Function with pointer arguments ANSI C

Question

I tried many different ways - and none seems to be the right one...

I want to pass the pointers up, down, left, right - so that I can change their values in the called function. Somehow I can't figure out the right syntaxes. And I couldn't find it either - as in I did try the suggestions I found in my textbooks and on the net, but... no luck.

//long find_neighbours( long, long *up, long *down, long *left, long *)
long find_neighbours( long, long, long, long, long);                  
int main (int argc, char *farg[])
{
    ...
    neighbours = find_neighbours(i, &up, &down, &left, &right);
    ...
}

//long find_neighbours( long center_number, long *up, long *down, long *right) 

long find_neighbours( long, long, long, long, long)
long center_number, *up, *down, *left, *right;
{
long x, y, neighbours;
 y = (long)(center_number/Lmax);
 x = center_number - (y*Lmax);

 if( !( x % center_number) ) //lqwa stena
 {
     *left = center_number + Lmax;
     *right = center_number+1;
 }   
 else
 {
    if( !((x+1)%center_number) ) //dqsna stena
    {
        *left = center_number-1;
        *right = center_number-Lmax;
    }
    else
    {   
        *left = center_number - 1;
        *right = center_number + 1;
    }
 }   

 if( y == 0 ) //lqwa stena
 {
     *up = 1 + x;
     *down = Lmax1 + x;
 }
 else
 {   
    if( y == Lmax1 ) //dqsna stena
    {
        *up = x;
        *down = Lmax - 2 +x;
    }
    else
    {   
        *down = center_number - Lmax;
        *up = center_number + Lmax;  
    }
 }   

 neighbours = Numb[*left] + Numb[*right] + Numb[*up] + Numb[*down];

 return (neighbours);
}


>>>$ gcc -lm perco_concCyclingv2.c -o square_clusters

>>>perco_concCyclingv2.c: In function ‘main’:
>>>perco_concCyclingv2.c:183:20: warning: passing argument 2 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                    neighbours = find_neighbours(i, &up, &down, &left, &right);
 >>>                   ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>> long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c:183:20: warning: passing argument 3 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                    neighbours = find_neighbours(i, &up, &down, &left, &right);
>>>                    ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>> long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c:183:20: warning: passing argument 4 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                    neighbours = find_neighbours(i, &up, &down, &left, &right);
>>>                    ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>> long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c:183:20: warning: passing argument 5 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                    neighbours = find_neighbours(i, &up, &down, &left, &right);
>>>                   ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>> long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c:235:21: warning: passing argument 2 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                     neighbours = find_neighbours(i, &up, &down, &left, &right);
>>>                     ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>> long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c:235:21: warning: passing argument 3 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                     neighbours = find_neighbours(i, &up, &down, &left, &right);
>>>                     ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>>long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c:235:21: warning: passing argument 4 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                     neighbours = find_neighbours(i, &up, &down, &left, &right);
>>>                     ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>> long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c:235:21: warning: passing argument 5 of ‘find_neighbours’ makes >>>integer from pointer without a cast [enabled by default]
>>>                     neighbours = find_neighbours(i, &up, &down, &left, &right);
>>>                     ^
>>>perco_concCyclingv2.c:20:6: note: expected ‘long int’ but argument is of type ‘long int >>>**’
>>> long find_neighbours( long, long, long, long, long);
>>>      ^
>>>perco_concCyclingv2.c: In function ‘find_neighbours’:
>>>perco_concCyclingv2.c:278:6: error: old-style parameter declarations in prototyped >>>function definition
>>> long find_neighbours( long, long, long, long, long)
>>>      ^
>>>perco_concCyclingv2.c:278:1: error: parameter name omitted
>>>long find_neighbours( long, long, long, long, long)
>>> ^
>>>perco_concCyclingv2.c:278:1: error: parameter name omitted
>>>perco_concCyclingv2.c:278:1: error: parameter name omitted
>>>perco_concCyclingv2.c:278:1: error: parameter name omitted
>>>perco_concCyclingv2.c:278:1: error: parameter name omitted
>>>perco_concCyclingv2.c:283:13: error: ‘center_number’ undeclared (first use in this >>>function)
>>>  y = (long)(center_number/Lmax);
>>>             ^
>>>perco_concCyclingv2.c:283:13: note: each undeclared identifier is reported only once >>>for each function it appears in
>>>perco_concCyclingv2.c:288:7: error: ‘left’ undeclared (first use in this function)
>>>      *left = center_number + Lmax;
>>>       ^
>>>perco_concCyclingv2.c:289:7: error: ‘right’ undeclared (first use in this function)
>>>      *right = center_number+1;
>>>       ^
>>>perco_concCyclingv2.c:307:7: error: ‘up’ undeclared (first use in this function)
>>>      *up = 1 + x;
>>>       ^
>>>perco_concCyclingv2.c:308:7: error: ‘down’ undeclared (first use in this function)
>>>      *down = Lmax1 + x;
>>>

Please, help... of any kind? Thank you...

Answer 1

Thanks - I found what I was doing wrong, after reading... everything, I didn't read - all times - the only thing that was wrong - and didn't even look at how I declared the variables in the first place. "Because it is so simple".

in "main":

long *x, *y, left, right, up, down, xp, yp; // the right way

long *x, *y, *left, *right, *up, *down, *xp, *yp; // the way it was left... 

I got totally confused by the errors and warnings - and never got back to the declaration line.

I'm sorry to have waisted your time. Thanks, everyone.

Answer 2

Whenever you need a function to modify its parameter, keep the following in mind:

1. Declare the function so that it accepts the type, the pointer (*) and the name of the variable.
2. Within the function, prefix assignments to the variable with *.
3. Within the callers of the function, prefix the variable with & when passing it into the function.

Here's a code snippet to demonstrate. It defines a function that sets its parameter to 100. It then calls this function on a variable whose value was 0, and prints the value of this variable after the call to show that it indeed was modified.

int f(int* x) {
    *x = 100;
}

int main(void) {
    int q = 0;
    f(&q);
    printf("%d\n", q);
    return 0;
}

I hope this helps.