"expression must have a constant value" visual studio error E0028 - c

Question

#define max 40
...
void transpose(int matrix[][max], int* row, int* col)
{
int data[*row][max]; //expression must have a constant value at *row.
for (int i = 0; i < *row; i++)
{
for (int j = 0; j < *col; j++)
{
data[i][j] = matrix[i][j];
}
}
int _col = *row; //this *row works fine.
*row = *col; //also this *row works fine.
*col = _col;
for (int i = 0; i < *row; i++) //this *row is fine too.
{
for (int j = 0; j < *col; j++)
{
matrix[i][j] = data[j][i];
}
}
}
int main()
{
...
if (...)
{
int row = 0, col = 0;
int matrix[30][max];
if (FunctionReadFile(Parameters[0], matrix, &row, &col))
{
...
transpose(matrix, &row, &col);
...
}
...
}
...
return 0;
}
I tried put 'const' before int but it still show this error at [*row], why does this error occured and how to fix it? Does declaring an dynamic array is the only to fix this problem, any possible solution easier?

Answer 1

Your compiler does not support VLA:s, so you should use dynamic allocation:
void transpose(int matrix[][max], int* row, int* col)
{
// Parenthesis matters. This is a pointer to array of size max. Without
// the parenthesis, it would be an array of pointers to int.
int (*data)[max] = malloc(max * sizeof (*data));
for (int i = 0; i < *row; i++)
{
for (int j = 0; j < *col; j++)
{
data[i][j] = matrix[i][j];
}
}
free(data);
}
Do note that I did not check if the allocation failed. You can do that with a simple check. If the pointer is NULL, then the allocation failed. Also, remember to free the memory when you're done with it as shown.

Answer 2

You can use _alloca() or _malloca() in MSVC for stack allocation.
Return Value
The _alloca routine returns a void pointer to the allocated space,
which is guaranteed to be suitably aligned for storage of any type of
object. If size is 0, _alloca allocates a zero-length item and
returns a valid pointer to that item.
A stack overflow exception is generated if the space cannot be
allocated. The stack overflow exception is not a C++ exception; it is
a structured exception. Instead of using C++ exception handling, you
must use Structured Exception Handling (SEH).
Remarks
_alloca allocates size bytes from the program stack. The allocated
space is automatically freed when the calling function exits (not when
the allocation merely passes out of scope). Therefore, do not pass the
pointer value returned by _alloca as an argument to free.
There are restrictions to explicitly calling _alloca in an exception
handler (EH). EH routines that run on x86-class processors operate in
their own memory frame: They perform their tasks in memory space that
is not based on the current location of the stack pointer of the
enclosing function. ...
For example:
void transpose(int matrix[][max], int* row, int* col)
{
int ( *data )[ max ] = _alloca( max * sizeof( *data ) );
for (int i = 0; i < *row; i++)
{
for (int j = 0; j < *col; j++)
{
data[i][j] = matrix[i][j];
}
}
...
Or, with _malloca(), which requires you to call _freea():
void transpose(int matrix[][max], int* row, int* col)
{
int ( *data )[ max ] = _malloca( max * sizeof( *data ) );
for (int i = 0; i < *row; i++)
{
for (int j = 0; j < *col; j++)
{
data[i][j] = matrix[i][j];
}
}
...
_freea( data );
...