Passing C array as char* function parameter

Question

I've got some code I'm mantaining with the following variable declaration:

char tmpry[40];

It's being used with this function:

char *SomeFunction(char *tmpryP) {
   // Do stuff.
}

The function call is:

SomeFunction(&tmpry[0]);

I'm pretty damn sure that's just the same as:

SomeFunction(tmpry);

I've even checked that the char* pointer in SomeFunction ends up pointing to the same memory location as the array in both cases.

My question is a sanity check as to whether the two function calls are identical (and therefore the original programmer was just being nasty)?

I think it would be good to pass a length also to SomeFunction, just in case... :) — AndersK
– AndersK, Commented Apr 8, 2009 at 6:25
The name of the character array is an alias for the address of the first element of the array. — David
– David, Commented Jul 15, 2009 at 16:07
Hello, I'm one of those "nasty" programmers who writes like this. Generally, I do this in C++ code, for the reason of lesser coupling to the array abstraction used at the moment. I.e. for an abstract array-like data structure (like std::vector<char>) with overloaded operator [] expressions x and &x[0] will get to mean very different things, and the second one is the one which fits better for my intent. Off course, all this doesn't matter so much for plain C, since you get almost none data abstraction in C anyway; thus the spelling with superior readability should be preferred. — ulidtko
– ulidtko, Commented Aug 20, 2013 at 11:08

Willi Ballenthin · Accepted Answer · 2009-04-08 04:47:47Z

12

they are exactly the same.

someArray[i]

means exactly

*(someArray + i)

where someArray in the second case is a pointer to the memory location. Similarly,

&someArray[i]

means exactly

(someArray + i)

In both these cases the terms are pointers to memory locations.

answered Apr 8, 2009 at 4:47

Willi Ballenthin

6,6746 gold badges41 silver badges54 bronze badges

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

Comments

ojblass · Accepted Answer · 2009-04-08 04:37:36Z

12

The two are equivalent and I think SomeFunction(tmpry); is more readable.

answered Apr 8, 2009 at 4:37

ojblass

21.8k23 gold badges89 silver badges137 bronze badges

Comments

Alex B · Accepted Answer · 2009-04-08 10:04:25Z

7

It may be significant, if SomeFunction is a macro and takes "sizeof" of one of its arguments, because sizeof(tmpry) may not necessarily be equal to sizeof(&tmpry[0]).

Otherwise, as others pointed out, they are exactly the same.

edited Apr 8, 2009 at 10:04

answered Apr 8, 2009 at 5:00

Alex B

85.4k49 gold badges213 silver badges284 bronze badges

3 Comments

sipsorcery Over a year ago

In this case there is no macro involved. However how would a macro treat them differently? Doesn't a macro work like a string search and replace so the function it produced would be equivalent to a normal function anyway?

Alex B Over a year ago

Results will differ if the macro takes sizeof(macro argument).

dreamlax Over a year ago

#define Macro(x) RealFunctionCall(x,sizeof(x)) // For example

mctylr · Accepted Answer · 2009-04-09 00:16:23Z

5

The C Programming FAQ section on arrays and pointers tackles this (and many other common C questions and confusions.

answered Apr 9, 2009 at 0:16

mctylr

5,20923 silver badges34 bronze badges

Comments

Jonathan Leffler · Accepted Answer · 2009-04-08 05:09:33Z

3

As everyone else said, the two notations are equivalent.

I would normally use the simpler one, unless there are multiple calls like this:

SomeFunction(&tmparry[0]);
SomeFunction(&tmparry[10]);
SomeFunction(&tmparry[NNN]);

Where ideally all the constants (magic numbers) would be enum (or #define) constants.

answered Apr 8, 2009 at 5:09

Jonathan Leffler

759k145 gold badges961 silver badges1.3k bronze badges

2 Comments

hasen Over a year ago

well, even then, tmparry + 10 is better than &tmparray[10]

Jonathan Leffler Over a year ago

I disagree - but it is a subjective decision.

Naveen · Accepted Answer · 2009-04-08 04:40:47Z

1

Both are one and the same although second one looks nice and clarifies the intention of passing the array to the function. But how does the SomeFunction know the size of the array being passed, is it always assumed as 40 ? I feel it is better to pass the size also as the parameter to SomeFunction.

answered Apr 8, 2009 at 4:40

Naveen

73.8k47 gold badges182 silver badges237 bronze badges

3 Comments

Jason Coco Over a year ago

I agree, you should pass the size as well as the array.

Naveen Over a year ago

I agree if it is a new code, but if the code is already written it may not be possible to change the datastructures so easily..

Jason Coco Over a year ago

Also, we have no idea if C++ is an option since the question said C array and the tag is simply C

sharptooth · Accepted Answer · 2009-04-08 04:41:21Z

1

These two variants are equivalent and passing like this

SomeFunction(tmpry);

looks cleaner.

answered Apr 8, 2009 at 4:41

sharptooth

171k110 gold badges545 silver badges1k bronze badges

Comments

Jiang Bian · Accepted Answer · 2009-04-08 14:55:02Z

1

The declaration

int a[10]; int *pa;

There is one difference between an array and a pointer that must be kept in mind. A pointer is a variable, so pa=a and pa++ are legal. But an array name is not a variable; construction like a=pa and a++ are illegal

As format parameters in a function definition, char s[] and char *s are equivalent;

From: The C Programming Language 2th, Page 99-100

answered Apr 8, 2009 at 14:55

Jiang Bian

2,0833 gold badges17 silver badges14 bronze badges

1 Comment

David Over a year ago

To quote from page 99: "When an array name is passed to a function, what is passed is the location of the initial element"

Collectives™ on Stack Overflow

Passing C array as char* function parameter

8 Answers 8

Comments

Comments

3 Comments

Comments

2 Comments

3 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

Comments

Comments

3 Comments

Comments

2 Comments

3 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related