Use malloc with structure

Question

I'm using malloc with structures but get some errors like

Error 1 error C2440: 'initializing' : cannot convert from 'void *' to 'my_vector *' c:\\lab3\\lab3\\linalg.cpp 19 lab3

I'm making MPI application and set all needed settings. I'm tried some solution but it doesn't helps.

linalg.cpp

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

#include <mpi.h>

#include "linalg.h"

void fatal_error(const char *message, int errorcode)
{
  printf("fatal error: code %d, %s\n", errorcode, message);
  fflush(stdout);
  MPI_Abort(MPI_COMM_WORLD, errorcode);
}

struct my_vector *vector_alloc(int size, double initial)
{
  struct my_vector *result = malloc(sizeof(struct my_vector) +
                                    (size-1) * sizeof(double));
  result->size = size;

  for(int i = 0; i < size; i++)
  {
    result->data[i] = initial;
  }

  return result;
}

void vector_print(FILE *f, struct my_vector *vec)
{
  for(int i = 0; i < vec->size; i++)
  {
    fprintf(f, "%.15lf ", vec->data[i]);
  }
  fprintf(f, "\n");
}

struct my_matrix *matrix_alloc(int rows, int cols, double initial)
{
  struct my_matrix *result = malloc(sizeof(struct my_matrix) + 
                                    (rows * cols - 1) * sizeof(double));

  result->rows = rows;
  result->cols = cols;

  for(int i = 0; i < rows; i++)
  {
    for(int j = 0; j < cols; j++)
    {
      result->data[i * cols + j] = initial;
    }
  }

  return result;
}

void matrix_print(FILE *f, struct my_matrix *mat)
{
  for(int i = 0; i < mat->rows; i++)
  {
    for(int j = 0; j < mat->cols; j++)
    {
      fprintf(f, "%lf ", mat->data[i * mat->cols + j]);
    }
    fprintf(f, "\n");
  }
}

struct my_matrix *read_matrix(const char *filename)
{
  FILE *mat_file = fopen(filename, "r");
  if(mat_file == NULL)
  {
    fatal_error("can't open matrix file", 1);
  }

  int rows;
  int cols;
  fscanf(mat_file, "%d %d", &rows, &cols);

  struct my_matrix *result = matrix_alloc(rows, cols, 0.0);
  for(int i = 0; i < rows; i++)
  {
    for(int j = 0; j < cols; j++)
    {
      fscanf(mat_file, "%lf", &result->data[i * cols + j]);
    }
  }

  fclose(mat_file);
  return result;
}

struct my_vector *read_vector(const char *filename)
{
  FILE *vec_file = fopen(filename, "r");
  if(vec_file == NULL)
  {
    fatal_error("can't open vector file", 1);
  }

  int size;
  fscanf(vec_file, "%d", &size);

  struct my_vector *result = vector_alloc(size, 0.0);
  for(int i = 0; i < size; i++)
  {
    fscanf(vec_file, "%lf", &result->data[i]);
  }

  fclose(vec_file);
  return result;
}

void write_vector(const char *filename, struct my_vector *vec)
{
  FILE *vec_file = fopen(filename, "w");
  if(vec_file == NULL)
  {
    fatal_error("can't open vector file", 1);
  }

  vector_print(vec_file, vec);

  fclose(vec_file);
}

I have problem at this place

struct my_vector *result = malloc(sizeof(struct my_vector) +
                                    (size-1) * sizeof(double));

Answer 1

Your code is probably being compiled with a C++ compiler. The fact that your code is named ".cpp" is also quite telling, that's the typical extension for C++.

In C++, you cannot convert from void * to other pointers without a cast:

my_vector *result = (my_vector *) malloc(...);

but you can drop the struct since that's implicit in C++. Also, in C++ you're supposed to use new :

my_vector *result = new my_vector[...];

which does away with the need to cast.

As you can see, C and C++ are not the same . You need a C compiler, if you want to program in C.

Answer 2

malloc returns void * . In C++ you must cast it to the appropriate pointer type:

struct my_vector *result = (struct my_vector *)malloc(...)

Answer 3

struct my_matrix *result = malloc(sizeof(struct my_matrix) + 
                                (rows * cols - 1) * sizeof(double));

In C++ there is no implicit conversion from void * to other pointer types, so you will have to cast the value returned by malloc . Note that you probably should use new[] instead.

This line suggests that you intend to write beyond the bounds of an array within struct my_matrix . That would cause undefined behaviour. It is possible to design your MPI structures to not use this so-called "struct hack". For some examples that you can model your design from, take a look at the headers of an open-source MPI library.

NB. I see you tagged this post as "C" and "C++". Please decide which language you are using. Trying to write code that is source-compatible between two different languages is a terrible idea. Headers, yes, function implementations, no.

Answer 4

The code looks like a legal C, so it will compile with a C compiler. However, it won't in C++. C++ has a stronger type checking rule. Casting the result of malloc (which is void* ) to the appropriate pointer type should work, however.

Answer 5

You are compiling your C code as C++ code. In C++, you need a cast for your specific situation (see the other answers for examples), whereas you don't need it in C.

Rename your files so they have *.c extensions and it should work.