Calling a function by name input by user

For Objective-C methods, you can use performSelector… or NSInvocation, e.g.

NSString *methodName = @"doSomething";
[someObj performSelector:NSSelectorFromString(methodName)];

For C functions in dynamic libraries, you can use dlsym(), e.g.

void *dlhandle = dlopen("libsomething.dylib", RTLD_LOCAL);
void (*function)(void) = dlsym(dlhandle, "doSomething");
if (function) {
    function();
}

For C functions that were statically linked, not in general. If the corresponding symbol hasn’t been stripped from the binary, you can use dlsym(), e.g.

void (*function)(void) = dlsym(RTLD_SELF, "doSomething");
if (function) {
    function();
}

Update: ThomasW wrote a comment pointing to a related question, with an answer by dreamlax which, in turn, contains a link to the POSIX page about dlsym. In that answer, dreamlax notes the following with regard to converting a value returned by dlsym() to a function pointer variable:

The C standard does not actually define behaviour for converting to and from function pointers. Explanations vary as to why; the most common being that not all architectures implement function pointers as simple pointers to data. On some architectures, functions may reside in an entirely different segment of memory that is unaddressable using a pointer to void.

With this in mind, the calls above to dlsym() and the desired function can be made more portable as follows:

void (*function)(void);
*(void **)(&function) = dlsym(dlhandle, "doSomething");
if (function) {
    (*function)();
}