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Logos is a component of the Theos development suite that allows method hooking code to be written easily and clearly, using a set of special preprocessor directives.


The syntax provided by Logos greatly simplifies the development of MobileSubstrate extensions ("tweaks") which can hook other methods throughout the OS. In this context, "method hooking" refers to a technique used to replace or modify methods of classes found in other applications on the OS.

Getting Logos

Logos is distributed with Theos, and you can use Logos' syntax in any Theos-built project without any extra setup. For more information about Theos, visit its page.

Command Line Interface

Logos can be invoked on the command line via logos.pl. Usage is as follows:

logos.pl [--config <setting=value>] <filename>

Below are two examples, one using the default generator (Substrate) and one using the internal generator:

logos.pl Tweak.xm
logos.pl --config generator=internal Tweak.xm

List of Logos Directives

Block level

The directives in this category open a block of code which must be closed by an %end directive (shown below). These should not exist within functions or methods.


%group Groupname

Groups are for conditional initialization or code organization. Grouping can be useful for managing backwards compatibility with older code.

Begin a hook group with the name Groupname. Groups cannot be inside another %group block. All ungrouped hooks are in the implicit "_ungrouped" group. The _ungrouped group is initialized for you if there are no other groups. You can use the %init directive to initialize it manually. Other groups must be initialized with the %init(Groupname) directive.

%group iOS8
	// your code here
%end // end hook
%end // end group ios8

%group iOS9
	// your code here
%end // end hook
%end // end group ios9

%ctor {
	if (kCFCoreFoundationVersionNumber > 1200) {
	} else {


%hook Classname

Open a hook block for the class named Classname.

Can be inside a %group block.

Here's a trivial example:

%hook SBApplicationController
-(void)uninstallApplication:(SBApplication *)application {
	NSLog(@"Hey, we're hooking uninstallApplication:!");
	%orig; // Call the original implementation of this method



Add a new method to a hooked class or subclass by adding this directive above the method definition. signature is the Objective-C type encoding for the new method; if it is omitted, one will be generated.

Must be inside a %hook block.

You should also declare the new method within an interface as Logos does not do that automatically. For example:

@interface TheClass (TweakMethods)

%hook TheClass
-(void)yourNewMethod { /* code */ }


%subclass Classname: Superclass <Protocol list>

Subclass block - the class is created at runtime and populated with methods. ivars are not yet supported (use associated objects). The %new specifier is needed for a method that doesn't exist in the superclass. To instantiate an object of the new class, you can use the %c operator.

Can be inside a %group block.

Here's an example:

%subclass MyObject : NSObject

- (id)init {
	self = %orig;
	[self setSomeValue:@"value"];
	return self;

//the following two new methods act as `@property (nonatomic, retain) id someValue;`
- (id)someValue {
	return objc_getAssociatedObject(self, @selector(someValue));

- (void)setSomeValue:(id)value {
	objc_setAssociatedObject(self, @selector(someValue), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);


%ctor {
	MyObject *myObject = [[%c(MyObject) alloc] init];
	NSLog(@"myObject: %@", [myObject someValue]);


%property (nonatomic|assign|retain|copy|weak|strong|getter|setter) Type name;

Add a property to a %subclass just like you would with @property to a normal Objective-C subclass as well as adding new properties to existing classes within %hook.

Must be inside a %subclass or %hook block.



Close a group/hook/subclass block.

Top level

The directives in this category should not exist within a group/hook/subclass block.



Set a logos configuration flag.

Configuration Flags

Key Values Notes
generator MobileSubstrate generate code that uses MobileSubstrate for hooking.
internal generate code that uses only internal Objective-C runtime methods for hooking.
warnings none suppress all warnings
default non-fatal warnings
error make all warnings fatal
dump yaml dump the internal parse tree in YAML format
perl dump the internal parse tree in a format suitable for evaluation as perl source.
dump to the perl source feature is removed since this commit.


%hookf(rtype, symbolName, args...) { … }

Generate a function hook for the function named symbolName. If the name is passed as a literal string then the function will be dynamically looked up.

// Given the function prototype
FILE *fopen(const char *path, const char *mode);
// The hook is thus made
%hookf(FILE *, fopen, const char *path, const char *mode) {
	NSLog(@"Hey, we're hooking fopen to deny relative paths!");
	if (path[0] != '/') {
		return NULL;
	return %orig; // Call the original implementation of this function

It is common for people to hook the function that its address is resolved at run-time like, MGGetBoolAnswer, for example:

bool (*orig_MGGetBoolAnswer)(CFStringRef);
bool fixed_MGGetBoolAnswer(CFStringRef string)
	if (CFEqual(string, CFSTR("StarkCapability"))) {
		return kCFBooleanTrue;
	return orig_MGGetBoolAnswer(string);

%ctor {
	MSHookFunction(((void *)MSFindSymbol(NULL, "_MGGetBoolAnswer")), (void *)fixed_MGGetBoolAnswer, (void **)&orig_MGGetBoolAnswer);

You can also do:

%hookf(bool, "_MGGetBoolAnswer", CFStringRef string)
	if (CFEqual(string, CFSTR("StarkCapability"))) {
		return true;
	return %orig;


%ctor { … }

Generate an anonymous constructor (of default priority).


%dtor { … }

Generate an anonymous deconstructor (of default priority).

Function level

The directives in this category should only exist within a function block.


%init([<class>=<expr>, …]);
%init(Group[, [+|-]<class>=<expr>, …]);

Initialize a group (or the default group). Passing no group name will initialize "_ungrouped", and passing class=expr arguments will substitute the given expressions for those classes at initialization time. The + sigil (as in class methods in Objective-C) can be prepended to the classname to substitute an expression for the metaclass. If not specified, the sigil defaults to -, to substitute the class itself. If not specified, the metaclass is derived from the class. The class name replacement is specially useful for classes that contain characters that can't be used as the class name token for the %hook directive, such as spaces and dots.


%hook SomeClass
-(id)init {
    return %orig;

%ctor {
    %init(SomeClass=objc_getClass("class with spaces or dots in the name"));


%class Class;
%class is deprecated. Do not use it in new code.

Forward-declare a class. Outmoded by %c, but still exists. Creates a $Class variable, and initializes it with the "_ungrouped" group.



Evaluates to Class at runtime. If the + sigil is specified, it evaluates to MetaClass instead of Class. If not specified, the sigil defaults to -, evaluating to Class.


%orig(arg1, …)

Call the original hooked method. It doesn't function in a %new'd method. It works in subclasses, strangely enough, because MobileSubstrate will generate a supercall closure at hook time. (If the hooked method doesn't exist in the class we're hooking, it creates a stub that just calls the superclass implementation.) args is passed to the original function - don't include self and _cmd, Logos does this for you.


%log([(<type>)<expr>, …]);

Dump the method arguments to syslog. Typed arguments included in %log will be logged as well.

File Extensions for Logos

Extension Process order
.x will be processed by Logos, then preprocessed and compiled as objective-c.
.xm will be processed by Logos, then preprocessed and compiled as objective-c++.
.xi will be preprocessed as objective-c first, then Logos will process the result, and then it will be compiled.
.xmi will be preprocessed as objective-c++ first, then Logos will process the result, and then it will be compiled.

xi or xmi files can use Logos directives in #define macros.

Splitting Logos Hooking Code Across Multiple Files

By default, the Logos pre-processor will only process one .xm file at build time. However, it is possible to split the Logos hooking code into multiple files.
First, the main file has to be renamed to an .xmi file. Then, other .xm files can be included in it using the #include directive. The Logos pre-processor will add those files to the main file before processing it.


Normally, it isn't possible to initialize hooking groups across multiple Logos files, but there is a workaround that can be used to unlock this functionality. This is done by wrapping group initializations inside of static methods that can then be called from other files.

Take a look at the following code. All it does is log a message when the SpringBoard application has finished launching. It is inside of a group called TweakGroup, which is initialized in a static function called InitGroup().

// Group.xm
#import "Shared.h"

%group TweakGroup
%hook SpringBoard

- (void)applicationDidFinishLaunching:(id)arg1 {
    NSLog(@"[Group Test] SpringBoard has finished launching");


extern "C" void InitGroup() {

As you may have noticed, there is an import for Shared.h at the top of Group.xm. That is simply a header file that will be imported into our main Logos file so that we may call the function there:

// Shared.h
extern "C" void InitGroup();

Finally, Shared.h can be imported into the Logos file that contains your constructor. Calling the static function will initialize the group from Group.xm and run its hooks:

// Tweak.xm
#import "Shared.h"

%ctor {
	NSLog(@"[Group Test] Our hook for SpringBoard should show up below this");

If done correctly and compiled without errors, this could should log two messages: one from the constructor and the other one from the method inside the group. Keep in mind that this doesn't apply to hooks that aren't inside of a group.

A few things to note:

  • This doesn't work in normal C, so you must use .xm files for your groups and constructor.
  • You have to pay attention to how many times you call the initialization as Logos will no longer tell you if it is called more than once.


You can use logify.pl to create a Logos source file from a header file that will log all of the functions of that header file. Here is an example of a very simple Logos tweak generated by logify.pl

Given a header file:

@interface SSDownloadAsset : NSObject
- (NSString *)finalizedPath;
- (NSString *)downloadPath;
- (NSString *)downloadFileName;
+ (id)assetWithURL:(id)url type:(int)type;
- (id)initWithURLRequest:(id)urlrequest type:(int)type;
- (id)initWithURLRequest:(id)urlrequest;
- (id)_initWithDownloadMetadata:(id)downloadMetadata type:(id)type;

You can find logify.pl at $THEOS/bin/logify.pl and you would use it as so:

$THEOS/bin/logify.pl ./SSDownloadAsset.h

The resulting output should be:

%hook SSDownloadAsset
- (NSString *)finalizedPath { %log; NSString * r = %orig; NSLog(@" = %@", r); return r; }
- (NSString *)downloadPath { %log; NSString * r = %orig; NSLog(@" = %@", r); return r; }
- (NSString *)downloadFileName { %log; NSString * r = %orig; NSLog(@" = %@", r); return r; }
+ (id)assetWithURL:(id)url type:(int)type { %log; id r = %orig; NSLog(@" = %@", r); return r; }
- (id)initWithURLRequest:(id)urlrequest type:(int)type { %log; id r = %orig; NSLog(@" = %@", r); return r; }
- (id)initWithURLRequest:(id)urlrequest { %log; id r = %orig; NSLog(@" = %@", r); return r; }
- (id)_initWithDownloadMetadata:(id)downloadMetadata type:(id)type { %log; id r = %orig; NSLog(@" = %@", r); return r; }

External links