System Calls - 2022.1 English - UG1399

System calls cannot be synthesized because they are actions that relate to performing some task upon the operating system in which the C/C++ program is running.

Vitis HLS ignores commonly-used system calls that display only data and that have no impact on the execution of the algorithm, such as printf() and fprintf(stdout,). In general, calls to the system cannot be synthesized and should be removed from the function before synthesis. Other examples of such calls are getc(), time(), sleep(), all of which make calls to the operating system.

Vitis HLS defines the macro __SYNTHESIS__ when synthesis is performed. This allows the __SYNTHESIS__ macro to exclude non-synthesizable code from the design.

In the following code example, the intermediate results from a sub-function are saved to a file on the hard drive. The macro __SYNTHESIS__ is used to ensure the non-synthesizable files writes are ignored during synthesis.

#include "hier_func4.h"

int shift_func(dint_t *in1, dint_t *in2, dout_t *outA, dout_t *outB)
{
 *outA = *in1 >> 1;
 *outB = *in2 >> 2;
}

void hier_func4(din_t A, din_t B, dout_t *C, dout_t *D)
{
 dint_t apb, amb;

 sumsub_func(&A,&B,&apb,&amb);
#ifndef __SYNTHESIS__
 FILE *fp1; // The following code is ignored for synthesis
 char filename[255];
 sprintf(filename,Out_apb_%03d.dat,apb);
 fp1=fopen(filename,w);
 fprintf(fp1, %d \n, apb);
 fclose(fp1);
#endif
 shift_func(&apb,&amb,C,D);
}

The __SYNTHESIS__ macro is a convenient way to exclude non-synthesizable code without removing the code itself from the function. Using such a macro does mean that the code for simulation and the code for synthesis are now different.