Not able to use register in C.

Gat Pelsinger · December 19, 2023

I came upon this register keyword in C and wanted to see how fast it is comparing to standard memory. Code-

#include <stdio.h>
#include <time.h>
#include <limits.h>

int main() {
    register unsigned long long x = 0ULL;
    clock_t start_time = clock();

    while (x < ULONG_LONG_MAX) {
        x++;
    }

    clock_t end_time = clock();

    printf("%llu\n", x);
    
    double elapsed_time = ((double)(end_time - start_time)) / CLOCKS_PER_SEC;
    printf("%lf seconds\n", elapsed_time);

    return 0;
}

But the problem is that I can't compile it without the optimization flags (o2 and o3). But this code compiles fine without any optimization flags, likely ignoring the register keyword. I use GCC-Mingw-w64 compiler. I get a LOT of errors when compiling with optimizations. If you want to read them-

C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.text+0x3d0): multiple definition of `WinMainCRTStartup'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.text+0x3d0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.text+0x3f0): multiple definition of `mainCRTStartup'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.text+0x3f0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.text+0x410): multiple definition of `atexit'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.text+0x410): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:cygming-crtbeg:(.text+0x430): multiple definition of `__gcc_register_frame'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/crtbegin.o:cygming-crtbeg:(.text+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:cygming-crtbeg:(.text+0x440): multiple definition of `__gcc_deregister_frame'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/crtbegin.o:cygming-crtbeg:(.text+0x10): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:main.c:(.text+0x4a4): multiple definition of `main'; C:\Users\ANISH_~1\AppData\Local\Temp\cciyTXdh.o:main.c:(.text+0x54): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x7b0): multiple definition of `.refptr.__mingw_initltsdrot_force'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__mingw_initltsdrot_force[.refptr.__mingw_initltsdrot_force]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x800): multiple definition of `.refptr.__native_startup_state'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__native_startup_state[.refptr.__native_startup_state]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.bss+0x0): multiple definition of `__mingw_module_is_dll'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.bss+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x750): multiple definition of `.refptr.__imp___initenv'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__imp___initenv[.refptr.__imp___initenv]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x8a0): multiple definition of `.refptr._matherr'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr._matherr[.refptr._matherr]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x870): multiple definition of `.refptr._dowildcard'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr._dowildcard[.refptr._dowildcard]+0x0): first defined
here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x7a0): multiple definition of `.refptr.__mingw_app_type'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__mingw_app_type[.refptr.__mingw_app_type]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x7e0): multiple definition of `.refptr.__mingw_oldexcpt_handler'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__mingw_oldexcpt_handler[.refptr.__mingw_oldexcpt_handler]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x840): multiple definition of `.refptr.__xi_a'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__xi_a[.refptr.__xi_a]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x8b0): multiple definition of `.refptr._newmode'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr._newmode[.refptr._newmode]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x7d0): multiple definition of `.refptr.__mingw_initltssuo_force'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__mingw_initltssuo_force[.refptr.__mingw_initltssuo_force]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x7f0): multiple definition of `.refptr.__native_startup_lock'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__native_startup_lock[.refptr.__native_startup_lock]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x730): multiple definition of `.refptr.__dyn_tls_init_callback'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__dyn_tls_init_callback[.refptr.__dyn_tls_init_callback]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x7c0): multiple definition of `.refptr.__mingw_initltsdyn_force'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__mingw_initltsdyn_force[.refptr.__mingw_initltsdyn_force]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x890): multiple definition of `.refptr._gnu_exception_handler'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr._gnu_exception_handler[.refptr._gnu_exception_handler]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.CRT+0x20): multiple definition of `__mingw_pcinit'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.CRT$XIAA+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x860): multiple definition of `.refptr._commode'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr._commode[.refptr._commode]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.CRT+0x8): multiple definition of `__mingw_pcppinit'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.CRT$XCAA+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x740): multiple definition of `.refptr.__image_base__'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__image_base__[.refptr.__image_base__]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x880): multiple definition of `.refptr._fmode'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr._fmode[.refptr._fmode]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x820): multiple definition of `.refptr.__xc_a'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__xc_a[.refptr.__xc_a]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x850): multiple definition of `.refptr.__xi_z'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__xi_z[.refptr.__xi_z]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x6f0): multiple definition of `.refptr._MINGW_INSTALL_DEBUG_MATHERR'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr._MINGW_INSTALL_DEBUG_MATHERR[.refptr._MINGW_INSTALL_DEBUG_MATHERR]+0x0): first defined here
C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: main.exe:crtexe.c:(.rdata+0x830): multiple definition of `.refptr.__xc_z'; C:/Users/USER/Downloads/x86_64-13.2.0-release-win32-seh-msvcrt-rt_v11-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.2.0/../../../../x86_64-w64-mingw32/lib/../lib/crt2.o:crtexe.c:(.rdata$.refptr.__xc_z[.refptr.__xc_z]+0x0): first defined here
collect2.exe: error: ld returned 1 exit status