default parameters in function call in C

Question

Lets say in a file I have some code in two files part of the same project

file1.c
int func1(int a, int b, int c, bool d)
{
        /* function body */
}

file2.c
extern func1(int a, int b, int c);

/* function call */
func1(runtime1, runtime2, runtime3);

What would be value bool d takes when being called from file2.c? I know this is really bad practice, but I am maintaining old code and somebody did this, I just want to know the default parameter or if it is implementation dependent. Please note also, that bool in this example is a typedef of the software, since this specific project does not support C99. Thank you.!

I'm not certain, but I would think that d would be anything that's on the stack at the moment of the call. — Some programmer dude
– Some programmer dude, Commented Feb 7, 2012 at 13:05
I am amazed that it links. I guess only way to know is to debug the program and even than you might have something on the stack. — Bort
– Bort, Commented Feb 7, 2012 at 13:13
Unless some specific compiler magic detected this foolish code and initialised it, could be "anything" each time it's called. No real way to find out without changing the code, might as well fix it, if you can do that. Slot in func1A(int, int,int) that calls func1(int,int,int, false) ? — Tony Hopkinson
– Tony Hopkinson, Commented Feb 7, 2012 at 13:16

Eregrith · Accepted Answer · 2012-02-07 13:06:52Z

5

The value would be undefined. When calling func1, its parameters go on the stack. If you call it with 1 parameter less, the stack will be sizeof(bool) bytes short of what the process expects. This will not crash your program as your stack and your heap are "facing", but if you try to access to d, you will access to whatever value is there on the stack -> rubbish.

answered Feb 7, 2012 at 13:06

Eregrith

4,39622 silver badges39 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Pedro Perez Over a year ago

Thank you for your answer. I believe it makes sense what you say.

Pedro Perez Over a year ago

I have checked with a simple program in Win32 under gcc and indeed, what you see is rubbish. Thank you everybody for your time and answers.

Fred Foo · Accepted Answer · 2012-02-07 13:11:00Z

4

The value is not just implementation-dependent; the entire behavior of the program is undefined. If you'd put the declaration for func1 in a header instead of in file2.c and you'd include that header in file1.c, as is good C practice, the compiler would refuse to compile this.

In practice, you'll likely observe d as having some arbitrary, unpredictable value, though your program might also crash mysteriously.

edited Feb 7, 2012 at 13:11

answered Feb 7, 2012 at 13:05

Fred Foo

365k80 gold badges765 silver badges852 bronze badges

Comments

ouah · Accepted Answer · 2012-02-07 13:47:16Z

1

This program is undefined behavior. As the program is undefined behavior the compiler has the right to refuse to compile it.

(C99, 6.2.7p2) "All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined."

The two function declarations in your program are not compatible; they don't have the same number of parameters.

(C99, 6.7.5.3p15) "For two function types to be compatible, both shall specify compatible return types. Moreover, the parameter type lists, if both are present, shall agree in the number of parameters and in use of the ellipsis terminator; corresponding parameters shall have compatible types."

edited Feb 7, 2012 at 13:47

answered Feb 7, 2012 at 13:37

ouah

147k16 gold badges287 silver badges338 bronze badges

Comments

asaelr · Accepted Answer · 2012-02-07 13:12:10Z

0

It's undefined behavior - it may have garbage value, and it may also crash, depend on the calling convention of your compiler and OS.

Edit: The other arguments may also be mixed, if they are been pushed from left to right.

answered Feb 7, 2012 at 13:12

asaelr

5,4581 gold badge19 silver badges22 bronze badges

Comments

Cacho Santa · Accepted Answer · 2012-04-16 23:02:38Z

0

It can be any value, garbage from the stack I believe if you call the method that way.

The program will have undefined behavior because you really do not know the value of the bool parameter. It may also crash during the execution.

Hope it helps.

edited Apr 16, 2012 at 23:02

answered Feb 7, 2012 at 13:07

Cacho Santa

6,9266 gold badges44 silver badges76 bronze badges

Collectives™ on Stack Overflow

default parameters in function call in C

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related