Calling Objective-C method from C code

Question

I'm trying to call a method implemented in Obj-C from C code as follows:

// MainViewController.m

- (void)Test
{
    [self outputLine:@"Called from MyCode.c"];
}
.
.
.
cFunc(id param);
.
.
.

// MyCode.c

void cFunc(id param)
{
    [param Test]; // compilation error: Parse issue: Expected expression
}

I guess it happens since the MainViewController is not declared in MyCode.c, but when I #include the MainViewController.h I get tons of other errors that suggests I'm totally wrong... How should I handle it correctly?

TIA

possible duplicate of Call Objective-C function from C function — One Man Crew
– One Man Crew, Commented Jul 11, 2013 at 13:47
@OneManCrew: That question covers a different issue, despite its title. I've edited it to make the title match the subject. I would be surprised, however, if there wasn't a duplicate of this somewhere. — jscs
– jscs, Commented Jul 11, 2013 at 20:55
although it really sounds trivial, even for an Obj-C newbie like me I was quite surprised not to find a similar answer, but maybe it's just my searching skills... — eagle_bear
– eagle_bear, Commented Jul 13, 2013 at 3:52

user529758 · Accepted Answer · 2013-07-11 13:47:51Z

4

You should compile the MyCode.c file as Objective-C. Objective-C is a superset of C, but it's not true the other way around. You can't have Objective-C code interspersed with your "pure C" code if you are intending to compile it as C.

answered Jul 11, 2013 at 13:47

user529758

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Community · Accepted Answer · 2017-05-23 12:21:04Z

0

Check: using objc_msgSend to call a Objective C function with named arguments

void cFunc(id param) {
    objc_msgSend(param, sel_getUid("Test"));
}

But, as per the link above, this is dangerous for a few reasons, if your arguments don't fit in registers (i.e. floats, structs, blah blah).

The accepted way of doing this is to cast objc_msgSend:

void cFunc(id param) {
    // Now let's pretend that you want to send someFloat to your method

    void (*objc_msgSendTyped)(id self, SEL _cmd, float bar) = (void*)objc_msgSend;

    float someFloat = 42.f;
    objc_msgSendTyped(param, sel_getUid("Test"), someFloat);
}

edited May 23, 2017 at 12:21

CommunityBot

11 silver badge

answered Jul 11, 2013 at 13:53

George Mitchell

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4 Comments

George Mitchell Over a year ago

Since you have no parameters this is an easy way to do it. Just don't use this if you have to pass parameters to the message, check @Ken's answer to the question I linked to.

Greg Parker Over a year ago

Never call objc_msgSend() without casting it to the correct function pointer type.

George Mitchell Over a year ago

That is true... but in this case it is fairly trivial because the OP's function was void anyway. I'll edit to reflect your suggestion though and to reemphasize the method in the link I listed.

user529758 Over a year ago

+1 because now it's edited, showing the proper way, and anyways I don't like "it's impossible"-style answers in general, which now mine apparently is.

Nico Cvitak · Accepted Answer · 2013-07-12 00:32:16Z

0

Just change your myCode.c to myCode.m :P

Don't be afraid to put C code in an Objective-C file.

answered Jul 12, 2013 at 0:32

Nico Cvitak

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