CLENQUEUEREADBUFFERR(3clc)

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version 1.0~svn33624 · 3 versions
, in ubuntu jammy, ubuntu noble

💖 CLENQUEUEREADBUFFERR (3clc) 💖

okay so bestie, this one's from the OpenCL Manual and it's lowkey the most underrated read command in the whole club. It's clEnqueueReadBufferRect and she's here to yoink a rectangular chunk of a buffer object straight into host memory. Rectangular!! Not just a boring straight line. It's giving geometry queen. ✨

🌸 NAME 🌸

clEnqueueReadBufferRect — enqueue commands to read from a rectangular region from a buffer object to host memory. fr fr, that's the whole vibe. 🎀

🦋 SYNOPSIS 🦋

cl_int clEnqueueReadBufferRect(cl_command_queue command_queue,
                               cl_mem buffer, cl_bool blocking_read,
                               const size_t * buffer_origin,
                               const size_t * host_origin,
                               const size_t * region,
                               size_t buffer_row_pitch,
                               size_t buffer_slice_pitch,
                               size_t host_row_pitch,
                               size_t host_slice_pitch, void *ptr,
                               cl_uint num_events_in_wait_list,
                               const cl_event *event_wait_list,
                               cl_event *event);

💅 PARAMETERS 💅

okay, meet the cast 👑

  • command_queue — the command-queue where this read command is getting queued up. heads up bestie: command_queue and buffer must have been created with the same OpenCL context, no mixing and matching, that's a whole mess.
  • buffer — a valid buffer object. like, actually valid. we've all been there. 🙈
  • blocking_read — is this read blocking or non-blocking? the moment of truth:
    • if blocking_read is CL_TRUE, i.e. the read command is blocking, then clEnqueueReadBufferRect does not return until the buffer data has been read and copied into the memory pointed to by ptr. she waits. patiently. 💖
    • if blocking_read is CL_FALSE, i.e. the read command is non-blocking, then clEnqueueReadBufferRect queues a non-blocking read command and just returns. the contents of the buffer that ptr points to cannot be used until the read command has completed. the event argument returns an event object you can use to query the execution status of the read command. once the read command is done, then the contents of the buffer that ptr points to are safe for the application to use. no peeking early, that's the rule ✋
  • buffer_origin — the (x, y, z) offset in the memory region associated with buffer. for a 2D rectangle region, the z value given by buffer_origin[2] should be 0. the offset in bytes is computed as:
buffer_origin[2] * buffer_slice_pitch + buffer_origin[1] * buffer_row_pitch + buffer_origin[0]
  • host_origin — the (x, y, z) offset in the memory region pointed to by ptr. for a 2D rectangle region, the z value given by host_origin[2] should be 0. the offset in bytes is computed as:
host_origin[2] * host_slice_pitch + host_origin[1] * host_row_pitch + host_origin[0]
  • region — the (width, height, depth) in bytes of the 2D or 3D rectangle being read or written. for a 2D rectangle copy, the depth value given by region[2] should be 1. yes she said read or written, she contains multitudes. 🫶
  • buffer_row_pitch — the length of each row in bytes to be used for the memory region associated with buffer. if buffer_row_pitch is 0, then buffer_row_pitch is computed as region[0]. auto-pilot mode, love that for her.
  • buffer_slice_pitch — the length of each 2D slice in bytes to be used for the memory region associated with buffer. if buffer_slice_pitch is 0, then buffer_slice_pitch is computed as region[1] * buffer_row_pitch. ✨
  • host_row_pitch — the length of each row in bytes to be used for the memory region pointed to by ptr. if host_row_pitch is 0, then host_row_pitch is computed as region[0].
  • host_slice_pitch — the length of each 2D slice in bytes to be used for the memory region pointed to by ptr. if host_slice_pitch is 0, then host_slice_pitch is computed as region[1] * host_row_pitch.
  • ptr — the pointer to buffer in host memory where data is to be read into. this is the destination, the final boss, the place the bits are going. 📦
  • event_wait_list, num_events_in_wait_list — these two specify events that need to complete before this particular command can even be executed. the deets:
    • if event_wait_list is NULL, then this particular command does not wait on any event to complete.
    • if event_wait_list is NULL, then num_events_in_wait_list must be 0.
    • if event_wait_list is not NULL, the list of events pointed to by event_wait_list must be valid and num_events_in_wait_list must be greater than 0.
    • the events specified in event_wait_list act as synchronization points.
    • the context associated with events in event_wait_list and command_queue must be the same. no cross-context shenanigans.
    • the memory associated with event_wait_list can be reused or freed after the function returns. go ahead bestie, free it, it's fine 💜
  • event — returns an event object that identifies this particular read command, and it can be used to query or queue a wait for this particular command to complete. event can be NULL, in which case it will not be possible for the application to query the status of this command or queue a wait for this command to complete. (rip, you gave up your tracking privileges.) if the event_wait_list and the event arguments are not NULL, then the event argument should not refer to an element of the event_wait_list array. keep them separate, they're not the same person. 🚫

⭐ NOTES ⭐

okay this part is important, listen up 📢

Calling clEnqueueReadBufferRect to read a region of the buffer object with the ptr argument value set to host_ptr and host_origin, buffer_origin values are the same — where host_ptr is a pointer to the memory region specified when the buffer object being read is created with CL_MEM_USE_HOST_PTR — must meet the same requirements given for clEnqueueReadBuffer(3clc).

Calling clEnqueueWriteBufferRect to update the latest bits in a region of the buffer object with the ptr argument value set to host_ptr and host_origin, buffer_origin values are the same — where host_ptr is a pointer to the memory region specified when the buffer object being written is created with CL_MEM_USE_HOST_PTR — must meet the following requirements in order to avoid undefined behavior:

bestie. UNDEFINED BEHAVIOR. we do not do undefined behavior in this house. read these three times. 😭
  • The host memory region given by (buffer_originregion) contains the latest bits when the enqueued write command begins execution.
  • The buffer object or memory objects created from this buffer object are not mapped.
  • The buffer object or memory objects created from this buffer object are not used by any command-queue until the write command has finished execution.

🔥 ERRORS 🔥

clEnqueueReadBufferRect returns CL_SUCCESS if the function is executed successfully. otherwise, she returns one of the following errors, and you're gonna wanna read them all:

  • CL_INVALID_COMMAND_QUEUE if command_queue is not a valid command-queue.
  • CL_INVALID_CONTEXT if the context associated with command_queue and buffer are not the same, or if the context associated with command_queue and events in event_wait_list are not the same.
  • CL_INVALID_MEM_OBJECT if buffer is not a valid buffer object.
  • CL_INVALID_VALUE if the region being read specified by (buffer_origin, region, buffer_row_pitch, buffer_slice_pitch) is out of bounds.
  • CL_INVALID_VALUE if ptr is a NULL value.
  • CL_INVALID_VALUE if any region array element is 0.
  • CL_INVALID_VALUE if buffer_row_pitch is not 0 and is less than region[0].
  • CL_INVALID_VALUE if host_row_pitch is not 0 and is less than region[0].
  • CL_INVALID_VALUE if buffer_slice_pitch is not 0 and is less than region[1] * buffer_row_pitch and not a multiple of buffer_row_pitch.
  • CL_INVALID_VALUE if host_slice_pitch is not 0 and is less than region[1] * host_row_pitch and not a multiple of host_row_pitch.
  • CL_INVALID_EVENT_WAIT_LIST if event_wait_list is NULL and num_events_in_wait_list > 0, or event_wait_list is not NULL and num_events_in_wait_list is 0, or if event objects in event_wait_list are not valid events.
  • CL_MISALIGNED_SUB_BUFFER_OFFSET if buffer is a sub-buffer object and the offset specified when the sub-buffer object is created is not aligned to CL_DEVICE_MEM_BASE_ADDR_ALIGN value for the device associated with queue.
  • CL_EXEC_STATUS_ERROR_FOR_EVENTS_IN_WAIT_LIST if the read and write operations are blocking and the execution status of any of the events in event_wait_list is a negative integer value.
  • CL_MEM_OBJECT_ALLOCATION_FAILURE if there is a failure to allocate memory for data store associated with buffer.
  • CL_INVALID_OPERATION if clEnqueueReadBufferRect is called on buffer which has been created with CL_MEM_HOST_WRITE_ONLY or CL_MEM_HOST_NO_ACCESS.
  • CL_OUT_OF_RESOURCES if there is a failure to allocate resources required by the OpenCL implementation on the device.
  • CL_OUT_OF_HOST_MEMORY if there is a failure to allocate resources required by the OpenCL implementation on the host.

📜 SPECIFICATION 📜

OpenCL Specification[1]

👀 SEE ALSO 👀

clEnqueueCopyBuffer(3clc), clEnqueueCopyBufferRect(3clc), clEnqueueWriteBuffer(3clc), clEnqueueWriteBufferRect(3clc), clEnqueueReadBuffer(3clc)

the whole rect-copy family, honestly. they all hang out together. 🫂

💜 AUTHORS 💜

The Khronos Group

Copyright © 2007-2011 The Khronos Group Inc.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and/or associated documentation files (the "Materials"), to deal in the Materials without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Materials, and to permit persons to whom the Materials are furnished to do so, subject to the condition that this copyright notice and permission notice shall be included in all copies or substantial portions of the Materials.

🌈 NOTES 🌈

  1. OpenCL Specification, page 74, section 5.2.2 - Reading, Writing and Copying Buffer Objects

aaaand that's the whole page bestie 💖 shoutout to The Khronos Group for the words, and the version stamp says 01/14/2021 — so this info is a lil vintage but she still slays. go read your rectangles, ily, drink water, and don't pass a NULL ptr. 🦋✨

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