Error: Invalid use of void - c++

Question

I'm compiling C++ source (main.cpp) with a C header (hps_linux.h). The code in hps_linux.h is:
#ifndef HPS_LINUX_H_
#define HPS_LINUX_H_
#include <stdbool.h>
#include <stdint.h>
#include "socal/hps.h"
int fd_dev_mem = 0;
void *h2f_lw_axi_master = NULL;
size_t h2f_lw_axi_master_span = ALT_LWFPGASLVS_UB_ADDR -ALT_LWFPGASLVS_LB_ADDR + 1;
size_t h2f_lw_axi_master_ofst = ALT_LWFPGASLVS_OFST;
#endif
hps_linux.h includes hps.h, that has the next defines:
#define ALT_LWFPGASLVS_OFST 0xff200000
#define ALT_LWFPGASLVS_ADDR ALT_CAST(void *, (ALT_CAST(char *, ALT_HPS_ADDR) + ALT_LWFPGASLVS_OFST))
#define ALT_LWFPGASLVS_LB_ADDR ALT_LWFPGASLVS_ADDR
#define ALT_LWFPGASLVS_UB_ADDR ALT_CAST(void *, ((ALT_CAST(char *, ALT_LWFPGASLVS_ADDR) + 0x200000) - 1))
My main.cpp includes hps_linux.h . I'm compiling like this:
gcc -Wall -std=gnu99 hps_linux.c -o hps_linux.o
g++ -Wall -std=c++0x main.cpp -o main
And it throws the next error:
hps_linux.h: error: invalid use of 'void'
In line:
size_t h2f_lw_axi_master_span = ALT_LWFPGASLVS_UB_ADDR -ALT_LWFPGASLVS_LB_ADDR + 1;
When I compile it with a main written in C (main.c), it works.

Answer 1

Looking at the source code here, we have:
#ifdef __ASSEMBLY__
# define ALT_CAST(type, ptr) ptr // <-- not (ptr)???
#else
# define ALT_CAST(type, ptr) ((type) (ptr))
#endif /* __ASSEMBLY__ */
Now, we have:
size_t h2f_lw_axi_master_span = ALT_LWFPGASLVS_UB_ADDR - ALT_LWFPGASLVS_LB_ADDR + 1;
Partially expanding the macros, we have:
size_t h2f_lw_axi_master_span = ALT_CAST(void *, ALT_CAST(char *, ...)) - ALT_CAST(void *, ALT_CAST(char *, ...)) + 1;
Now, if __ASSEMBLY__ is set, this line is:
size_t h2f_lw_axi_master_span = ... - ... + 1;
Both ... expressions are integer, so this is OK. However, if __ASSEMBLY__ is not set, this line is:
size_t h2f_lw_axi_master_span = (void *) (...) - (void *) (...) + 1;
Subtracting two void pointers is not defined, so the compiler gives up.
Therefore, you need to ensure __ASSEMBLY__ is defined; one way would be:
g++ -Wall -D__ASSEMBLY__ -std=c++0x main.cpp -o main
However, this might cause problems as you should be relying on the earlier header files to set it correctly.
Update: A quick search of the git archive does not show __ASSEMBLY__ being set, and given the name it suggests it should be a compiler built-in...