -S 0x12345678

# Show kernel limits for semaphores
cat /proc/sys/kernel/sem
# Output format: SEMMSL SEMMNS SEMOPM SEMMNI
# SEMMSL = max semaphores per set
# SEMMNS = max total semaphores system-wide
# SEMOPM = max operations per semop() call
# SEMMNI = max number of semaphore sets