feat: cross-compilation guide

Author: HackerSmacker

README.md

@@ -304,6 +304,9 @@ This will install the headers and library files into $PREFIX. You can override t
 # PREFIX=/opt/concord make install
 ```
 
+### Cross-compiling Concord
+To cross-compile Concord, see the manual [here](docs/guides/cross_compiling.md).
+
 ### Included dependencies
 
 The following are `stable` and well documented dependencies that are packaged with Concord and can be included to your projects:

docs/guides/cross_compiling.md

@@ -0,0 +1,55 @@
+# Cross Compiling Concord
+
+Unlike most Autoconf-based projects, Concord uses normal Makefiles. As such,
+cross-compilation can be rather daunting at first. Fortunately, once you 
+understand it, you won't have any trouble (unless your compiler malfunctions
+or is misconfigured). 
+
+## Step 1: getting a cross compiler
+Debian-based Linux distros have cross compilers available in their package
+repositories. For example, to get an ARM cross compiler suitable for building
+for an old 32-bit Raspberry Pi, run `apt-get install gcc-arm-linux-gnueabihf`.
+Search the Debian package repositories for "gcc" to see other cross compilers
+available. 
+
+If you aren't on Debian, you can use cross-compilers from [here](https://mirrors.edge.kernel.org/pub/tools/crosstool/), although, do note that these compilers
+might take a fair bit of work to get working perfectly. If you do use one of
+these, ensure that `$PATH` gets updated to include the compiler's bin directory,
+or you won't be able to invoke the compiler.
+
+## Step 2: cross-compiling Concord
+For this example, we will assume that your host system is a standard x86_64
+PC, and uses the target triplet `x86_64-pc-linux-gnu`. We will be
+cross-compiling to a 64-bit Raspberry Pi 4, with the target triplet
+`aarch64-linux-gnu`. Now, when GCC cross-compilers are made, all of the built
+executables for the compiler are prefixed with the target tripet. This means
+that our actual C compiler command is `aarch64-linux-gnu-gcc`. This will be
+our `CC` value.
+
+The gencodecs system that Concord uses requires that the system C compiler is
+available. This means we have to set `HOSTCC` when we compile. Since our
+target triplet is `x86_64-pc-linux-gnu`, this means that our host C compiler
+is named `x86_64-pc-linux-gnu-gcc`. However, since this is the primary C
+compiler on the system, we can just use `gcc` for `CC`. 
+
+Finally, we will install Concord to the Raspberry Pi by directly accessing its
+root filesystem via NFS. The Pi's Linux image has been mounted read-write into
+`/mnt`. Since we want Concord to go into `/usr/local`, our target `PREFIX`
+path for `make install` is `/mnt/usr/local`. 
+
+Okay, we can now run the commands to actually compile.
+
+```console
+cd gencodecs
+make HOSTCC=gcc CC=aarch64-linux-gnu-gcc
+cd ..
+make CC=aarch64-linux-gnu-gcc
+make install PREFIX=/mnt/usr/local
+```
+
+Easy as that! Granted, you can perform a compilation with fewer typed commands
+if you `export` those three variables beforehand, but, this guide was written
+with maximum learning in mind (and demonstrating why gencodecs needs the
+split compilation procedure). 
+
+

gencodecs/Makefile

@@ -4,6 +4,7 @@
 # available here to be set.
 CPP = cc -E
 CC = cc
+HOSTCC = cc
 
 TOP = ..
 
@@ -63,7 +64,7 @@ DOXYGEN_DESC = "/**\n @file $@\n @author Cogmasters\n @brief Generated code\n*/"
 all: $(OUT_O)
 
 $(PP): $(PP).c
-	$(CC) $(CFLAGS) $< -o $@
+	$(HOSTCC) $(CFLAGS) $< -o $@
 
 $(OUT_O): $(OUT_C) $(OUT_H)
 	$(CC) -c $(CFLAGS) $< -o $@