'\" t
.\"     Title: clEnqueueReadBuffer
.\"    Author: The Khronos Group
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 01/14/2021
.\"    Manual: OpenCL Manual
.\"    Source: The Khronos Group
.\"  Language: English
.\"
.TH "CLENQUEUEREADBUFFER" "3clc" "01/14/2021" "The Khronos Group" "OpenCL Manual"
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.SH "NAME"
clEnqueueReadBuffer \- Enqueue commands to read from a buffer object to host memory\&.
.SH ""
.HP \w'cl_int\ clEnqueueReadBuffer('u
.BI "cl_int clEnqueueReadBuffer(cl_command_queue\ " "command_queue" ", cl_mem\ " "buffer" ", cl_bool\ " "blocking_read" ", size_t\ " "offset" ", size_t\ " "size" ", void\ *" "ptr" ", cl_uint\ " "num_events_in_wait_list" ", const\ cl_event\ *" "event_wait_list" ", cl_event\ *" "event" ");"
.SH "PARAMETERS"
.PP
\fIcommand_queue\fR
.RS 4
Refers to the command\-queue in which the read command will be queued\&.
\fIcommand_queue\fR
and
\fIbuffer\fR
must be created with the same OpenCL context\&.
.RE
.PP
\fIbuffer\fR
.RS 4
Refers to a valid buffer object\&.
.RE
.PP
\fIblocking_read\fR
.RS 4
Indicates if the read operations are
\fIblocking\fR
or non\-blocking\&.
.sp
If
\fIblocking_read\fR
is
\fBCL_TRUE\fR
i\&.e\&. the read command is blocking,
\fBclEnqueueReadBuffer\fR
does not return until the buffer data has been read and copied into memory pointed to by
\fIptr\fR\&.
.sp
If
\fIblocking_read\fR
is
\fBCL_FALSE\fR
i\&.e\&. the read command is
\fInon\-blocking\fR,
\fBclEnqueueReadBuffer\fR
queues a
\fInon\-blocking\fR
read command and returns\&. The contents of the buffer that
\fIptr\fR
points to cannot be used until the read command has completed\&. The
\fIevent\fR
argument returns an event object which can be used to query the execution status of the read command\&. When the read command has completed, the contents of the buffer that
\fIptr\fR
points to can be used by the application\&.
.RE
.PP
\fIoffset\fR
.RS 4
The offset in bytes in the buffer object to read from\&.
.RE
.PP
\fIsize\fR
.RS 4
The size in bytes of data being read\&.
.RE
.PP
\fIptr \fR
.RS 4
The pointer to buffer in host memory where data is to be read into\&.
.RE
.PP
\fI \fR\fI\fR\fI event_wait_list \fR, \fI \fR\fI\fR\fI num_events_in_wait_list \fR
.RS 4

\fIevent_wait_list\fR
and
\fInum_events_in_wait_list\fR
specify events that need to complete before this particular command can be executed\&. If
\fIevent_wait_list\fR
is NULL, then this particular command does not wait on any event to complete\&. If
\fIevent_wait_list\fR
is NULL,
\fInum_events_in_wait_list\fR
must be 0\&. If
\fIevent_wait_list\fR
is not NULL, the list of events pointed to by
\fIevent_wait_list\fR
must be valid and
\fInum_events_in_wait_list\fR
must be greater than 0\&. The events specified in
\fIevent_wait_list\fR
act as synchronization points\&. The context associated with events in
\fIevent_wait_list\fR
and
\fIcommand_queue\fR
must be the same\&. The memory associated with
\fIevent_wait_list\fR
can be reused or freed after the function returns\&.
.RE
.PP
\fI event \fR
.RS 4
Returns an event object that identifies this particular read command and can be used to query or queue a wait for this particular command to complete\&.
\fIevent\fR
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\&. If the
\fIevent_wait_list\fR
and the event arguments are not NULL, the
\fIevent\fR
argument should not refer to an element of the
\fIevent_wait_list\fR
array\&.
.RE
.SH "NOTES"
.PP
Calling
\fBclEnqueueReadBuffer\fR
to read a region of the buffer object with the
\fIptr\fR
argument value set to
\fIhost_ptr\fR
+
\fIoffset\fR, where
\fIhost_ptr\fR
is a pointer to the memory region specified when the buffer object being read is created with
\fBCL_MEM_USE_HOST_PTR\fR, must meet the following requirements in order to avoid undefined behavior:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}

All commands that use this buffer object or a memory object (buffer or image)
created from this buffer object have finished execution before the read command
begins execution\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}

The buffer object or memory objects created from this buffer object are not mapped\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}

The buffer object or memory objects created from this buffer object are not used
by any command\-queue until the read command has finished execution\&.
.RE
.SH "ERRORS"
.PP
\fBclEnqueueReadBuffer\fR
returns
CL_SUCCESS
if the function is executed successfully\&. Otherwise, it returns one of the following errors:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_INVALID_COMMAND_QUEUE if \fIcommand_queue\fR
is not a valid command\-queue\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_INVALID_CONTEXT if the context associated with
\fIcommand_queue\fR and \fIbuffer\fR are not the same
or if the context associated with \fIcommand_queue\fR and events in
\fIevent_wait_list\fR are not the same\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_INVALID_MEM_OBJECT if \fIbuffer\fR is not
a valid buffer object\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_INVALID_VALUE if the region being read specified by
(\fIoffset\fR, \fIsize\fR) is out of bounds or if
\fIptr\fR is a NULL value or if size is 0\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_INVALID_EVENT_WAIT_LIST
if \fIevent_wait_list\fR is NULL and
\fInum_events_in_wait_list\fR greater than
0, or \fIevent_wait_list\fR is not NULL and
\fInum_events_in_wait_list\fR is 0, or if event objects in
\fIevent_wait_list\fR are not valid events\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_MISALIGNED_SUB_BUFFER_OFFSET if
\fIbuffer\fR is a sub\-buffer object and \fIoffset\fR
specified when the sub\-buffer object is created is not aligned to
\fBCL_DEVICE_MEM_BASE_ADDR_ALIGN\fR value for device associated
with \fIqueue\fR\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
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 \fIevent_wait_list\fR is a negative integer value\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_MEM_OBJECT_ALLOCATION_FAILURE if there is a failure to
allocate memory for data store associated with \fIbuffer\fR\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_INVALID_OPERATION if
\fBclEnqueueReadBuffer\fR is called on \fIbuffer\fR
which has been created with \fBCL_MEM_HOST_WRITE_ONLY\fR or
\fBCL_MEM_HOST_NO_ACCESS\fR\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_OUT_OF_RESOURCES if there is a failure to allocate
resources required by the OpenCL implementation on the device\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CL_OUT_OF_HOST_MEMORY if there is a failure to allocate
resources required by the OpenCL implementation on the host\&.
.RE
.SH "SPECIFICATION"
.PP
\m[blue]\fBOpenCL Specification\fR\m[]\&\s-2\u[1]\d\s+2
.SH "SEE ALSO"
.PP
\fBclEnqueueCopyBuffer\fR(3clc),
\fBclEnqueueWriteBuffer\fR(3clc),
\fBclEnqueueWriteBufferRect\fR(3clc),
\fBclEnqueueReadBufferRect\fR(3clc)
.SH "AUTHORS"
.PP
\fBThe Khronos Group\fR
.SH "COPYRIGHT"
.br
Copyright \(co 2007-2010 The Khronos Group Inc.
.br
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.
.sp
.SH "NOTES"
.IP " 1." 4
OpenCL Specification
.RS 4
\%page 72, section 5.2.2 - Reading, Writing, Copying Buffer Objects
.RE