How to return an array in c - c

Question

I know this question has been asked before but cannot find it in the same manner as I will describe it here:
Its all about returning an one-dimensional array in c-language. In java its very easy:
double[] myFunction() {
double[] v = new double[10];
return v;
}
I know that the implementation in c is not the same. But as an array element can be considered as a pointer to the first element in that array I thought one could do the following implementation:
double (*myFunction()) {
double v[10];
return v;
}
This compiles fine in gcc but when I make a call of the function I get a compilation error.
SO my Question - how does one return a one-dimensional vector in c-language?
Thanks

Answer 1

sample code
#include <stdio.h>
#include <stdlib.h>
double *myFunction1(){
return malloc(10*sizeof(double));
}
double (*myFunction2())[10]{
double (*p)[10];
p = malloc(sizeof(double[10]));
return p;
}
typedef struct d10 {
double vec[10];
} D10;
D10 myFunction3(){//Make little sense
D10 v = {{0}};
v.vec[9]=1.25;
return v;
}
int main(){
double *v1 = myFunction1();
double (*v2)[10] = myFunction2();
D10 v3= myFunction3();
//do something
printf("%lf\n", v3.vec[9]);
v1[0] = 3.14;
(*v2)[0] = 3.14 * 2;
free(v1);
free(v2);
return 0;
}

Answer 2

Corrected:
You can't pass an array and you can't return an array you only pass an address to it or return a pointer to it first element:
double *someFunction()
{
double *output;
p = (double *)malloc(size*sizeof(double)); // size should be defined
//... do some process with the content pointed to by output
return output; // return the address pointed to by output
}
Or pass a pointer to it first element and do some process on the content pointed to.
void someFunction(double *output, int size)
{
//... do some process with the content pointed to by output
}

Answer 3

For one, declaring v in the function makes the array live only in that function. Once the function returns, that array is popped off the stack, and is likely to be modified after successive function calls.
The proper C solution is to use malloc to allocate an amount of space that you can use as an array, and return the pointer to that array.
double * myFunc(){
double * v = malloc(10*size of(double));
return v;
}
In Java, it was safe to return an array because the garbage collector could detect that the array was still in use, after it was popped off the stack.

Answer 4

how does one return a one-dimensional vector in c-language?
Technically speaking, one doesn't. C functions cannot return array types.
The main problem you are running into is that, in the C code, v only exists for the lifetime of the function; once the function exits, v no longer exists, so any pointer you return will not be valid.
For this to work, you'll have to do something similar to what the Java code is doing; you'll need to allocate the memory for the array from the heap (i.e., allocate it dynamically):
double *myfunction()
{
double *v = malloc( sizeof *v * 10 );
return v;
}
Note that C does not clean up after you, so you'll have to explicitly free that memory when you're done with it, such as:
int main( void )
{
double *arr = myfunction();
/**
* do stuff with arr
*/
free( arr );
return 0;
}