You can use a constant such as const int,
or constexpr with pragmas or directives. For
example:
const int MY_DEPTH=1024;
#pragma HLS stream variable=my_var depth=MY_DEPTH
You can also use macros in the C code to implement this functionality. The key to using macros is to use a level of hierarchy in the macro. This allows the expansion to be correctly performed. The code can be made to compile as follows:
#include <hls_stream.h>
using namespace hls;
#define PRAGMA_SUB(x) _Pragma (#x)
#define PRAGMA_HLS(x) PRAGMA_SUB(x)
#define STREAM_IN_DEPTH 8
void foo (stream<int> &InStream, stream<int> &OutStream) {
// Legal pragmas
PRAGMA_HLS(HLS stream depth=STREAM_IN_DEPTH variable=InStream)
#pragma HLS stream depth=8 variable=OutStream
}