"unrecognized selector sent to class" when calling category method from a library

Question

Problem

This question may seem a bit long, but I try to give as much information as possible, since I am really staggered by this.

I am currently working an a library which should automate XML document parsing. But I am running into a problem now testing the library for the first time.

I have a library class called CSXDocumentLayout which represents the layout of a document. This class contains the private method - (NSError *)readLayoutDocument:(NSString *)fpath called from an init method.

/* MARK: Reading in Layouts */
- (NSError *)readLayoutDocument:(NSString *)fpath {
    CSXDocumentLayout *layout;
    CSXXMLParser *parser;
    BOOL state;

    layout = [CSXDocumentLayout layoutDocumentLayout];

    parser = [[CSXXMLParser alloc] initWithDocumentLayouts:
              [NSArray arrayWithObject:layout]];
    parser.file = fpath;

    state = [parser parse];

    if(state == NO || parser.error != nil) {
        return parser.error;
    }
    return nil;
}

This method will read in an XML document representing the layout of an other XML document. It is parsed by the class CSXXMLParser, which I want to test.

I create an object representing a layout document with +[CSXDocumentLayout layoutDocumentLayout]. This method is implemented in the category CSXDocumentLayout (CSXLayoutObject).

Below is my test file:

#import <Foundation/Foundation.h>
#import <CeasyXML.h>

int main(int argc, const char **argv) {
    NSAutoreleasePool *pool = [NSAutoreleasePool new];

    NSString *file;

    file = [NSString stringWithUTF8String:__FILE__];
    file = [file stringByDeletingLastPathComponent];
    file = [file stringByAppendingPathComponent:@"Layout.xml"];

    CSXDocumentLayout *layout;
    NSError *error;

    layout = [[CSXDocumentLayout alloc] initWithLayoutDocument:file 
                                                         error:&error];
    if(layout == nil) {
        NSLog(@"Could not create layout: %@", error);
        exit(0);
    }

    NSLog(@"Layout:\n%@", layout);

    [pool release];
    return 0;
}

This file compiles to a separate executable linked to my static library libceasyxml.a. Everything compiles just fine without any warnings.

But when I run it I get a unrecognized selector sent to class exception:

2012-05-02 16:59:47.620 TestApp[1887:a0f] +[CSXDocumentLayout layoutDocumentLayout]: unrecognized selector sent to class 0x1000064c8
2012-05-02 16:59:47.791 TestApp[1887:a0f] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[CSXDocumentLayout layoutDocumentLayout]: unrecognized selector sent to class 0x1000064c8'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x00007fff83e47784 __exceptionPreprocess + 180
    1   libobjc.A.dylib                     0x00007fff84604f03 objc_exception_throw + 45
    2   CoreFoundation                      0x00007fff83ea11a0 __CFFullMethodName + 0
    3   CoreFoundation                      0x00007fff83e198ef ___forwarding___ + 751
    4   CoreFoundation                      0x00007fff83e15a38 _CF_forwarding_prep_0 + 232
    5   TestApp                             0x0000000100001512 -[CSXDocumentLayout(Private) readLayoutDocument:] + 49
    6   TestApp                             0x00000001000010d4 -[CSXDocumentLayout initWithLayoutDocument:error:] + 96
    7   TestApp                             0x0000000100001017 main + 179
    8   TestApp                             0x0000000100000f5c start + 52
    9   ???                                 0x0000000000000001 0x0 + 1
)

I find it very disturbing that I cannot call the class method +[CSXDocumentLayout(CSXLayoutObject) layoutDocumentLayout], though I can call both -[CSXDocumentLayout initWithLayoutDocument:error:], and -[CSXDocumentLayout(Private) readLayoutDocument:].

Research

I checked if the method is defined in the output files by running nm file and it is, well partly:

In libceasyxml.a, it is defined (nm libceasyxml.a)

...
libceasyxml.a(CSXDocumentLayout+CSXLayoutObject.o):
0000000000000100 t +[CSXDocumentLayout(CSXLayoutObject) classAttributeLayout]
00000000000020e0 s +[CSXDocumentLayout(CSXLayoutObject) classAttributeLayout].eh
000000000000056b t +[CSXDocumentLayout(CSXLayoutObject) documentElementLayout]
0000000000002180 s +[CSXDocumentLayout(CSXLayoutObject) documentElementLayout].eh
0000000000000402 t +[CSXDocumentLayout(CSXLayoutObject) layoutDocumentLayout]
0000000000002148 s +[CSXDocumentLayout(CSXLayoutObject) layoutDocumentLayout].eh
0000000000000200 t +[CSXDocumentLayout(CSXLayoutObject) layoutElementLayout]
0000000000002110 s +[CSXDocumentLayout(CSXLayoutObject) layoutElementLayout].eh
0000000000000000 t +[CSXDocumentLayout(CSXLayoutObject) nameAttributeLayout]
00000000000020b0 s +[CSXDocumentLayout(CSXLayoutObject) nameAttributeLayout].eh
0000000000002098 s EH_frame1
0000000000001c49 s LC0
...

In TestApp, it is NOT defined (nm TestApp), actually I can't find any method with the category name CSXLayoutObject.

...
0000000100001271 t -[CSXDocumentLayout setDocumentClassString:]
00000001000013a8 t -[CSXDocumentLayout setElements:]
0000000100001490 t -[CSXDocumentLayout setName:]
00000001000014db t -[CSXDocumentLayout(Private) readLayoutDocument:]
0000000100004060 t -[CSXElementList dealloc]
00000001000040b0 t -[CSXElementList elements]
...

Answer 1

I suspect you have run into a known problem of categories embedded in static libraries.

If I am right, try to compile with the linker flags -ObjC (and if that is not enough, also -all_load) and your problem should disappear.