Toolchains

The GNU toolchain consists of these components:

• Binutils: Assembler, linker, (ld) and other tools
• GCC: Compilers for C, C++, Objective-C, Objective-C++, Java, Fortran, Ada, and Go. They all use a common back-end which produces assembler code for the GNU assembler (GAS)
• C library: Standardized API based on POSIX (interface to the operating system kernel from applications)

GNU prefix tuple

GNU uses a prefix to identify the various combinations of it’s tools:

• CPU: The CPU architecture
  • arm, mips, x86_64, …
  • may add postfix for endianes mipsel (little-endian MIPS), armeb (big-endian ARM), …
• Vendor: The provider of the toolchain
  • buildroot, poky, unknown, …
  • Sometimes it is left out
• Kernel:
  • linux, …
• Operating system (user space):
  • gnu, uclibcgnu, …
  • ABI can be appended (ARM): gnueabi, gnueabihf, uclibcgnueabi, uclibcgnueabihf, …

Print the tuple: gcc -dumpmachine

C library

There are several C libraries

• glibc: standard GNU C library. It is big and not very configurable, but it is the most complete implementation
• uClibc: initially for uClinux. Allows you to fine-tune its features, smaller than glibc
• musl libc: small, new C library designed for embedded systems

gcc as Cross Compiler

gcc version: arm-cortex_a8-linux-gnueabi-gcc --version

How gcc was configured, use: arm-cortex_a8-linux-gnueabi-gcc -v

The output contains (among other information):

• --with-sysroot=/home/luki/x-tools/arm-cortex_a8-linux-gnueabihf/arm-cortex_a8-linux-gnueabihf/sysroot: the default sysroot directory
• --enable-languages=c,c++: C and C++ languages are enabled
• --with-arch=armv7-a: code is generated using the ARM v7a instruction set
• --with-cpu=cortex-a8 and --with-tune=cortex-a8: code is tweaked for Cortex A8
• --with-float=hard: Generate opcodes for the floating point unit and uses the VFP registers for parameters
• --enable-threads=posix: Enable POSIX threads

Print architecture-specific options: arm-cortex_a8-linux-gnueabihf-gcc --target-help

The sysroot, library, and header files

Sysroot directory contains

• libraries
• header files
• other configuration files.

Print default sysroot by using -print-sysroot

Sysroot can be changed

Subdirectories:

• lib: shared objects for the C library and the dynamic linker/loader (ld-linux)
• usr/lib: the static library archives for the C library and other libraries
• usr/include: headers for all the libraries
• usr/bin: utility programs that run on the target (ldd, …)
• /usr/share: Used for localization and internationalization
• sbin: ldconfig utility (and others)

Binutils and other tools

• addr2line: Convert program addresses into filenames and numbers (decoding addresses from a crash report)
• ar: create static libraries (archives)
• as: assembler
• c++filt: demangle C++ symbols
• cpp: C preprocessor
• elfedit: update ELF headers
• gcov: coverage tool
• gdb: debugger
• gprof: profiling
• ld: linker
• nm: list symbols from object files
• objcopy: copy and translate object files
• objdump: display information from object files
• ranlib: creates or modifies an index in a static library (make the linking faster)
• readelf: display information in ELF object files
• size: list section sizes
• strings: display printable characters in files
• strip: strip the debug symbols

Looking at the components of the C library

• libc: the main C library
• libm: math functions
• libpthread: POSIX threads
• librt: real-time extensions for POSIX

libc is always linked, the others have to be explicitly linked.

Shared libraries

Compile with -fPIC and link with -shared option

See also: ldconfig