# Setting up a Linux Toolchain

## Choosing the C-Library

The C-Library is an interface layer between applications and the system kernel implemented in the C-Language,
which conforms to the POSIX standards.
It can be looked at as an abstraction layer for programms running on a system, even if the programm itself
is not implemented in C.
Python, Java and many more languages eventually call functions from the C-Library,
which makes it an essential part of the toolchain.

There are multible C-Libraries to choose from:

* glibc
* musl libc
* uClibc-ng
* eglibc

The GNU standard C-Library is **glibc**.
Depending on your system specs, you might choose a different C-Library.
If you are very limited on RAM and Disk space, **musl libc** may be your go-to, since it uses very little resources.
**uClibc-ng** only if you are using uClinux and otherwise glibc.

![clib image](../img/whichclibtochoose.jpg)

## Installation

### Requirements

For the installation of the toolchain you first need some tools:

```bash
sudo apt-get install autoconf automake bison bzip2 cmake flex g++ gawk gcc \
gettext git gperf help2man libncurses5-dev libstdc++6 libtool \
libtool-bin make patch python3-dev rsync texinfo unzip wget xz-utils
```

### Building a toolchain

In this example of a toolchain build process, we are using crosstool-NG. Start with installing the dependencies mentioned above.

Starting off, you want to clone the crosstool-ng git repository:
```bash
git clone https://github.com/crosstool-ng/crosstool-ng.git
```

After the download has finished, install the package from source. In this example, we are using version 1.24.0:
```bash
cd crosstool-ng
git checkout crosstool-ng-1.24.0
./bootstrap
./configure --prefix=${PWD}
make
make install
```

The configure script with the "--prefix" option defines the build path for the installation. In this case, we install it into the current working directory, which is the git repository we just cloned.

Under "bin/", there are the just built binaries. To start the crosstool, run
```bash
bin/t-ng
```

The following section, describes the toolchain build for the beaglebone black board:

At first, we select a a target configuration:
```bash
bin/ct-ng arm-cortex_a8-linux-gnueabi
```

Other samples can be listed with
```bash
bin/ct-ng list-samples
```

After all this, every config specific detail can be customized by using the menuconfig:
```bash
bin/ct-ng menuconfig
```

For the Beaglebone Black we change the following settings:
- In Paths and misc options, disable Render the toolchain read-only (`CT_PREFIX_DIR_RO`).
- In Target options | Floating point, select hardware (FPU) (`CT_ARCH_FLOAT_HW`).
- In Target options, enter neon for Use specific FPU.

Now we can use crosstool-ng to build the environment we just specified:
```bash
bin/ct-ng build
```


*source:* Mastering Embedded Linux Programming - Third Edition By Frank Vasquez, Chris Simmonds