The errno=28, No space left on device error in the context of the semget() system call in Linux typically means that the system has run out of space for creating new semaphores. This is not referring to disk space, but rather to the limits set for system-wide semaphore usage. Here are a few potential causes and solutions:

  1. System-Wide Semaphore Limits Reached:
    The system has a limit on the number of semaphore sets and the number of semaphores per set. These limits are set by kernel parameters and can be viewed and adjusted using the sysctl command.
    To check the current limits:
sysctl -a | grep sem

You should see output similar to:

kernel.sem = 250 32000 32 128

The parameters in kernel.sem are:

  • The maximum number of semaphores per semaphore set.
  • The maximum number of semaphores that the system can allocate.
  • The maximum number of operations that can be performed in a single semop call.
  • The maximum number of semaphore sets.
  1. Increasing the Limits:
    If the system limits are too low, you can increase them. For example, to increase the maximum number of semaphore sets and semaphores, you can modify the kernel.sem parameter.
    Edit the /etc/sysctl.conf file and add or modify the line:
kernel.sem = 250 32000 100 256

Then, apply the changes with:

sysctl -p
  1. Check Current Usage:
    You can also check the current usage of semaphores to see if the limits are being approached:
ipcs -s

This will display a list of all active semaphores and their IDs.

  1. Cleaning Up Unused Semaphores:
    Sometimes, semaphores are left allocated after a process has terminated. You can clean these up manually using the ipcrm command:
ipcrm sem <semid>
  1. Code Issues:
    Ensure that your code is correctly handling semaphores, including releasing them when they are no longer needed.

By adjusting the system limits and ensuring proper semaphore usage in your application, you can resolve the No space left on device error related to semaphores.