| /* |
| * Copyright (C) 2004-2021 Savoir-faire Linux Inc. |
| * |
| * Author: Tristan Matthews <tristan.matthews@savoirfairelinux.com> |
| * |
| * This program is free software; you can redistribute it and/or modify |
| * it under the terms of the GNU General Public License as published by |
| * the Free Software Foundation; either version 3 of the License, or |
| * (at your option) any later version. |
| * |
| * This program is distributed in the hope that it will be useful, |
| * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| * GNU General Public License for more details. |
| * |
| * You should have received a copy of the GNU General Public License |
| * along with this program; if not, write to the Free Software |
| * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. |
| */ |
| |
| #ifndef SHM_HEADER_H_ |
| #define SHM_HEADER_H_ |
| |
| #include <cstdint> |
| #include <semaphore.h> |
| |
| /* Implementation note: double-buffering |
| * Shared memory is divided in two regions, each representing one frame. |
| * First byte of each frame is guaranteed to be aligned on 16 bytes. |
| * One region is marked as readable: this region can be safely read. |
| * The other region is writeable: only the producer can use it. |
| */ |
| |
| struct SHMHeader |
| { |
| sem_t mutex; // lock it before any operations on these fields |
| sem_t frameGenMutex; // unlocked by producer when frameGen is modified |
| unsigned frameGen; // monotonically incremented when a producer changes readOffset |
| unsigned frameSize; // size in bytes of 1 frame |
| unsigned mapSize; // size to map if you need all the data |
| unsigned readOffset; // offset of readable frame in data |
| unsigned writeOffset; // offset of writable frame in data |
| uint8_t data[]; // the whole shared memory |
| }; |
| |
| #endif |