wl_shm_buffer_begin_access (3) - Linux Man Pages
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void wl_shm_buffer_begin_access (struct wl_shm_buffer *buffer)Mark that the given SHM buffer is about to be accessed
- buffer The SHM buffer
An SHM buffer is a memory-mapped file given by the client. According to POSIX, reading from a memory-mapped region that extends off the end of the file will cause a SIGBUS signal to be generated. Normally this would cause the compositor to terminate. In order to make the compositor robust against clients that change the size of the underlying file or lie about its size, you should protect access to the buffer by calling this function before reading from the memory and call wl_shm_buffer_end_access afterwards. This will install a signal handler for SIGBUS which will prevent the compositor from crashing.
After calling this function the signal handler will remain installed for the lifetime of the compositor process. Note that this function will not work properly if the compositor is also installing its own handler for SIGBUS.
If a SIGBUS signal is received for an address within the range of the SHM pool of the given buffer then the client will be sent an error event when wl_shm_buffer_end_access is called. If the signal is for an address outside that range then the signal handler will reraise the signal which would will likely cause the compositor to terminate.
It is safe to nest calls to these functions as long as the nested calls are all accessing the same buffer. The number of calls to wl_shm_buffer_end_access must match the number of calls to wl_shm_buffer_begin_access. These functions are thread-safe and it is allowed to simultaneously access different buffers or the same buffer from multiple threads.
void wl_shm_buffer_end_access (struct wl_shm_buffer *buffer)Ends the access to a buffer started by wl_shm_buffer_begin_access
- buffer The SHM buffer
This should be called after wl_shm_buffer_begin_access once the buffer is no longer being accessed. If a SIGBUS signal was generated in-between these two calls then the resource for the given buffer will be sent an error.
void * wl_shm_buffer_get_data (struct wl_shm_buffer *buffer)Get a pointer to the memory for the SHM buffer
- buffer The buffer object
Returns a pointer which can be used to read the data contained in the given SHM buffer.
As this buffer is memory-mapped, reading it from may generate SIGBUS signals. This can happen if the client claims that the buffer is larger than it is or if something truncates the underlying file. To prevent this signal from causing the compositor to crash you should call wl_shm_buffer_begin_access and wl_shm_buffer_end_access around code that reads from the memory.
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