setjmp.h: add some historical detail about the _JBLEN values.
Test: treehugger
Change-Id: Iadc16347be9bb186d9e7ada1e6f44bd2c36b04f2
diff --git a/libc/include/setjmp.h b/libc/include/setjmp.h
index 141e925..6d047ae 100644
--- a/libc/include/setjmp.h
+++ b/libc/include/setjmp.h
@@ -47,14 +47,33 @@
#include <sys/cdefs.h>
#if defined(__aarch64__)
+/**
+ * The size in words of an arm64 jmp_buf. Room for callee-saved registers,
+ * including floating point, stack pointer and program counter, various
+ * internal implementation details, and leaving some free space.
+ *
+ * Coincidentally matches OpenBSD, though they also save/restore the
+ * floating point status register too.
+ */
#define _JBLEN 32
#elif defined(__arm__)
+/** The size in words of an arm32 jmp_buf. Inherited from OpenBSD. */
#define _JBLEN 64
#elif defined(__i386__)
+/** The size in words of an x86 jmp_buf. Inherited from OpenBSD. */
#define _JBLEN 10
#elif defined(__riscv)
+/**
+ * The size in words of a riscv64 jmp_buf. Room for callee-saved registers,
+ * including floating point, stack pointer and program counter, various
+ * internal implementation details, and leaving some free space.
+ *
+ * Coincidentally matches OpenBSD, though they also save/restore the
+ * floating point status register too.
+ */
#define _JBLEN 32
#elif defined(__x86_64__)
+/** The size in words of an x86-64 jmp_buf. Inherited from OpenBSD. */
#define _JBLEN 11
#endif