add use cases in README

README.md

@@ -1,4 +1,46 @@
 # ufuncgraph
-Funcgraph for userspace processes.
 
-Construction in progress...
+bpf(2)-based ftrace(1)-like function graph tracer for userspace processes.
+
+Limits:
+
+1. Only Works for Go programs for now;
+2. `.symtab` ELF section is required, and `.debug_info` section is preferable;
+3. The Go binary must be built using static linking;
+3. Running on x86-64 little-endian Linux only;
+4. Kernel version has to support bpf(2) and uprobe, and I developped it on 5.14.0;
+
+# Example
+
+Let's trace dockerd to see how it handles `docker stop`:
+
+Type the command:
+
+```bash
+sudo ufuncgraph -d 1 ./bundles/binary-daemon/dockerd '!runtime.*' '!context.*' '!*vendor*' 'github.com/docker/docker/daemon.(*Daemon).containerStop(id=+0(+64(%rdi)):c256, name=+0(+200(%rdi)):c256, name_len=+208(%rdi):s32)'
+```
+
+Explanations:
+
+1. `-d 1`: search the functions called by `github.com/docker/docker/daemon.(*Daemon).containerStop`, and only search for one layer;
+2. `!runtime.*`: ignore the functions matching the wildcard `runtime.*`;
+3. `!context.*`: ignore the functions matching the wildcard `context.*`;
+4. `!*vendor*`: ignore the functions matching the wildcard `*vendor*`;
+5. `github.com/docker/docker/daemon.(*Daemon).containerStop(id=+0(+64(%rdi)):c256, name=+0(+200(%rdi)):c256, name_len=+208(%rdi):s32)`: attach the function `github.com/docker/docker/daemon.(*Daemon).containerStop`, and fetch the arguments `id`, `name` and `name_len` using [Linux uprobe_tracer](https://docs.kernel.org/trace/uprobetracer.html)'s FETCHARGS syntax.
+
+And will get the results:
+
+![docker-stop-tracing](https://raw.githubusercontent.com/jschwinger233/ufuncgraph/master/assets/docker-stop-tracing.jpg)
+
+# Use cases
+
+1. Wall time profiling;
+2. Execution flow observing;
+
+## Wall time profiling case
+
+Chinese version: https://roamresearch.com/#/app/FEZ/page/C-xt1C2M1
+
+## Execution flow observing
+
+Chinese version: https://roamresearch.com/#/app/FEZ/page/ya-t0xN8m

internal/eventmanager/handler.go

@@ -21,7 +21,7 @@ func (m *EventManager) Handle(event bpf.UfuncgraphEvent) (err error) {
 		}
 		if userSpecified {
 			if err = m.PrintStack(event.StackId); err != nil {
-				return
+				return err
 			}
 		}
 		m.ClearStack(event)