Does C support optional null parameters?

Question

In Python, I'm used to things like

def send_command(command, modifier = None):

and then the modifier argument is optional, and the absence of the argument can be differentiated from an argument of 0. Is there similar functionality in C? I'm inexperienced with C, and Googling, but can't find a clear statement of how to use optional parameters in C. It seems you can assign them similarly, like this:

void send_command(uint8_t command, uint8_t modifier = 0) {

so the second argument is optional and defaults to 0 if not used? (Edit: No, this is invalid C anyway)

But can the function distinguish between send_command(SOMETHING) and send_command(SOMETHING, 0)? Ideally, the second parameter could be any uint8 value, including 0.

Maybe NULL is different from 0?

void send_command(uint8_t command, uint8_t modifier = NULL) {

See: [function overloading in C](http://stackoverflow.com/questions/479207/function-overloading-in-c) — Sjoerd, Feb 07 '12 at 16:30

score 19 · Accepted Answer · answered Feb 07 '12 at 16:30

19

C does not support optional parameters. Nor does it support function overloading which can often be used to similar effect.

answered Feb 07 '12 at 16:30

David Heffernan

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That might be why I'm having trouble finding it. So my example that assigns default value of 0 would only be valid in C++? – endolith Feb 07 '12 at 16:33
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Correct. That is known as a default argument in C++. – David Heffernan Feb 07 '12 at 16:36

score 14 · Answer 2 · answered Feb 07 '12 at 16:50

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Optional parameters are possible in C99 with variadic macros:

#define JUST3(a, b, c, ...) (a), (b), (c)
#define FUNC(...) func(JUST3(__VA_ARGS__, 0, 0))

Now FUNC(x) expands to func((x), (0), (0)), FUNC(x,y) expands to func((x), (y), (0)), etc.

answered Feb 07 '12 at 16:50

R.. GitHub STOP HELPING ICE

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I don't whether to cry or laugh – pmg Feb 07 '12 at 16:57
You don't want to see overloading, do you? ;-) – R.. GitHub STOP HELPING ICE Feb 07 '12 at 17:01
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And unfortunately, `FUNC(x,y,z,t)` expands to `func((x), (y), (z))` instead of an error. The important thing, I feel, is to just *slightly* punish people who want overloading. When they least expect it. – Steve Jessop Feb 07 '12 at 17:10
Bonus points to whomever figures out how to make `FUNC(x,y,z,t)` generate an error. I suspect it's not too hard. – R.. GitHub STOP HELPING ICE Feb 07 '12 at 17:13
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Totally reasonable. I think the best answer is "No, C doesn't have them [unless you like evil preprocessor hacks]." – R.. GitHub STOP HELPING ICE Feb 07 '12 at 17:26
This doesn't differentiate between missing argument and 0 anyway, does it? – endolith Feb 07 '12 at 17:28
The macros I gave cause the function to receive zeros for the remaining arguments if they're omitted. They could easily be changed to use a different default value if you prefer. – R.. GitHub STOP HELPING ICE Feb 07 '12 at 17:34
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It'd be tricky to imitate `void send_command(int command = 1; int modifier = 0)`, though, since whatever value you add immediately after `__VA_ARGS__` will be used as the default for either parameter. This is a poor version of "default arguments" or "overloading". Obviously R.. knows that, this is presented mostly for interest. The warnings not to actually use this should probably be heeded! – Steve Jessop Feb 08 '12 at 10:22
Haha Steve you blew my cover. I was trying not to mention the fact that it won't work for the first argument. ;-) – R.. GitHub STOP HELPING ICE Feb 08 '12 at 16:23

score 3 · Answer 3 · answered Feb 07 '12 at 16:37

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As others have said, C does not have optional parameters.

As for the difference between NULL and 0, there isn't much of one.

answered Feb 07 '12 at 16:37

Samuel Edwin Ward

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That depends. Defining NULL to mean `(void *)0` will, with most compilers, result in a "conversion from pointer to integer without a cast" warning when passed in an integer argument. I find it rather odd that the C++ committee chose to go with a plain 0, which defeats even the most basic type checking that would've occurred had they retained the C definition. – Tom Lint Jul 19 '20 at 10:41

Jens Gustedt · Answer 4 · 2012-02-07T18:28:45.193

As others said C doesn't support default arguments of functions directly. But there are ways to do this with macros. P99 has convenient "meta"-macros that make this feature relatively easy to specify. As an example to avoid to repeatedly have to specify the second argument of the pthread_mutex_init function:

P99_PROTOTYPE(int, pthread_mutex_init, pthread_mutex_t*, pthread_mutexattr_t const*);
#define pthread_mutex_init(...) P99_CALL_DEFARG(pthread_mutex_init, 2, __VA_ARGS__)
P99_DECLARE_DEFARG(pthread_mutex_init, , (pthread_mutexattr_t*)0);

and straight forward to use afterwards

pthread_mutex_init(&my_mutex);

The semantic of evaluation of the default argument here is the same as for C++, that is the evaluation context of the default argument is the context of the declaration. There is also the possibility to specify this in a way that the context of evaluation is the context of the macro invocation.

score 0 · Answer 5 · answered Feb 07 '12 at 16:32

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The C Programming Language has no optional parameters. C++ does, but "the mother of many modern programming languages" C does not...

answered Feb 07 '12 at 16:32

Roy Dictus

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Does C support optional null parameters?

5 Answers5