doc: add match function call in C

website/catalog/c/index.md

@@ -2,5 +2,6 @@
 
 This page curates a list of example ast-grep rules to check and to rewrite C code.
 
+<!--@include: ./match-function-call.md-->
 <!--@include: ./rewrite-method-to-function-call.md-->
 <!--@include: ./yoda-condition.md-->

website/catalog/c/match-function-call.md

@@ -0,0 +1,47 @@
+## Match Function Call in C
+
+* [Playground Link](/playground.html#eyJtb2RlIjoiQ29uZmlnIiwibGFuZyI6ImMiLCJxdWVyeSI6InRlc3QoJCQkKSIsInJld3JpdGUiOiIiLCJjb25maWciOiJydWxlOlxuICBwYXR0ZXJuOiBcbiAgICBjb250ZXh0OiAkTSgkJCQpO1xuICAgIHNlbGVjdG9yOiBjYWxsX2V4cHJlc3Npb24iLCJzb3VyY2UiOiIjZGVmaW5lIHRlc3QoeCkgKDIqeClcbmludCBhID0gdGVzdCgyKTtcbmludCBtYWluKCl7XG4gICAgaW50IGIgPSB0ZXN0KDIpO1xufSJ9)
+
+### Description
+
+One of the common questions of ast-grep is to match function calls in C.
+
+A plain pattern like `test($A)` will not work. This is because [tree-sitter-c](https://github.com/tree-sitter/tree-sitter-c)
+parse the code snippet into `macro_type_specifier`, see the [pattern output](https://ast-grep.github.io/playground.html#eyJtb2RlIjoiUGF0Y2giLCJsYW5nIjoiYyIsInF1ZXJ5IjoidGVzdCgkJCQpIiwicmV3cml0ZSI6IiIsImNvbmZpZyI6InJ1bGU6XG4gIHBhdHRlcm46IFxuICAgIGNvbnRleHQ6ICRNKCQkJCk7XG4gICAgc2VsZWN0b3I6IGNhbGxfZXhwcmVzc2lvbiIsInNvdXJjZSI6IiNkZWZpbmUgdGVzdCh4KSAoMip4KVxuaW50IGEgPSB0ZXN0KDIpO1xuaW50IG1haW4oKXtcbiAgICBpbnQgYiA9IHRlc3QoMik7XG59In0=).
+
+To avoid this ambiguity, ast-grep lets us write a [contextual pattern](/guide/rule-config/atomic-rule.html#pattern), which is a pattern inside a larger code snippet.
+We can use `context` to write a pattern like this: `test($A);`. Then, we can use the selector `call_expression` to match only function calls.
+
+### YAML
+
+```yaml
+id: match-function-call
+language: c
+rule:
+  pattern:
+    context: $M($$$);
+    selector: call_expression
+```
+
+### Example
+
+<!-- highlight matched code in curly-brace {lineNum} -->
+```c{2,4}
+#define test(x) (2*x)
+int a = test(2);
+int main(){
+    int b = test(2);
+}
+```
+
+### Caveat
+
+Note, tree-sitter-c parses code differently when it receives code fragment. For example,
+
+* `test(a)` is parsed as `macro_type_specifier`
+* `test(a);` is parsed as `expression_statement -> call_expression`
+* `int b = test(a)` is parsed as `declaration -> init_declarator -> call_expression`
+
+The behavior is controlled by how the tree-sitter parser is written. And tree-sitter-c behaves differently from [tree-sitter-cpp](https://github.com/tree-sitter/tree-sitter-cpp).
+
+Please file issues on tree-sitter-c repo if you want to change the behavior. ast-grep will respect changes and decision from upstream authors.

website/catalog/index.md

@@ -7,6 +7,7 @@ Explore the power of ast-grep with these rewriting rules that can transform your
 Feel free to join our [Discord](https://discord.gg/4YZjf6htSQ) channel and ask @ast-grep-bot to explain the rules for you line by line!
 
 * [C](/catalog/c/)
+  * [Match Function Call](/catalog/c/#match-function-call)
   * [Rewrite Method to Function Call](/catalog/c/#rewrite-method-to-function-call)
   * [Rewrite Check to Yoda Condition](/catalog/c/#rewrite-check-to-yoda-condition)
 * [Go](/catalog/go/)