ltrace: example to trace dlopen-ed libraries

src/trace_profile/ltrace.md

@@ -7,6 +7,8 @@ ltrace [opts] [prg]
   -o <file> ... log output into <file>
   -l <filter> . show who calls into lib matched by <filter>
   -C .......... demangle
+  -x <filter> . which symbol table entry points to trace
+                (can be of form sym_pattern@lib_pattern)
 ```
 
 # Example
@@ -15,3 +17,17 @@ List which program/libs call into `libstdc++`:
 ```bash
 ltrace -l '*libstdc++*' -C -o ltrace.log ./main
 ```
+
+Trace symbols from `dlopen(3)`ed libraries.
+```bash
+# Assume libfoo.so would be dynamically loaded via dlopen.
+ltrace -x '@libfoo.so'
+
+# Trace all dlopened symbols.
+ltrace -x '*'
+# Trace all symbols from dlopened libraries which name match the
+# pattern "liby*".
+ltrace -x '@liby*'
+# Trace symbol "foo" from all dlopened libraries matching the pattern.
+ltrace -x 'foo@liby*'
+```