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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.

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    OS X no longer comes with GCC. It comes with clang, and clang does support cross-compiling to iOS. In fact – just drop the .c files into your project at the same place you would put your .m files. They'll be compiled just fine along with the Objective-C code (remember, Objective-C is a proper superset of C. If you can cross-compile Objective-C, you can cross-compile C as well.) Commented Feb 6, 2015 at 0:34

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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.

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Thanks Marko! I was able to confirm exactly what you mentioned myself before your post (thanks for confirming regardless!). I went through and replaced '/bin/gcc' with 'xcrun -sdk iphoneos clang -arch armv7' and was able to compile everything for arm. I know there is a way to make a universal library, which I'll do next and combine armv7, arm64 and x86 libraries together.
@Dan I have been hunting for that command. Thanks! xcrun -sdk iphoneos clang -arch armv7 main.c

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