Reference Guide
Preface
I. Hardware
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1. 1. Processor Cores
  Expand section "1. Processor Cores" Collapse section "1. Processor Cores"
  1. 1.1. Caches
  2. 1.2. Interconnects
2. 2. Memory Allocation
  Expand section "2. Memory Allocation" Collapse section "2. Memory Allocation"
  1. 2.1. Demand Paging
  2. 2.2. Using mlock to Avoid Page I/O
3. 3. Hardware Interrupts
  Expand section "3. Hardware Interrupts" Collapse section "3. Hardware Interrupts"
II. Application Architecture
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1. 4. Threads and Processes
2. 5. Priorities and Policies
3. 6. Affinity
  Expand section "6. Affinity" Collapse section "6. Affinity"
  1. 6.1. Using the taskset Command to Set Processor Affinity
  2. 6.2. Using the sched_setaffinity() System Call to Set Processor Affinity
4. 7. Thread Synchronization
  Expand section "7. Thread Synchronization" Collapse section "7. Thread Synchronization"
5. 8. Sockets
  Expand section "8. Sockets" Collapse section "8. Sockets"
  1. 8.1. Socket Options
6. 9. Shared Memory
7. 10. Shared Libraries
III. Library Services
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1. 11. Setting the Scheduler
  Expand section "11. Setting the Scheduler" Collapse section "11. Setting the Scheduler"
  1. 11.1. Using chrt to Set the Scheduler
  2. 11.2. Preemption
  3. 11.3. Using Library Calls to Set Priority
    
    Expand section "11.3. Using Library Calls to Set Priority" Collapse section "11.3. Using Library Calls to Set Priority"
    
    11.3.1. sched_getscheduler
    
    11.3.2. sched_setscheduler
    
    11.3.3. sched_getparam and sched_setparam
    
    11.3.4. sched_get_priority_min and sched_get_priority_max
    
    11.3.5. sched_rr_get_interval
2. 12. Creating Threads and Processes
3. 13. Mmap
4. 14. System Calls
  Expand section "14. System Calls" Collapse section "14. System Calls"
  1. 14.1. sched_yield
  2. 14.2. getrusage()
5. 15. Timestamping
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  1. 15.1. Hardware Clocks
    
    Expand section "15.1. Hardware Clocks" Collapse section "15.1. Hardware Clocks"
    
    15.1.1. Reading Hardware Clock Sources
  2. 15.2. POSIX Clocks
    
    Expand section "15.2. POSIX Clocks" Collapse section "15.2. POSIX Clocks"
    
    15.2.1. CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE
    
    15.2.2. Using clock_getres() to Compare Clock Resolution
    
    15.2.3. Using C Code to Compare Clock Resolution
    
    15.2.4. Using the time Command to Compare Cost of Reading Clocks
16. More Information
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1. 16.1. Reporting Bugs
A. Revision History
Legal Notice

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Chapter 9. Shared Memory

One of the main advantages of program threads is that all threads created in one process context share the same address space. This means that all data structures become accessible to them. However, it is not always appropriate for applications to use threads. And in these cases, processes might need to share part of the address space. This can be achieved on both ordinary and realtime kernels by using shared memory.

The original mechanism for sharing a memory region between two processes was the System V IPC shmem set of calls. These calls are quite capable, but overly complicated and cumbersome for the vast majority of use cases. For this reason, they have been deprecated on the Red Hat Enterprise Linux for Real Time kernel and should no longer be used.

Red Hat Enterprise Linux for Real Time uses POSIX shared memory calls, such as shm_open and mmap.

Note

For more information, or for further reading, the following man pages are related to the information given in this section:

shm_open(3)
shm_overview(7)
mmap(2)