trace-cmd: Handle zeroing of set_ftrace_pid on older kernels

Older kernels require a "-1" be passed into set_ftrace_pid to zero
it out. Newer kernels need just an empty string.

The sad part is that using the wrong one will cause the write to fail.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/trace-cmd.c b/trace-cmd.c
index 1365a73..be1241c 100644
--- a/trace-cmd.c
+++ b/trace-cmd.c
@@ -267,6 +267,7 @@ static void reset_max_latency(void)
 static void update_ftrace_pid(const char *pid)
 {
         char *path;
+        int ret;
         int fd;
 
         path = get_tracing_file("set_ftrace_pid");
@@ -277,7 +278,15 @@ static void update_ftrace_pid(const char *pid)
         if (fd < 0)
                 return;
 
-        if (write(fd, pid, strlen(pid)) < 0)
+        ret = write(fd, pid, strlen(pid));
+
+        /*
+         * Older kernels required "-1" to disable pid
+         */
+        if (ret < 0 && !strlen(pid))
+                ret = write(fd, "-1", 2);
+
+        if (ret < 0)
                 die("error writing to %s", path);
 
         close(fd);