Q: Returning a matrix from a fuction in C I'm making a program where I have to work constantly with matrices in functions, this is one of the many functions, this function is supposed to take open an external file which is a data set, where the data is separated with tabulations, it opens the file and saves the data in a matrix M, I know this matrix is composed of 6 columns, but the row number is unknown,  I know the error is where I declare the matrix, it has to be declared with pointers since the function returns the matrix.
//type float** since it will return a matrix
float **carga_archivo(char *nombre_archivo)       
{
  float **M=(float **)malloc(6*sizeof(float*));  //error should be here
  int i=0;
  FILE *archivo;                      //FILE type pointer to open the external file
  archivo=fopen(nombre__archivo,"r"); //Opens the file in the address indicated
                                      //"nombre_de_archivo" is a variable
  while(!feof(archivo))               //Browses the file row per row till the end of it
  {
      //saves the data in its corresponding place in the matrix
      fscanf(archivo,"%f\t%f\t%f\t%f\t%f\t%f\n",
          &M[0][i],&M[1][i],&M[2][i],&M[3][i],&M[4][i],&M[5][i]);
      i++;
  }
  tam=i;
  fclose (archivo);                  //closes the file
  return M;
}

What I need is the right way to declare the matrix.
P.S. I documented the main lines in the code in case it could help someone who needs something similar. 
Any correction is welcome.
Update:
Applied some changes proposed in comments, and worked better, here  is the new code I nade for that function
float **carga_archivo(char *nombre_archivo)
{
int i=0;
float P[300][6];
FILE *archivo;
archivo=fopen(nombre_archivo,"r");
 while(!feof(archivo))
{
i++;
       //this was just so the feof function could browse row per row 
       //instead of character per character
    scanf("%f\t%f\t%f\t%f\t%f\t%f\n",
               &P[0][i],&P[1][i],&P[2][i],&P[3][i],&P[4][i],&P[5][i]);
    printf("%i\n",i);
}
tam=i;
printf("%i",tam);
int filas = 6;
int columnas = tam;
float **M;

M = (float **)malloc(filas*sizeof(float*));

for (i=0;i<filas;i++)
    M[i] = (float*)malloc(columnas*sizeof(float));

for (i = 0; i < columnas; ++i)
    fscanf(archivo,"%f\t%f\t%f\t%f\t%f\t%f\n",
             &M[0][i],&M[1][i],&M[2][i],&M[3][i],&M[4][i],&M[5][i]);
fclose (archivo);
return M;
}

The new problem is when the function is called, the program actually compiles, but when its running and the function is called the program crashes and stops.
Here Is the part of the code that  calls to that function.
int main()
{
int i,j;
char *nombre_archivo="Agua_Vapor.txt";
float **agua_vapor=carga_archivo(nombre_archivo);
for (i = 0; i < 6; i++)
{
    for (j = 0; i < tam; i++)
        printf("%f   ", agua_vapor[i][j]);
    printf("\n");
}
return 0;
}


A: Your program has undefined behaviour, because you're populating memory referenced by uninitialised pointers.
Since you know there are always 6 columns, a simple approach is to store the matrix as row-major instead of column-major (your example is column-major).  This means you can store the matrix data as one large piece of memory and use realloc when necessary.  You may want to make a simple struct for this too.
struct matrix {
    int rows, cols;
    float ** data;
};

Then create it dynamically.
struct matrix * matrix_alloc( int rows, int cols )
{
    int i;

    struct matrix * m = malloc(sizeof(struct matrix));
    m->rows = rows;
    m->cols = cols;
    m->data = malloc(rows * sizeof(float*));
    m->data[0] = malloc(rows * cols * sizeof(float));

    for( i = 1; i < rows; i++ ) {
        m->data[i] = m->data[i-1] + cols; 
    }
    return m;   
}

void matrix_free( struct matrix * m )
{
    free( m->data[0] );
    free( m->data );
    free( m );
}

Now, when you decide you need to add storage for more rows:
void matrix_set_row_dimension( struct matrix * m, int rows )
{
    float **new_index, *new_block;
    new_index = realloc(m->data, rows * sizeof(float**));
    new_block = realloc(m->data[0], rows * m->cols * sizeof(float));
    if( new_index && new_block )
    {
        int i = m->rows;
        m->rows = rows;
        m->data = new_index;

        /* if block address changed, prepare to reindex entire block */
        if( m->data[0] != new_block )
        {
            m->data[0] = new_block;
            i = 1;
        }

        /* reindex */
        for( ; i < rows; i++ ) {
            m->data[i] = m->data[i-1] + cols;
        }
    }
}

So, now as you populate the matrix...
struct matrix * m = matrix_alloc( 10, 6 );  /* Start with 10 rows */
int row = 0;
while( 1 ) {
    /* Double matrix row count if not large enough */
    if( row == m->rows )
    {
        matrix_set_row_dimension( m, m->rows * 2 );

        /* Check for error here */
    }

    /* Now the matrix has enough storage to continue adding */
    m->data[row][0] = 42;
    m->data[row][1] = 42;
    m->data[row][2] = 42;
    m->data[row][3] = 42;
    m->data[row][4] = 42;
    m->data[row][5] = 42;

    row++;     
}

