[gnome-build-meta.wiki] Create Compiling C programs into GNOME OS

[Tanvir Roshid <tanvirroshid src gnome org>]

commit 4f636cbae4e7cbcea25f765d69a22c979bd470f1
Author: Tanvir Roshid <tanvirroshid786123 gmail com>
Date:   Fri Sep 16 12:06:22 2022 +0000

    Create Compiling C programs into GNOME OS

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+### Intuition
+
+We are going to be defining a manual element. Manual elements are described as:
+
+- The most basic build element does nothing but allows users to add custom build commands to the array 
understood by the BuildElement
+
+Additional reading link: https://docs.buildstream.build/1.6/elements/manual.html#module-elements.manual.
+
+In effect, all our element will be doing is:
+
+- Link to the source code, i.e., the raw C files
+- Link to a makefile that runs commands to compile the file. These commands contain variables, i.e., 
destinations and prefixes for the final executable
+- Link to a compiler that can run said makefile. In our case we will link to the GCC compiler available via 
Freedesktop SDK
+- Run the commands using the config key that we will define
+- Link everything we've done to a file relatively low in the dependency chain. This was explained in the 
general intuition section above.
+
+### Commands to run
+
+Run the following commands to create the necessary directory structure, assuming you are in the root 
directory of the GnomeOS project:
+
+```
+mkdir -p elements/custom/hello/src
+```
+
+Add these lines to define your `Makefile` in `hello/src`. As you can see when the compiler invokes the 
Makefile the DESTDIR and PREFIX variables must be passed in.
+
+```
+# Sample makefile for hello.c
+#
+.PHONY: all install
+
+all: hello
+
+install:
+       install -d ${DESTDIR}${PREFIX}/bin
+       install -m 755 hello ${DESTDIR}${PREFIX}/bin
+
+hello: hello.c
+       $(CC) -Wall -o $@ $<
+```
+
+Create a new C file, `hello.c` with these lines
+```
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+  printf("Hello World\n");
+  return 0;
+}
+```
+
+Create our manual element which defines the commands that Buildstream should run processing this .bst file 
by adding these lines to `hello/hello.bst`.
+
+```
+kind: manual
+description: |
+
+  Building manually
+
+# Depend on the base system
+depends:
+- freedesktop-sdk.bst:bootstrap-import.bst
+
+# Stage the files/src directory for building
+sources:
+  - kind: local
+    path: elements/custom/hello/src
+
+# Now configure the commands to run
+config:
+
+  build-commands:
+  - make PREFIX="%{prefix}"
+
+  install-commands:
+  - make -j1 PREFIX="%{prefix}" DESTDIR="%{install-root}" install
+```
+
+The final step is to link the hello.bst file we created into the dependency chain so that Buildstream is 
aware of our new file and can process the commands. As mentioned, we wish to define such a dependency very 
high in the chain so that prior cache can be used and that the majority of elements do not have to be rebuilt 
from cache. A recommended location is `deps.bst`.
+
+Dependencies can be added like this:
+```
+- custom/hello/hello.bst
+```
+
+### Building and running the image
+
+Build using the tutorial [here](Preliminary%20build%20instructions).
+
+Run using the tutorial [here](Emulating GNOME OS)
+
+You should be able to find the new executable inside the root directory of the OS inside the bin folder i.e. 
run the following inside GnomeOS.
+
+```
+cd /
+./bin/hello
+```
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