[sysprof] examples: add example app for marks

[Christian Hergert <chergert src gnome org>]

commit dbb6b9143f7faaf74ac6c3a25ff86da5d16f23bf
Author: Christian Hergert <chergert redhat com>
Date:   Mon Jun 3 11:39:56 2019 -0700

    examples: add example app for marks

 examples/app.c                                  | 79 +++++++++++++++++++++++++
 examples/meson.build                            |  3 +
 meson.build                                     |  1 +
 src/libsysprof-capture/sysprof-capture-writer.c |  7 ++-
 4 files changed, 88 insertions(+), 2 deletions(-)
---
diff --git a/examples/app.c b/examples/app.c
new file mode 100644
index 0000000..e3bfb1e
--- /dev/null
+++ b/examples/app.c
@@ -0,0 +1,79 @@
+/* This code is public domain */
+
+/* The following is an example of how to use the SYSPROF_TRACE_FD
+ * environment set by Sysprof to add marks or other information
+ * to the capture from your application.
+ */
+
+#define _GNU_SOURCE
+
+#include <sched.h>
+#include <signal.h>
+#include <sysprof-capture.h>
+#include <unistd.h>
+
+static void do_some_work (SysprofCaptureWriter *);
+
+int
+main (int   argc,
+      char *argv[])
+{
+  SysprofCaptureWriter *writer;
+
+  /* Ignore SIGPIPE because we might get a pipe from the environment and we
+   * don't want to trap if write() is used on it.
+   */
+  signal (SIGPIPE, SIG_IGN);
+
+  /* This will check for SYSPROF_TRACE_FD=N, parse the FD number, and use it as
+   * the backing file for the trace data. It may be a file, socket, pipe,
+   * memfd, etc. To reduce chances of the FD being used a second time, this
+   * function will also set the environment variable to "" by overwriting the
+   * first byte to \0.
+   */
+  writer = sysprof_capture_writer_new_from_env (0);
+
+  /* Nothing to do */
+  if (writer == NULL)
+    {
+      g_printerr ("SYSPROF_TRACE_FD not found, exiting.\n");
+      return 1;
+    }
+
+  do_some_work (writer);
+
+  sysprof_capture_writer_unref (writer);
+
+  return 0;
+}
+
+static void
+do_some_work (SysprofCaptureWriter *writer)
+{
+  const gint64 duration_usec = G_USEC_PER_SEC / 60L;
+
+  /* Wait a few seconds to get started */
+  g_usleep (G_USEC_PER_SEC * 3);
+
+  for (guint i = 0; i < 120; i++)
+    {
+      gint64 begin_time_nsec = SYSPROF_CAPTURE_CURRENT_TIME;
+      gint64 real_duration;
+      gint64 end_time_nsec;
+
+      g_usleep (duration_usec);
+
+      end_time_nsec = SYSPROF_CAPTURE_CURRENT_TIME;
+      real_duration = end_time_nsec - begin_time_nsec;
+
+      sysprof_capture_writer_add_mark (writer,
+                                       begin_time_nsec,            /* Begin time in nsec */
+                                       sched_getcpu (),            /* -1 to ignore */
+                                       getpid (),                  /* -1 to ignore */
+                                       real_duration,              /* duration in nsec */
+                                       "Example",                  /* Group name, 23 chars+\0 */
+                                       "Sleep",                    /* Name, 39 chars+\0 */
+                                       "Anscillary message data"); /* UTF-8 Message data, limited to
+                                                                      64kb-frame size. */
+    }
+}
diff --git a/examples/meson.build b/examples/meson.build
new file mode 100644
index 0000000..124ac24
--- /dev/null
+++ b/examples/meson.build
@@ -0,0 +1,3 @@
+app = executable('example-app', 'app.c',
+  dependencies: [libsysprof_capture_dep],
+)
diff --git a/meson.build b/meson.build
index 4c8b857..b61911c 100644
--- a/meson.build
+++ b/meson.build
@@ -174,6 +174,7 @@ configure_file(
 
 subdir('src')
 subdir('data')
+subdir('examples')
 subdir('help')
 subdir('po')
 
diff --git a/src/libsysprof-capture/sysprof-capture-writer.c b/src/libsysprof-capture/sysprof-capture-writer.c
index 0440d78..d7ef09d 100644
--- a/src/libsysprof-capture/sysprof-capture-writer.c
+++ b/src/libsysprof-capture/sysprof-capture-writer.c
@@ -426,8 +426,8 @@ sysprof_capture_writer_new_from_fd (int   fd,
   g_assert (fd != -1);
   g_assert (buffer_size % _sysprof_getpagesize() == 0);
 
-  if (ftruncate (fd, 0) != 0)
-    return NULL;
+  /* This is only useful on files, memfd, etc */
+  ftruncate (fd, 0);
 
   self = g_new0 (SysprofCaptureWriter, 1);
   self->ref_count = 1;
@@ -1287,6 +1287,9 @@ sysprof_capture_writer_new_from_env (gsize buffer_size)
   if (!(fdstr = g_getenv ("SYSPROF_TRACE_FD")))
     return NULL;
 
+  /* Make sure the clock is initialized */
+  sysprof_clock_init ();
+
   if (!(fd = atoi (fdstr)))
     return NULL;