System Calls - System Calls - 2026.1 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.

AMD 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.