Make the three C lessons compile on Linux, and mask every machine measurement

The first Linux CI run failed three compiled lessons that pass on macOS: F15-L06, P02-L10 and
P04-L06 call clock_gettime(CLOCK_MONOTONIC) under -std=c11, and glibc hides POSIX from a
strict ISO C build unless it is asked for before the first header. macOS shows it regardless.
Each now defines _DEFAULT_SOURCE, as P04-L09 already did; stub and solution scores unchanged.

The reproducibility check also relied on luck for 13 lines: timings it only masked when two
runs happened to differ. F15-L06 prints "us/tick" and F15-L07 "rollouts/s", now recognised as
measurements; T03-L02 printed seconds and nanoseconds as bare numbers in table cells, which
gate 13 forbids, and now prints them with their units.

brand/social-preview.png: the 1280 x 640 card for sharing the repository, drawn by
tools/brand_lockups.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: TarrySingh

brand/social-preview.png

@@ -8,6 +8,9 @@
 // watch the TODOs turn into PASSes. Everything below the three exercises is the harness; it
 // is given, and it is worth reading, because it is the shape every control loop has.
 
+/* clock_gettime and CLOCK_MONOTONIC are POSIX, not ISO C. Under -std=c11, glibc (Linux)
+   hides them unless asked before the first header; macOS shows them regardless. */
+#define _DEFAULT_SOURCE
 #include <mujoco/mujoco.h>
 
 #include <math.h>

flagships/humanoid-lab/lessons/F15-L06-control-loop-in-c/lesson.c

@@ -5,6 +5,9 @@
 //
 //     make SRC=solutions/lesson.c BIN=solutions/lesson_bin test
 
+/* clock_gettime and CLOCK_MONOTONIC are POSIX, not ISO C. Under -std=c11, glibc (Linux)
+   hides them unless asked before the first header; macOS shows them regardless. */
+#define _DEFAULT_SOURCE
 #include <mujoco/mujoco.h>
 
 #include <math.h>

flagships/humanoid-lab/lessons/F15-L06-control-loop-in-c/solutions/lesson.c

@@ -375,7 +375,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "dab1775e0dda",
+   "id": "293502b9957b",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -396,10 +396,10 @@
     "\n",
     "\n",
     "if __name__ == \"__main__\":\n",
-    "    print(f\"{'bytes':>8} {'symbols':>9} {'seconds':>8} {'ns/symbol/merge':>16}\")\n",
+    "    print(f\"{'bytes':>8} {'symbols':>9} {'time':>9} {'per symbol, per merge':>22}\")\n",
     "    for _row in python_scaling():\n",
-    "        print(f\"{_row['bytes']:8d} {_row['symbols']:9d} {_row['seconds']:8.3f}\"\n",
-    "              f\" {_row['ns_per_symbol_per_merge']:16.1f}\")\n",
+    "        print(f\"{_row['bytes']:8d} {_row['symbols']:9d} {_row['seconds']:7.3f} s\"\n",
+    "              f\" {_row['ns_per_symbol_per_merge']:19.1f} ns\")\n",
     "    print(\"\\nSeconds roughly double when the corpus doubles. The last column is flat, and it\")\n",
     "    print(\"is the constant you are about to attack: the cost of one symbol, one time, here.\")"
    ]
@@ -921,7 +921,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "f978bb688e2c",
+   "id": "d058e7643bb7",
    "metadata": {
     "lines_to_next_cell": 1
    },
@@ -945,10 +945,10 @@
     "    cpp_rows = cpp_scaling()\n",
     "    py_rows = python_scaling()\n",
     "    print(\"per symbol, per merge — the only column that can tell a constant from a curve\")\n",
-    "    print(f\"{'symbols':>9} {'python ns':>11} {'c++ ns':>9} {'ratio':>9}\")\n",
+    "    print(f\"{'symbols':>9} {'python':>11} {'c++':>11} {'ratio':>9}\")\n",
     "    for p, c in zip(py_rows, cpp_rows):\n",
-    "        print(f\"{c['symbols']:9d} {p['ns_per_symbol_per_merge']:11.1f}\"\n",
-    "              f\" {c['ns_per_symbol_per_merge']:9.1f}\"\n",
+    "        print(f\"{c['symbols']:9d} {p['ns_per_symbol_per_merge']:8.1f} ns\"\n",
+    "              f\" {c['ns_per_symbol_per_merge']:8.1f} ns\"\n",
     "              f\" {p['ns_per_symbol_per_merge'] / c['ns_per_symbol_per_merge']:8.1f}x\")\n",
     "    print(\"\\nRead the two middle columns DOWN, not across. Each is roughly flat as the corpus\")\n",
     "    print(\"grows, and that flatness is the straight line — the same line in both languages.\")\n",

lessons/T03-L02-bpe-merge-loop-in-cpp/lesson.ipynb

@@ -337,10 +337,10 @@ def python_scaling(fractions=(0.25, 0.5, 1.0), merges: int = 30) -> list:
 
 
 if __name__ == "__main__":
-    print(f"{'bytes':>8} {'symbols':>9} {'seconds':>8} {'ns/symbol/merge':>16}")
+    print(f"{'bytes':>8} {'symbols':>9} {'time':>9} {'per symbol, per merge':>22}")
     for _row in python_scaling():
-        print(f"{_row['bytes']:8d} {_row['symbols']:9d} {_row['seconds']:8.3f}"
-              f" {_row['ns_per_symbol_per_merge']:16.1f}")
+        print(f"{_row['bytes']:8d} {_row['symbols']:9d} {_row['seconds']:7.3f} s"
+              f" {_row['ns_per_symbol_per_merge']:19.1f} ns")
     print("\nSeconds roughly double when the corpus doubles. The last column is flat, and it")
     print("is the constant you are about to attack: the cost of one symbol, one time, here.")
 
@@ -788,10 +788,10 @@ def _show_scaling() -> None:
     cpp_rows = cpp_scaling()
     py_rows = python_scaling()
     print("per symbol, per merge — the only column that can tell a constant from a curve")
-    print(f"{'symbols':>9} {'python ns':>11} {'c++ ns':>9} {'ratio':>9}")
+    print(f"{'symbols':>9} {'python':>11} {'c++':>11} {'ratio':>9}")
     for p, c in zip(py_rows, cpp_rows):
-        print(f"{c['symbols']:9d} {p['ns_per_symbol_per_merge']:11.1f}"
-              f" {c['ns_per_symbol_per_merge']:9.1f}"
+        print(f"{c['symbols']:9d} {p['ns_per_symbol_per_merge']:8.1f} ns"
+              f" {c['ns_per_symbol_per_merge']:8.1f} ns"
               f" {p['ns_per_symbol_per_merge'] / c['ns_per_symbol_per_merge']:8.1f}x")
     print("\nRead the two middle columns DOWN, not across. Each is roughly flat as the corpus")
     print("grows, and that flatness is the straight line — the same line in both languages.")

lessons/T03-L02-bpe-merge-loop-in-cpp/lesson.py

@@ -252,10 +252,10 @@ def python_scaling(fractions=(0.25, 0.5, 1.0), merges: int = 30) -> list:
 
 
 if __name__ == "__main__":
-    print(f"{'bytes':>8} {'symbols':>9} {'seconds':>8} {'ns/symbol/merge':>16}")
+    print(f"{'bytes':>8} {'symbols':>9} {'time':>9} {'per symbol, per merge':>22}")
     for _row in python_scaling():
-        print(f"{_row['bytes']:8d} {_row['symbols']:9d} {_row['seconds']:8.3f}"
-              f" {_row['ns_per_symbol_per_merge']:16.1f}")
+        print(f"{_row['bytes']:8d} {_row['symbols']:9d} {_row['seconds']:7.3f} s"
+              f" {_row['ns_per_symbol_per_merge']:19.1f} ns")
     print("\nSeconds roughly double when the corpus doubles. The last column is flat, and it")
     print("is the constant you are about to attack: the cost of one symbol, one time, here.")
 
@@ -631,10 +631,10 @@ def _show_scaling() -> None:
     cpp_rows = cpp_scaling()
     py_rows = python_scaling()
     print("per symbol, per merge — the only column that can tell a constant from a curve")
-    print(f"{'symbols':>9} {'python ns':>11} {'c++ ns':>9} {'ratio':>9}")
+    print(f"{'symbols':>9} {'python':>11} {'c++':>11} {'ratio':>9}")
     for p, c in zip(py_rows, cpp_rows):
-        print(f"{c['symbols']:9d} {p['ns_per_symbol_per_merge']:11.1f}"
-              f" {c['ns_per_symbol_per_merge']:9.1f}"
+        print(f"{c['symbols']:9d} {p['ns_per_symbol_per_merge']:8.1f} ns"
+              f" {c['ns_per_symbol_per_merge']:8.1f} ns"
               f" {p['ns_per_symbol_per_merge'] / c['ns_per_symbol_per_merge']:8.1f}x")
     print("\nRead the two middle columns DOWN, not across. Each is roughly flat as the corpus")
     print("grows, and that flatness is the straight line — the same line in both languages.")

lessons/T03-L02-bpe-merge-loop-in-cpp/solutions/lesson_solution.py

@@ -16,6 +16,9 @@
  * of the notebook measure both on your machine, and those measured numbers are the only ones
  * you should ever quote.
  */
+/* clock_gettime and CLOCK_MONOTONIC are POSIX, not ISO C. Under -std=c11, glibc (Linux)
+   hides them unless asked before the first header; macOS shows them regardless. */
+#define _DEFAULT_SOURCE
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>

programmes/document-intelligence/lessons/P02-L10-streaming-scanner-in-c/lesson.c

@@ -16,6 +16,9 @@
  * of the notebook measure both on your machine, and those measured numbers are the only ones
  * you should ever quote.
  */
+/* clock_gettime and CLOCK_MONOTONIC are POSIX, not ISO C. Under -std=c11, glibc (Linux)
+   hides them unless asked before the first header; macOS shows them regardless. */
+#define _DEFAULT_SOURCE
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>

programmes/document-intelligence/lessons/P02-L10-streaming-scanner-in-c/solutions/lesson.c

@@ -12,6 +12,9 @@
 // Moved or copied this checkout? Run `make clean` first. See the common-mistakes section of
 // the notebook for why.
 
+/* clock_gettime and CLOCK_MONOTONIC are POSIX, not ISO C. Under -std=c11, glibc (Linux)
+   hides them unless asked before the first header; macOS shows them regardless. */
+#define _DEFAULT_SOURCE
 #include <float.h>
 #include <math.h>
 #include <setjmp.h>

programmes/model-risk/lessons/P04-L06-reproduce-the-number-in-c/lesson.c

@@ -5,6 +5,9 @@
 //
 //     make SRC=solutions/lesson.c BIN=solutions/lesson_bin test
 
+/* clock_gettime and CLOCK_MONOTONIC are POSIX, not ISO C. Under -std=c11, glibc (Linux)
+   hides them unless asked before the first header; macOS shows them regardless. */
+#define _DEFAULT_SOURCE
 #include <float.h>
 #include <math.h>
 #include <setjmp.h>