gitlab-ci: fix testing whether a variable with a given name is set or not
[mesa.git] / docs / libGL.txt
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2
3
4 Introduction
5 ------------
6
7 This document describes the implementation of the XFree86 4.0 libGL.so
8 library defined by the Linux/OpenGL Base specification found at
9 http://reality.sgi.com/opengl/linux/linuxbase.html.
10
11 The documentation is divided into two sections:
12 User's Guide
13 Driver Developer's Guide
14
15 Author: Brian Paul (brian@precisioninsight.com)
16 Date: February 2000
17
18
19
20 User's Guide
21 ------------
22
23 Using libGL.so
24
25 The libGL.so library defines the gl- and glX-prefixed functions needed to
26 run OpenGL programs. OpenGL client applications should link with the
27 -lGL option to use it.
28
29 libGL.so serves two primary functions: GLX protocol generation for indirect
30 rendering and loading/management of hardware drivers for direct rendering.
31
32 When libGL.so initializes itself it uses the DRI to determine the
33 appropriate hardware driver for each screen on the local X display.
34 The hardware drivers are expected to be in the /usr/X11R6/lib/modules/dri/
35 directory. Drivers are named with the convention <name>_dri.so where
36 <name> is a driver such as "radeon", "i965", "nouveau", etc.
37
38 The LIBGL_DRIVERS_DIR environment variable may be used to specify a
39 different DRI modules directory, overriding /usr/X11R6/lib/modules/dri/.
40 This environment variable is ignored in setuid programs for security
41 reasons.
42
43 When libGL.so is unable to locate appropriate hardware drivers it will
44 fall back to using indirect GLX rendering.
45
46 To aid in solving problems, libGL.so will print diagnostic messages to
47 stderr if the LIBGL_DEBUG environment variable is defined.
48
49 libGL.so is thread safe. The overhead of thread safety for common,
50 single-thread clients is negligible. However, the overhead of thread
51 safety for multi-threaded clients is significant. Each GL API call
52 requires two calls to pthread_get_specific() which can noticeably
53 impact performance. Warning: libGL.so is thread safe but individual
54 DRI drivers may not be. Please consult the documentation for a driver
55 to learn if it is thread safe.
56
57
58
59 Indirect Rendering
60
61 You can force indirect rendering mode by setting the LIBGL_ALWAYS_INDIRECT
62 environment variable to `true`. Hardware acceleration will not be used.
63
64
65
66 libGL.so Extensibility
67
68 libGL.so is designed to be extended without upgrading. That is,
69 drivers may install new OpenGL extension functions into libGL.so
70 without requiring libGL.so to be replaced. Clients of libGL.so should
71 use the glXGetProcAddressEXT() function to obtain the address of
72 functions by name. For more details of GLX_ARB_get_proc_address see
73 http://oss.sgi.com/projects/ogl-sample/registry/ARB/get_proc_address.spec
74
75 libGL.so is also designed with flexibility such that it may be used
76 with many generations of hardware drivers to come.
77
78
79
80
81 Driver Developer's Guide
82 ------------------------
83
84 This section describes the requirements to make an XFree86 4.0
85 libGL.so-compatible hardware driver. It is not intended for end
86 users of libGL.so.
87
88
89 XFree86 source files
90
91 libGL.so is built inside XFree86 with sources found in xc/lib/GL/.
92 Specifically, libGL.so is built from:
93
94 xc/lib/GL/glx/*.c
95 xc/lib/dri/XF86dri.c
96 xc/lib/dri/dri_glx.c
97 xc/lib/GL/mesa/src/glapi.c
98 xc/lib/GL/mesa/src/glapitemp.h
99 xc/lib/GL/mesa/src/glapitable.h
100 xc/lib/GL/mesa/src/glapioffsets.h
101 xc/lib/GL/mesa/src/glapinoop.c
102 xc/lib/GL/mesa/src/glheader.h
103 xc/lib/GL/mesa/src/glthread.c
104 xc/lib/GL/mesa/src/glthread.h
105 xc/lib/GL/mesa/src/X86/glapi_x86.S
106 xc/lib/GL/mesa/src/X86/assyntax.h
107
108 Understand that the mesa/src/gl*.[ch] files are not tied to Mesa. They
109 have no dependencies on the rest of Mesa and are designed to be reusable
110 in a number of projects.
111
112 The glapi_x86.X and assyntax.h files implement x86-optimized dispatch
113 of GL functions. They are not required; C-based dispatch can be used
114 instead, with a slight performance penalty.
115
116
117
118 Driver loading and binding
119
120 When libGL.so initializes itself (via the __glXInitialize function) a
121 call is made to driCreateDisplay(). This function uses DRI facilities
122 to determine the driver file appropriate for each screen on the local
123 display. Each screen's driver is then opened with dlopen() and asked
124 for its __driCreateScreen() function. The pointers to the __driCreateScreen()
125 functions are kept in an array, indexed by screen number, in the
126 __DRIdisplayRec struct.
127
128 When a driver's __driCreateScreen() function is called, it must initialize
129 a __DRIscreenRec struct. This struct acts as the root of a tree of
130 function pointers which are called to create and destroy contexts and
131 drawables and perform all the operations needed by the GLX interface.
132 See the xc/lib/GL/glx/glxclient.h file for details.
133
134
135
136 Dynamic Extension Function Registration
137
138 In order to provide forward compatibility with future drivers, libGL.so
139 allows drivers to register new OpenGL extension functions which weren't
140 known when libGL.so was built.
141
142 The register_extensions() function in xc/lib/GL/dri/dri_glx.c is called
143 as soon as libGL.so is loaded. This is done with gcc's constructor
144 attribute. This mechanism will likely have to be changed for other compilers.
145
146 register_extensions() loops over all local displays and screens, determines
147 the DRI driver for each, and calls the driver's __driRegisterExtensions()
148 function, if present.
149
150 The __driRegisterExtensions() function can add new entrypoints to libGL
151 by calling:
152
153 GLboolean _glapi_add_entrypoint(const char *funcName, GLuint offset)
154
155 The parameters are the name of the function (such as "glFoobarEXT") and the
156 offset of the dispatch slot in the API dispatch table. The return value
157 indicates success (GL_TRUE) or failure (GL_FALSE).
158
159 _glapi_add_entrypoint() will synthesize entrypoint code in assembly
160 language. Assembly languages is required since parameter passing
161 can't be handled correctly using a C-based solution.
162
163 The address of the new entrypoint is obtained by calling the
164 glXGetProcAddressARB() function.
165
166 The dispatch offset number MUST be a number allocated by SGI in the same
167 manner in which new GL_* constants are allocated. Using an arbitrary
168 offset number will result in many problems.
169
170
171
172 Dispatch Management
173
174 When a GL context is made current, the driver must install its dispatch
175 table as the current dispatch table. This is done by calling
176
177 void _glapi_set_dispatch(struct _glapi_table *dispatch);
178
179 This will install the named dispatch table for the calling thread.
180 The current dispatch table for a thread can be obtained by calling
181
182 struct _glapi_table *_glapi_get_dispatch(void);
183
184 For higher performance in the common single-thread case, the global
185 variable _glapi_Dispatch will point to the current dispatch table.
186 This variable will be NULL when in multi-thread mode.
187
188
189
190 Context Management
191
192 libGL.so uses the XFree86 xthreads package to manage a thread-specific
193 current context pointer. See __glXGet/SetCurrentContext() in glext.c
194
195 Drivers may use the _glapi_set/get_context() functions to maintain
196 a private thread-specific context pointer.
197