README: started call function section.

README.md

@@ -1414,19 +1414,52 @@ Linux
 
 </details>
 
+<details>
+	<summary><h4>call</h4> - call your own defined functions</summary>
+
+```
+$(call variable,param,param,…)
+```
 
+- `variable` - the name of the function to call. 
+- `param` - strings that will serve as arguments of the function defined in `variable`.
 
+If you're constantly writing the same complex expressions, you can define a function of your own and assign it the expression. When the times to expand the function, each `param` is assigned to the temporary variables `$(1)`, `$(2)`, etc. As for `$(0)` it receives the variable in `variable`.
 
-<!-- 
-	Functions for generic purpose
-	
-	$(shell command)
-	$(call variable,param,param,…)
- -->
+Let's assume you need to compile a few sub-Makefiles and echo a message to alert it has been done. Something like this:
+
+```Makefile
+all:
+	echo "Compiling folder-1"
+	$(MAKE) -C folder-1
+
+	echo "Compiling folder-2"
+	$(MAKE) -C folder-2
+
+	echo "Compiling folder-3"
+	$(MAKE) -C folder-3
+```
+
+The example below, saved on [code/26-call-example](code/26-call-example) refactors the Makefile above by defining an expression that holds both the `echo` and the `make` commands:
+
+```Makefile
+SUBFOLDERS = folder-1 folder-2 folder-3
+
+define compile
+echo "Compiling $(1)"
+$(MAKE) -C $(1)
+endef
+
+all:
+	$(foreach f, $(SUBFOLDERS), $(call compile, $(f)))
+```
+
+</details>
 
 
 ------------------------------------------------------------------
 
+
 # <a name="useful-topics">Useful Topics</a>
 
 I don't think those topics fit either in the beginner's guide or in the advanced topics. However, I think they are useful to know and can be used to improve your Makefiles.

code/26-call-example/Makefile

@@ -0,0 +1,16 @@
+SUBFOLDERS = folder-1 folder-2 folder-3
+
+define compile
+	$(info Compiling $(1))
+	$(MAKE) -C $(1)
+endef
+
+all:
+	$(foreach folder, $(SUBFOLDERS), $(call compile,$(folder)))
+
+clean:
+	$(MAKE) clean -C folder-1
+	$(MAKE) clean -C folder-2
+	$(MAKE) clean -C folder-3
+
+.SILENT:

code/26-call-example/folder-1/Makefile

@@ -0,0 +1,9 @@
+CC = cc
+CFLAGS = -Wall -Werror -Wextra
+FILE = hello-1
+
+all:
+	$(CC) $(CFLAGS) $(FILE).c -o $(FILE).o
+
+clean:
+	$(RM) $(FILE).o

code/26-call-example/folder-1/hello-1.c

@@ -0,0 +1,19 @@
+/* ************************************************************************** */
+/*                                                                            */
+/*                                                        :::      ::::::::   */
+/*   hello-1.c                                          :+:      :+:    :+:   */
+/*                                                    +:+ +:+         +:+     */
+/*   By: ncarvalh <ncarvalh@student.42.fr>          +#+  +:+       +#+        */
+/*                                                +#+#+#+#+#+   +#+           */
+/*   Created: 2023/08/02 16:37:19 by ncarvalh          #+#    #+#             */
+/*   Updated: 2023/08/02 16:37:39 by ncarvalh         ###   ########.fr       */
+/*                                                                            */
+/* ************************************************************************** */
+
+#include <stdio.h>
+
+int main(void)
+{
+	printf("hello-1\n");
+	return (0);
+}

code/26-call-example/folder-2/Makefile

@@ -0,0 +1,9 @@
+CC = cc
+CFLAGS = -Wall -Werror -Wextra
+FILE = hello-2
+
+all:
+	$(CC) $(CFLAGS) $(FILE).c -o $(FILE).o
+
+clean:
+	$(RM) $(FILE).o

code/26-call-example/folder-2/hello-2.c

@@ -0,0 +1,19 @@
+/* ************************************************************************** */
+/*                                                                            */
+/*                                                        :::      ::::::::   */
+/*   hello-2.c                                          :+:      :+:    :+:   */
+/*                                                    +:+ +:+         +:+     */
+/*   By: ncarvalh <ncarvalh@student.42.fr>          +#+  +:+       +#+        */
+/*                                                +#+#+#+#+#+   +#+           */
+/*   Created: 2023/08/02 16:37:19 by ncarvalh          #+#    #+#             */
+/*   Updated: 2023/08/02 16:43:05 by ncarvalh         ###   ########.fr       */
+/*                                                                            */
+/* ************************************************************************** */
+
+#include <stdio.h>
+
+int main(void)
+{
+	printf("hello-2\n");
+	return (0);
+}

code/26-call-example/folder-3/Makefile

@@ -0,0 +1,9 @@
+CC = cc
+CFLAGS = -Wall -Werror -Wextra
+FILE = hello-3
+
+all:
+	$(CC) $(CFLAGS) $(FILE).c -o $(FILE).o
+
+clean:
+	$(RM) $(FILE).o

code/26-call-example/folder-3/hello-3.c

@@ -0,0 +1,19 @@
+/* ************************************************************************** */
+/*                                                                            */
+/*                                                        :::      ::::::::   */
+/*   hello-3.c                                          :+:      :+:    :+:   */
+/*                                                    +:+ +:+         +:+     */
+/*   By: ncarvalh <ncarvalh@student.42.fr>          +#+  +:+       +#+        */
+/*                                                +#+#+#+#+#+   +#+           */
+/*   Created: 2023/08/02 16:37:19 by ncarvalh          #+#    #+#             */
+/*   Updated: 2023/08/02 16:43:02 by ncarvalh         ###   ########.fr       */
+/*                                                                            */
+/* ************************************************************************** */
+
+#include <stdio.h>
+
+int main(void)
+{
+	printf("hello-3\n");
+	return (0);
+}