Hacking up an armv7s library

NOTE: Please take care with this. I obviously cannot test if this will actually work on a new iPhone 5 device! I provide no warranty if you submit having used this and it doesn’t actually work on the new device. Please think twice before submitting an app which you have used this method to create. You don’t have to submit an armv7s binary. Just set your “Architectures” build setting to armv7 only and submit the resulting binary.

UPDATE: It worked! I tested an app that I’d used this method to build an armv7s slice with. It ran fine on my iPhone 5 :-D.

Well the iPhone 5 has been announced and it just so happens that the architecture it uses is what they’re calling armv7s. This brings in yet another architecture to the mix alongside armv6 and armv7. And I bet you are wondering why you’re getting linker errors when building for armv7s when using external libraries. It’s because those external libraries do not have armv7s versions!

If you run file on the library then you’ll see that there is no armv7s version. For example:

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$ file libUAirship-1.2.1.a libUAirship-1.2.1.a: Mach-O universal binary with 3 architectures libUAirship-1.2.1.a (for architecture armv7):    current ar archive random library libUAirship-1.2.1.a (for architecture armv6):    current ar archive random library libUAirship-1.2.1.a (for architecture i386): current ar archive random library

So what can you do? You could wait for the library to be updated, or you could just follow these steps…

So what’s the deal?

Well, the problem is that when the linker does its merry business linking together all your object files, it is told what architecture to link for. Each of your libraries’ .a files will most likely be what are called “fat” meaning they have more than one architecture in them. But the linker won’t be able to find the armv7s version since it doesn’t exist in there.

But, we know that armv7s is a superset of armv7 (it’s just got a new version of the floating point unit so only adds new instructions). So what we can do is to copy the armv7 part of the library and add it again but tell it that it’s for armv7s. That sounds simple, but there’s more to it than that.

Inside each architecture’s portion of the fat library is something called an object file archive. This contains a collection of .o files that were combined together to form the library. Inside each .o is the code for each method. The linker uses these to build the final app binary, picking all the methods it needs to create the app. The problem is that these .o files also have a header to say what architecture they’re for.

Inside this header (called a Mach-O header) is a field for the CPU type and the CPU subtype. ARM is CPU type 12, armv7 is CPU subtype 9 and armv7s is CPU subtype 11. So, all we need to do is toggle all the 9s to 11s, right? Yup! But that’s easier said than done.

My solution is a script that strips out the armv7 portion of the fat library and then unpacks the archive into its constituent .o files. Then I wrote a little C program to do the 9 => 11 toggling which is run on each of the .o files. Then finally the new .o files are packaged up into a new portion which is re-added to the fat library.

Simple!

So, if you’re ready to get going then read on…

Do you need this to submit an app?

No.

Do not use this unless you really understand what you’re doing. You do not need to submit with an armv7s binary. Just set your Architectures build setting to armv7 only and submit the resulting binary.

A program you’ll need

The first thing you’ll need is the following program written in C:

armv7sconvert.c

/*  * armv7sconvert <infile> <outfile>  * Switches CPU subsystem type to armv7s  *  * By Matt Galloway - http://www.galloway.me.uk  *   * Based on g3spot.c from http://redbutton.sourceforge.net (c) Simon Kilvington, 2009  */ #include <stdio.h> #include <stdlib.h> #include <stdarg.h> #include <string.h> #include <errno.h> #include <mach-o/loader.h> void fatal(char *msg, ...); void fatal(char *msg, ...) { va_listap; va_start(ap, msg); vfprintf(stderr, msg, ap); fprintf(stderr, "\n"); va_end(ap); exit(EXIT_FAILURE); } int main(int argc, char *argv[]) { if (argc != 3) { fatal("Syntax: %s <in_file> <out_file>", argv[0]); } char *inName = argv[1]; char *outName = argv[2]; FILE *inFile = fopen(inName, "r"); if (inFile == NULL) { fatal("Unable to read %s: %s", inName, strerror(errno)); } /* find out how big it is */ fseek(inFile, 0, SEEK_END); longsize = ftell(inFile); rewind(inFile); printf("%s: %lu bytes\n", inName, size); /* read it all into memory */ unsigned char *buf = malloc(size); if (buf == NULL) { fatal("Out of memory"); } if (fread(buf, 1, size, inFile) != size) { fatal("Error trying to read %s: %s", inName, strerror(errno)); } if (fclose(inFile)) { fatal("Error trying to close %s: %s", inName, strerror(errno)); } structmach_header *mach_hdr = (struct mach_header *) (buf); printf("Mach magic: 0x%08x\n", mach_hdr->magic); if (mach_hdr->magic != MH_MAGIC) { fatal("Wrong magic number (expecting 0x%08x)", MH_MAGIC); } printf("CPU type: %d\n", ntohl(mach_hdr->cputype)); printf("CPU sub-type: %d\n", ntohl(mach_hdr->cpusubtype)); printf("*** Changing to CPU sub-type 11\n"); mach_hdr->cpusubtype = 11; printf("Saving as %s\n", outName); FILE *