Innteresting. It seems that the `output` collection is much more

reliable as a source of truth about which monitors and screens are
actually in use.  Each one comes with its own collection of `crtc`,
but you can just skip the ones that return `null`, giving you a
reliable list of the "active and visible" screens that the user is
currently looking at.  Excellent!  This means that I'm one step closer
to having a viable solution!

I also discovered the [xcb_util](https://xcb.freedesktop.org/XcbUtil/)
library of helpful utilities has a function called
`xcb_aux_get_screen` for getting the root screen because everyone was
weirded out by that list traversal algorithm.

The source code to [xedgewarp](https://github.com/Airblader/xedgewarp)
was invaluable in revealing these secrets to me.

So far this little toy compiles down to only 35KB, and that includes
using `std::cout` and `std::vector`!  I wonder how big the Rust
version will be.  Yeah, yeah, I know, it cheats by having lots of
itself hidden in the kernel.

Next up: Actually knowing what the rotation status is.

.clang-format

@@ -0,0 +1,178 @@
+---
+Language:        Cpp
+# BasedOnStyle:  LLVM
+AccessModifierOffset: -2
+AlignAfterOpenBracket: Align
+AlignArrayOfStructures: None
+AlignConsecutiveMacros: None
+AlignConsecutiveAssignments: None
+AlignConsecutiveBitFields: None
+AlignConsecutiveDeclarations: None
+AlignEscapedNewlines: Right
+AlignOperands:   Align
+AlignTrailingComments: true
+AllowAllArgumentsOnNextLine: true
+AllowAllConstructorInitializersOnNextLine: true
+AllowAllParametersOfDeclarationOnNextLine: true
+AllowShortEnumsOnASingleLine: true
+AllowShortBlocksOnASingleLine: Never
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: All
+AllowShortLambdasOnASingleLine: All
+AllowShortIfStatementsOnASingleLine: Never
+AllowShortLoopsOnASingleLine: false
+AlwaysBreakAfterDefinitionReturnType: None
+AlwaysBreakAfterReturnType: None
+AlwaysBreakBeforeMultilineStrings: false
+AlwaysBreakTemplateDeclarations: MultiLine
+AttributeMacros:
+  - __capability
+BinPackArguments: true
+BinPackParameters: true
+BraceWrapping:
+  AfterCaseLabel:  false
+  AfterClass:      false
+  AfterControlStatement: Never
+  AfterEnum:       false
+  AfterFunction:   false
+  AfterNamespace:  false
+  AfterObjCDeclaration: false
+  AfterStruct:     false
+  AfterUnion:      false
+  AfterExternBlock: false
+  BeforeCatch:     false
+  BeforeElse:      false
+  BeforeLambdaBody: false
+  BeforeWhile:     false
+  IndentBraces:    false
+  SplitEmptyFunction: true
+  SplitEmptyRecord: true
+  SplitEmptyNamespace: true
+BreakBeforeBinaryOperators: None
+BreakBeforeConceptDeclarations: true
+BreakBeforeBraces: Attach
+BreakBeforeInheritanceComma: false
+BreakInheritanceList: BeforeColon
+BreakBeforeTernaryOperators: true
+BreakConstructorInitializersBeforeComma: false
+BreakConstructorInitializers: BeforeColon
+BreakAfterJavaFieldAnnotations: false
+BreakStringLiterals: true
+ColumnLimit:     100
+CommentPragmas:  '^ IWYU pragma:'
+CompactNamespaces: false
+ConstructorInitializerAllOnOneLineOrOnePerLine: false
+ConstructorInitializerIndentWidth: 4
+ContinuationIndentWidth: 4
+Cpp11BracedListStyle: true
+DeriveLineEnding: true
+DerivePointerAlignment: false
+DisableFormat:   false
+EmptyLineAfterAccessModifier: Never
+EmptyLineBeforeAccessModifier: LogicalBlock
+ExperimentalAutoDetectBinPacking: false
+FixNamespaceComments: true
+ForEachMacros:
+  - foreach
+  - Q_FOREACH
+  - BOOST_FOREACH
+IfMacros:
+  - KJ_IF_MAYBE
+IncludeBlocks:   Preserve
+IncludeCategories:
+  - Regex:           '^"(llvm|llvm-c|clang|clang-c)/'
+    Priority:        2
+    SortPriority:    0
+    CaseSensitive:   false
+  - Regex:           '^(<|"(gtest|gmock|isl|json)/)'
+    Priority:        3
+    SortPriority:    0
+    CaseSensitive:   false
+  - Regex:           '.*'
+    Priority:        1
+    SortPriority:    0
+    CaseSensitive:   false
+IncludeIsMainRegex: '(Test)?$'
+IncludeIsMainSourceRegex: ''
+IndentAccessModifiers: false
+IndentCaseLabels: false
+IndentCaseBlocks: false
+IndentGotoLabels: true
+IndentPPDirectives: None
+IndentExternBlock: AfterExternBlock
+IndentRequires:  false
+IndentWidth:     4
+IndentWrappedFunctionNames: false
+InsertTrailingCommas: None
+JavaScriptQuotes: Leave
+JavaScriptWrapImports: true
+KeepEmptyLinesAtTheStartOfBlocks: true
+LambdaBodyIndentation: Signature
+MacroBlockBegin: ''
+MacroBlockEnd:   ''
+MaxEmptyLinesToKeep: 1
+NamespaceIndentation: None
+ObjCBinPackProtocolList: Auto
+ObjCBlockIndentWidth: 2
+ObjCBreakBeforeNestedBlockParam: true
+ObjCSpaceAfterProperty: false
+ObjCSpaceBeforeProtocolList: true
+PenaltyBreakAssignment: 2
+PenaltyBreakBeforeFirstCallParameter: 19
+PenaltyBreakComment: 300
+PenaltyBreakFirstLessLess: 120
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+PenaltyIndentedWhitespace: 0
+PointerAlignment: Left
+PPIndentWidth:   -1
+ReferenceAlignment: Pointer
+ReflowComments:  true
+ShortNamespaceLines: 1
+SortIncludes:    CaseSensitive
+SortJavaStaticImport: Before
+SortUsingDeclarations: true
+SpaceAfterCStyleCast: false
+SpaceAfterLogicalNot: false
+SpaceAfterTemplateKeyword: true
+SpaceBeforeAssignmentOperators: true
+SpaceBeforeCaseColon: false
+SpaceBeforeCpp11BracedList: false
+SpaceBeforeCtorInitializerColon: true
+SpaceBeforeInheritanceColon: true
+SpaceBeforeParens: ControlStatements
+SpaceAroundPointerQualifiers: Default
+SpaceBeforeRangeBasedForLoopColon: true
+SpaceInEmptyBlock: false
+SpaceInEmptyParentheses: false
+SpacesBeforeTrailingComments: 1
+SpacesInAngles:  Never
+SpacesInConditionalStatement: false
+SpacesInContainerLiterals: true
+SpacesInCStyleCastParentheses: false
+SpacesInLineCommentPrefix:
+  Minimum:         1
+  Maximum:         -1
+SpacesInParentheses: false
+SpacesInSquareBrackets: false
+SpaceBeforeSquareBrackets: false
+BitFieldColonSpacing: Both
+Standard:        Latest
+StatementAttributeLikeMacros:
+  - Q_EMIT
+StatementMacros:
+  - Q_UNUSED
+  - QT_REQUIRE_VERSION
+TabWidth:        8
+UseCRLF:         false
+UseTab:          Never
+WhitespaceSensitiveMacros:
+  - STRINGIZE
+  - PP_STRINGIZE
+  - BOOST_PP_STRINGIZE
+  - NS_SWIFT_NAME
+  - CF_SWIFT_NAME
+...
+

Makefile

@@ -12,3 +12,6 @@ build/xrandr: build/Makefile src/xrandr.cpp CMakeLists.txt
 
 clean:
 	rm -fr build
+
+run: build/xrandr
+	./build/xrandr

README.md

@@ -19,7 +19,10 @@ obviously X Windows.
 
 In progress.
 
-**DETAILS:**
+DETAILS:
+
+**Notes:**
+
 
 **BUILDING:**
 

docs/Progress.md

@@ -0,0 +1,40 @@
+Resources used in the production of this program:
+
+- [The XCB Reference from FreeDesktop](https://xcb.freedesktop.org/manual/index.html)
+- [The Xorg Basic Programming with the XCB
+  Library](https://www.x.org/releases/X11R7.6/doc/libxcb/tutorial/index.html), 
+- [The KDE Source Code for
+  XCBWrapper](https://github.com/KDE/libkscreen/tree/master/backends),
+- [xedgewarp](https://github.com/Airblader/xedgewarp)
+  [Vlad Zahorodnii](https://github.com/zzag) et. al., ongoing.
+- [C++ in a
+  Nutshell](https://www.oreilly.com/library/view/c-in-a/059600298X/)
+  Ray Lischner, O'Reilly Publishing, 2003
+
+The sheer variety of sources I needed to cobble together an
+understanding how the XCB library works is an example of what teachers
+call [extrinsic
+load](https://betterhumans.pub/skyrocket-your-learning-top-3-studying-techniques-based-on-cognitive-load-theory-1641b5e56508),
+an unnecessary extra load on learning because the documentation is
+poorly organized and lacks instruction.  *Basic Programming with XCB*
+is a
+*[tutorial](https://www.writethedocs.org/videos/eu/2017/the-four-kinds-of-documentation-and-why-you-need-to-understand-what-they-are-daniele-procida/)*
+oriented toward commonplace tasks, but peters out before getting to an
+XCB extensions such as RandR.  The reference is a *reference*; it
+doesn't help you understand how to use the library.  `XCBWrapper` is
+an excellent example of how to use XCB, but it uses some fairly
+high-level C++ to accomplish all that it does, and untangling the
+relationship between the wrapper template, the macro that does wrapper
+declarations, and the XCB Reference; once you find the insight that
+XCB's declarations are all derived from a massive XML file, you can
+start to understand that XCBWrapper exploits the patterns produced by
+the derivative file, but it requires insight and effort that's
+unrelated to understanding XCB in the first place.
+
+This project does show the usual trajectory of one of my learning
+exercises, especially since I'm fond of delving in places where no man
+has documented before.  I've dumped a ton of stuff into my brain and
+now it's all starting to make sense.  I also note that I'm doing
+_better_ than a lot of the open-source examples, in that I'm batching
+many of my requests before processing them.  I'm not batching storing
+the replies yet, but I don't see why that couldn't happen.

notes.md

@@ -0,0 +1,8 @@
+- A Display has Screens
+- A Screen has Monitors & Windows
+- The root Screen is accessed by traversing to the head of the list of
+  screens using an interator.
+- The root screen's root window is used as the parameter for
+  'get_screen_resources'.
+- The screen_resources object is contains 'screen_resources_crtcs',
+  which you again get via an iterator. These are cookies.

src/xrandr.cpp

@@ -1,14 +1,154 @@
+#include "xcb/randr.h"
+#include "xcb/xcb.h"
+#include "xcb/xcb_aux.h"
 #include <iostream>
-#include <xcb/xcb.h>
+#include <memory>
+#include <vector>
 
-int main(int argc, const char *argv[]) {
-  xcb_connection_t *xConnection = xcb_connect(nullptr, nullptr);
-  for (auto iter = xcb_setup_roots_iterator(xcb_get_setup(xConnection));
-       iter.rem; xcb_screen_next(&iter)) {
-    xcb_screen_t *screen = iter.data;
-    std::cout << "Screen " << iter.index << " (" << screen->width_in_pixels
-              << ", " << screen->height_in_pixels << ")" << std::endl;
-  }
-  xcb_disconnect(xConnection);
-  return 0;
+xcb_randr_get_screen_resources_reply_t* get_screen_resources(xcb_connection_t* connection,
+                                                             xcb_window_t rootWindow) {
+    return xcb_randr_get_screen_resources_reply(
+        connection, xcb_randr_get_screen_resources(connection, rootWindow), nullptr);
+}
+
+std::vector<xcb_randr_crtc_t> get_crtc_cookies(xcb_randr_get_screen_resources_reply_t* resources) {
+    std::vector<xcb_randr_crtc_t> reply;
+    xcb_randr_crtc_t* crtcs = xcb_randr_get_screen_resources_crtcs(resources);
+    const int crtcsCount = xcb_randr_get_screen_resources_crtcs_length(resources);
+    for (int i = 0; i < crtcsCount; ++i) {
+        reply.push_back(crtcs[i]);
+    }
+    return reply;
+}
+
+void display_one_crtc(xcb_randr_get_crtc_info_reply_t* crtc) {
+    std::cout << "(x: " << crtc->x << ", y: " << crtc->y << ") (width: " << crtc->width
+              << ", height: " << crtc->height << ") status:" << unsigned(crtc->status) << std::endl;
+}
+
+void display_crtc_info(xcb_connection_t* connection, std::vector<xcb_randr_crtc_t>& crtc_cs) {
+    std::vector<xcb_randr_get_crtc_info_cookie_t> cookies;
+    xcb_generic_error_t* error = nullptr;
+    for (const auto& crtc : crtc_cs) {
+        cookies.push_back(xcb_randr_get_crtc_info(connection, crtc, XCB_CURRENT_TIME));
+    }
+    for (const auto& cookie : cookies) {
+        xcb_randr_get_crtc_info_reply_t* reply =
+            xcb_randr_get_crtc_info_reply(connection, cookie, &error);
+        if (error) {
+            free(error);
+        } else {
+            display_one_crtc(reply);
+            free(reply);
+        }
+    }
+}
+
+void display_one_output(xcb_connection_t* connection, xcb_randr_get_output_info_reply_t* output,
+                        xcb_timestamp_t timestamp) {
+    xcb_randr_get_crtc_info_reply_t* crtc = xcb_randr_get_crtc_info_reply(
+        connection, xcb_randr_get_crtc_info(connection, output->crtc, timestamp), NULL);
+    if (!crtc) {
+        return;
+    }
+    display_one_crtc(crtc);
+    free(crtc);
+}
+
+void display_outputs(xcb_connection_t* connection,
+                     xcb_randr_get_screen_resources_reply_t* resources, xcb_timestamp_t timestamp) {
+    std::vector<xcb_randr_get_output_info_cookie_t> cookies;
+    xcb_generic_error_t* error = nullptr;
+
+    int len = xcb_randr_get_screen_resources_outputs_length(resources);
+    xcb_randr_output_t* randr_outputs = xcb_randr_get_screen_resources_outputs(resources);
+
+    for (int i = 0; i < len; ++i) {
+        cookies.push_back(xcb_randr_get_output_info(connection, randr_outputs[i], timestamp));
+    }
+
+    for (const auto& cookie : cookies) {
+        xcb_randr_get_output_info_reply_t* reply =
+            xcb_randr_get_output_info_reply(connection, cookie, &error);
+        if (error) {
+            free(error);
+            continue;
+        } else {
+            display_one_output(connection, reply, timestamp);
+            free(reply);
+        }
+    }
+}
+
+// Lesson of the day: All _reply_t* objects must be freed.
+//
+//             xcb_randr_rotation_t)
+//             }
+//             get_rotation(xcb_connection_t* connection, xcb_randr_crtc_t crtc) {
+//                 xcb_generic_error_t* error = nullptr;
+//                 xcb_randr_get_crtc_info_cookie_t cookie =
+//                     xcb_randr_get_crtc_info(connection, crtc, XCB_CURRENT_TIME);
+//                 xcb_randr_get_crtc_info_reply_t* reply =
+//                     xcb_randr_get_crtc_info_reply(connection, cookie, &error);
+//                 xcb_randr_rotation_t rotation =
+//                 (xcb_randr_rotation_t)reply->rotation; free(reply); return rotation;
+//             }
+//
+
+xcb_randr_query_version_reply_t* getVersion(xcb_connection_t* connection) {
+    xcb_generic_error_t* error = nullptr;
+    xcb_randr_query_version_reply_t* version = xcb_randr_query_version_reply(
+        connection,
+        xcb_randr_query_version(connection, XCB_RANDR_MAJOR_VERSION, XCB_RANDR_MINOR_VERSION),
+        &error);
+    if (error) {
+        std::cout << "Error code:" << error->error_code << std::endl;
+        free(error);
+        return NULL;
+    }
+
+    if (!version) {
+        return NULL;
+    }
+
+    return version;
+}
+
+void display_screen_info(int screen_id, xcb_screen_t* screen) {
+    std::cout << "Screen " << screen_id << " (" << screen->width_in_pixels << ", "
+              << screen->height_in_pixels << ")" << std::endl;
+}
+
+void display_randr_version(xcb_randr_query_version_reply_t* version) {
+    if (!version) {
+        std::cout << "Something went wrong retrieving the version number" << std::endl;
+        exit(0);
+    }
+
+    std::cout << "Server reports RandR version (" << version->major_version << "."
+              << version->minor_version << ")" << std::endl;
+}
+
+// This gives us a connection and a screen object.
+
+int main(int argc, const char* argv[]) {
+    int default_screen_id;
+    xcb_connection_t* xConnection = xcb_connect(nullptr, &default_screen_id);
+
+    auto screen = xcb_aux_get_screen(xConnection, default_screen_id);
+    display_screen_info(default_screen_id, screen);
+
+    xcb_randr_query_version_reply_t* version = getVersion(xConnection);
+    display_randr_version(version);
+
+    xcb_randr_get_screen_resources_reply_t* screen_resources =
+        get_screen_resources(xConnection, screen->root);
+
+    xcb_timestamp_t timestamp = screen_resources->config_timestamp;
+
+    display_outputs(xConnection, screen_resources, timestamp);
+
+    free(screen_resources);
+    xcb_disconnect(xConnection);
+    return 0;
 }

studies/xcb_good_usage.cc

@@ -0,0 +1,54 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/time.h>
+#include <xcb/xcb.h>
+
+double get_time(void) {
+    struct timeval timev;
+    gettimeofday(&timev, NULL);
+    return (double)timev.tv_sec + (((double)timev.tv_usec) / 1000000);
+}
+
+int main() {
+    int count = 500;
+    xcb_atom_t* atoms = (xcb_atom_t*)malloc(count * sizeof(atoms));
+    xcb_intern_atom_cookie_t* cookies =
+        (xcb_intern_atom_cookie_t*)malloc(count * sizeof(xcb_intern_atom_cookie_t));
+    char** names;
+    double end, diff_x;
+
+    /* init names */
+    names = (char**)malloc(count * sizeof(char*));
+    for (int i = 0; i < count; ++i) {
+        char buf[100];
+        sprintf(buf, "NAME%d", i);
+        names[i] = strdup(buf);
+    }
+
+    auto start = get_time();
+    xcb_connection_t* connection = xcb_connect(NULL, NULL);
+    for (int i = 0; i < count; ++i) {
+        cookies[i] = xcb_intern_atom(connection, 0, strlen(names[i]), names[i]);
+    }
+
+    for (int i = 0; i < count; i++) {
+        xcb_intern_atom_reply_t* r = xcb_intern_atom_reply(connection, cookies[i], 0);
+        if (r) {
+            atoms[i] = r->atom;
+            free(r);
+        }
+    }
+    xcb_disconnect(connection);
+    end = get_time();
+
+    diff_x = end - start;
+    printf("XCB (good) use time : %f\n", diff_x);
+
+    for (int i = 0; i < count; ++i)
+        free(names[i]);
+    free(names);
+    free(atoms);
+    free(cookies);
+    return 0;
+}

studies/xcb_poor_usage.cc

@@ -0,0 +1,44 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/time.h>
+#include <xcb/xcb.h>
+
+double get_time(void) {
+    struct timeval timev;
+    gettimeofday(&timev, NULL);
+    return (double)timev.tv_sec + (((double)timev.tv_usec) / 1000000);
+}
+
+int main() {
+    int count = 500;
+    xcb_atom_t* atoms = (xcb_atom_t*)malloc(count * sizeof(atoms));
+    char** names;
+    double end, diff_x;
+
+    /* init names */
+    names = (char**)malloc(count * sizeof(char*));
+    for (int i = 0; i < count; ++i) {
+        char buf[100];
+        sprintf(buf, "NAME%d", i);
+        names[i] = strdup(buf);
+    }
+
+    auto start = get_time();
+    xcb_connection_t* connection = xcb_connect(NULL, NULL);
+    for (int i = 0; i < count; ++i) {
+        atoms[i] = xcb_intern_atom_reply(
+                       connection, xcb_intern_atom(connection, 0, strlen(names[i]), names[i]), NULL)
+                       ->atom;
+    }
+    xcb_disconnect(connection);
+    end = get_time();
+
+    diff_x = end - start;
+    printf("XCB (poor) use time : %f\n", diff_x);
+
+    for (int i = 0; i < count; ++i)
+        free(names[i]);
+    free(names);
+    return 0;
+}

studies/xlib_demo.cc

@@ -0,0 +1,43 @@
+#include <X11/Xlib.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/time.h>
+
+double get_time(void) {
+    struct timeval timev;
+    gettimeofday(&timev, NULL);
+    return (double)timev.tv_sec + (((double)timev.tv_usec) / 1000000);
+}
+
+int main() {
+    int count = 500;
+    Atom* atoms_x = (Atom*)malloc(count * sizeof(atoms_x));
+    char** names;
+    double end, diff_x;
+
+    /* init names */
+    names = (char**)malloc(count * sizeof(char*));
+    for (int i = 0; i < count; ++i) {
+        char buf[100];
+        sprintf(buf, "NAME%d", i);
+        names[i] = strdup(buf);
+    }
+
+    auto start = get_time();
+    auto disp = XOpenDisplay(getenv("DISPLAY"));
+    for (int i = 0; i < count; ++i)
+        atoms_x[i] = XInternAtom(disp, names[i], 0);
+    end = get_time();
+    XCloseDisplay(disp);
+
+    diff_x = end - start;
+    printf("Xlib use time : %f\n", diff_x);
+
+    free(atoms_x);
+
+    for (int i = 0; i < count; ++i)
+        free(names[i]);
+    free(names);
+    return 0;
+}