malloc of a structure in C ( ported from C++ )

Question

I would like to reserve some memory space on the heap and access it with a pointer.

The code run fine in C++ but I cannot compile it in C.

#include <string.h>
#include <stdlib.h>

#define IMG_WIDTH 320

struct cluster_s
{
  uint16_t size;
  uint16_t xMin;
  uint16_t xMax;
  uint16_t yMin;
  uint16_t yMax;
};

static struct cluster_s* detectPills(const uint16_t newPixel[])
{

  static struct cluster_s **pixel = NULL;
  static struct cluster_s *cluster = NULL;

  if(!pixel){
    pixel = (cluster_s**) malloc(IMG_WIDTH * sizeof(struct cluster_s*));
    if(pixel == NULL){
      return NULL;
    }
  }
  if(!cluster){
    cluster = (cluster*) malloc((IMG_WIDTH+1) * sizeof(struct cluster_s));
    if(cluster == NULL){
      return NULL;
    }
    for(int i=0; i<IMG_WIDTH;i++){
      memset(&cluster[i], 0, sizeof(cluster[i]));
      pixel[i] = &cluster[i];
    }
  }
(...)
}

which gives me the following compilation error:

error: 'cluster_s' undeclared (first use in this function) pixel = (cluster_s**) malloc(IMG_WIDTH * sizeof(struct *cluster_s));

If I comment out the two malloc calls, I am able to compile it. I also tried to remove the cast before malloc and got the compilation error:

In function _sbrk_r': sbrkr.c:(.text._sbrk_r+0xc): undefined reference to_sbrk' collect2: error: ld returned 1 exit status

EDIT: The proposed answers are correct, the problem comes from the linker which do not find sbrk

Answer 1

This

I also tried to remove the cast before malloc and got the compilation error:

and this

The code run fine in C++ but I cannot compile it in C.

contradict each other.

The first one means that you are trying to compile the program as a C++ program.

To make the program to compile as a C++ program and as a C program there are two approaches.

The first one is everywhere in the program to use the type specifier struct cluster_s instead of just cluster_s . For example

pixel = (struct cluster_s**) malloc(IMG_WIDTH * sizeof(struct cluster_s*));
         ^^^^^^^^^^^^^^^^  
//...
cluster = (struct cluster*) malloc((IMG_WIDTH+1) * sizeof(struct cluster_s));
           ^^^^^^^^^^^^^^

The second one is to introduce an alias for the type specifier struct cluster_s like

typedef struct cluster_s cluster_s;

Answer 2

Replace

struct cluster_s
{
   ...
};

with

typedef struct cluster_s 
{
    ....
}cluster_s;

In C++, a struct and a Class are analogous and can be used interchangeably for the most part. So cluster_s and struct cluster_s can both be used.

In C, cluster_s is not defined. The change above will define a type of the same name.

You can see the answer When should you use a class vs a struct in C++? for the differences between a class and a struct.