how to pass arrays between functions

Question

Can someone help me? I having a problem with a C program. Here it is:

In my main I call a function (func A) where two of the arguments are a second function (fun B) and "user data" (that in principle can be a single number, char or array). This "user data" is also an argument of the function B. I have everything working when the "user data" is a single integer but now I need to use it as an array. So the present working structure is like this:

static int FunB(...,void *userdata_)  
{  
   int *a=userdata_;  
   ...  
   (here I use *a that in this case will be 47)  
   ...  
}  

int main()  
{  
   int b=47;  
   funcA(...,FunB,&b)  
}

So now I want b in the main as an array (for example {3,45} ) in order to pass more that one single "data" to the function B.

Thanks

AnT stands with Russia · Accepted Answer · 2011-04-16 01:22:39Z

2

There are at least two ways to do it.

First

static int FunB(..., void *userdata_)  
{  
   int *a = userdata_;  
   /* Here `a[0]` is 3, and `a[1]` is 45 */
   ...  
}  

int main()  
{  
   int b[] = { 3, 45 };  
   funcA(..., FunB, b); /* Note: `b`, not `&b` */
}

Second

static int FunB(..., void *userdata_)  
{  
   int (*a)[2] = userdata_;  
   /* Here `(*a)[0]` is 3, and `(*a)[1]` is 45 */
   ...  
}  

int main()  
{  
   int b[] = { 3, 45 };  
   funcA(..., FunB, &b); /* Note: `&b`, not `b` */
}

Chose which one you like more. Note that the second variant is specifically tailored for the situations when the array size is fixed and known at compile time (exactly 2 in this case). In such situations the second variant is actually preferable.

If the array size is not fixed then you have to use the first variant. And of course you have to pass that size to FunB somehow.

Pay attention how the array is passed to funcA (either as b or as &b) and how it is accessed in FunB (either as a[i] or as (*a)[i]) in both variants. If you fail to do it properly the code might compile but won't work.

edited Apr 16, 2011 at 1:22

answered Apr 16, 2011 at 1:16

AnT stands with Russia

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3 Comments

aschepler Over a year ago

Isn't (void*) b == (void*) &b?

AnT stands with Russia Over a year ago

@aschepler: It is. Yet, purely conceptually I see no reason to introduce a disagreement between the points where the array is passed and where it is received and used. If you expect to receive a pointer-to-array, you should pass a pointer-to-array. If you expect to receive a pointer-to-element, you should pass a pointer-to-element.

aschepler Over a year ago

Yes, I understand why you wrote the two examples the way you did. I was just confused by "If you fail to do it properly the code might compile but won't work."

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