Compiling C Source for iOS

Question

I have some existing source code that is written in C that I want to build and include in my iOS project. The entire source package is very large and is built using existing Makefiles and GCC. It is producing static libraries (.a files) that I would love to move over to my iOS project. However, the static libraries the Makefile produces is for x86 processors, which obviously won't work on iOS.

Is there a way I can switch GCC to build for ARMv7/ARM64 instead, without making changes to the existing source (in most cases)? I know there is the -march switch for GCC or you can download ARM specific GCC compilers, so I know the general concept of building for a different architecture than the build machine.

To build for ARM on Mac OS, will I have to download a different GCC compiler or is that capability built into the default GCC?

I'm sorry for the lack of understanding of basic concepts here; I'm primarily a Java and Objective-C developer, so building source for different architectures is a mostly foreign concept to me.

Answer 1

Whilst GCC supports a good many CPU architecture and platforms, it is usually built for a single one. To compile for ARM you generally need an ARM-cross-compiling GCC targeted appropriately.

The default system compiler for MacOSX and iOS for all architectures is clang and has been for some time (the last version of GCC apple shipped in dev tools is creaking and obsolete, and definitely won't support ARMv8).

The usual way of getting clang is to install Xcode (free from the App Store). There's a option in the installer (and in the UI of Xcode) to install the command-line tool package. This installs sym-links in /usr/bin to the compiler, and installs a bunch of other stuff you might expect such as make.

clang is (mostly) command-line compatible with gcc, and furthermore, you'll find that if you run gcc from the command-line on a Mac with dev-tools installed, you in fact get clang.

$ gcc --version
Configured with: --    prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix 

clang comes with ARMv7, ARMv8, i686, x86_64 on MacOSX, and can be configured to compile for any of these from the command line (See documentation)

Given the above, there's a fair chance your code will compile with minimal changes to compiler-flags using the existing makefile. You might want to read the documentation for lipo - which allows you to produce multi-architecture binaries.