iOS_codeSamples

This repository is a companion for LLVM for iOS. It contains config files, sample code that compiles and runs on iOS, scripts, tips and tricks to make things work.

Compilation

To compile a project:

• for each source file, clang --config ~/clang.cfg source.c. This will produce source.ll
• assemble the compiled sources with: llvm-link -o=executable.bc *.ll This will produce the linked file.
• to start the program, use lli executable.bc <flags to the main program>

Configuration files

There are 2 configuration files in this repository: clang.cfg and gnu.cfg. The former is for standard projects:

 -S -emit-llvm -I ~/lib/clang/7.0.0/include -isysroot ~ -Wno-nullability-completeness  -D_FORTIFY_SOURCE=0

The latter is for complex projects, using configure:

 -S -emit-llvm -I ../lib -I .. -I ../src -isysroot ~ -I ~/lib/clang/7.0.0/include -Wno-nullability-completeness -DHAVE_CONFIG_H -D_FORTIFY_SOURCE=0 

The big difference is the -DHAVE_CONFIG_H which tells the compiler to look for a config.h file, which you have to create: cp config.hin config.h and edit the config.h file.

(-isysroot ~ has the same effect as -I ~/usr/include, but it will also work when compiling Objective-C).

Sample code

The directory sampleCode contains some simple projects you can compile:

• hello.c just experiments with input and output methods.
• pwd.c is the BSD implementation of the shell command pwd. It prints the current working directory.
• bsd_find contains the Free BSD implementation of the shell command find, with minor edits (see fork, below),
• bsd_diff contains the Free BSD implementation of the shell command diff. It is a good command to test the difference between JIT and interpreter.

Additionally, tree works with no modifications, except you have to link it with libc_replacement/qsort.c (see below, "Function replacement").

iOS "forbidden" functions

Some functions (fork(), system()...) are not available in iOS due to sandbox restrictions. Some are intercepted at compile time, others at run time. For the functions intercepted at run time, they are implemented as external functions in lli.cpp. If I forgot one, tell me and I'll add it.

For the functions intercepted at compile time, you'll have to edit the headers. For system(), that's in: ~/usr/include/stdlib.h, line 193. This line:

__API_AVAILABLE(macos(10.0)) __IOS_PROHIBITED

is responsible for system() exclusion. Replace it with:

__API_AVAILABLE(macos(10.0), ios(11.0))

fork() requires special attention. lli catches it so it doesn't stop the program, but it can't actually fork. And exec() doesn't exit. fork() returns 0, so the interpreter will always go through the child branch. You'll have to edit the code to make sure it doesn't throw an exception when exec() returns and goes through the parent branch. For example, here is the original code:

	switch (pid = fork()) {
	case 0: {
		execvp(plan->e_argv[0], plan->e_argv);
		warn("%s", plan->e_argv[0]);
		_exit(1); 
	}
	}
	pid = waitpid(pid, &status, 0);
	if (pid != -1 && WIFEXITED(status) && !WEXITSTATUS(status))
		return (1);
	return (0);

And here is the modified version:

	switch (pid = fork()) {
	case 0: {
		execvp(plan->e_argv[0], plan->e_argv);
		/* warn("%s", plan->e_argv[0]);
		_exit(1); */
	}
	}
	pid = waitpid(pid, &status, 0);
	if (pid != -1 && WIFEXITED(status) && !WEXITSTATUS(status))
		return (1);
	return (0);

Function replacement

The LLVM interpreter (lli) calls external functions using libFFI. For some reason, external functions that operate on pointers (qsort, bsearch) cause the interpreter to crash.

The directory libc_replacement contains source files for these functions. If your code uses one of these functions, compile the relevant source file and link with it. For now, there is qsort and bsearch. It will expand as more problematic functions are discovered.

How to install scripts from the scripts folder

1. Copy the script you want to install from the Scripts folder into ~
2. tar -xvzf {the file you just copied}
3. Check if the script got installed by opening the Scripts view controller