Add libproc2-test, and explain why we don't use libproc2

README.md

@@ -271,6 +271,13 @@ accept
 
 * Use case reports and feedback
 
+Implementation Notes
+--------------------
+
+* We don't use [procps/libproc2](https://man7.org/linux/man-pages/man3/procps_pids.3.html) because
+  procps_pids_select(), for some reason, always parses /proc/$pid/status.
+  This is relatively expensive, and we don't need it.
+
 Changelog
 ---------
 

contrib/libproc2-test/.gitignore

@@ -0,0 +1 @@
+/out

contrib/libproc2-test/Makefile

@@ -0,0 +1,5 @@
+out: Makefile *.c
+	gcc *.c -Wall -Wextra -g -o out -lproc2
+
+format:
+	clang-format --style=file -i *.c

contrib/libproc2-test/libproc2-test.c

@@ -0,0 +1,30 @@
+#include <libproc2/pids.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char* argv[])
+{
+    enum pids_item items[] = { PIDS_STATE };
+    struct pids_info* info;
+    struct pids_stack* stack;
+    struct pids_fetch* fetched;
+
+    int ret = procps_pids_new(&info, items, 1);
+    if (ret != 0) {
+        printf("new failured, ret=%d\n", ret);
+        exit(4);
+    }
+
+    unsigned pidlist[] = { 1 };
+    fetched = procps_pids_select(info, pidlist, 1, PIDS_SELECT_PID_THREADS);
+    if (!fetched) {
+        printf("select error\n");
+        exit(3);
+    }
+    if (fetched->counts->total != 1) {
+        exit(2);
+    }
+    stack = fetched->stacks[0];
+    char state = PIDS_VAL(0, s_ch, stack, info);
+    printf("%c\n", state);
+}