The Ware House

uptime prototype

With the basic functionality available so far, I can write something in the vein of the Unix uptime command.

$ man -k uptime
uptime (1)           - Tell how long the system has been running.

I am going to make a quick prototype first to validate the concept.


I already have a one second based System Tick interrupt routine, so I just need to make sure it updates a count of seconds. I make a copy of usart1tx.c as uplow.1.c to make the changes. I use a number suffix for the filename as I anticipate making several revisions.

volatile unsigned uptime ;  /* seconds elapsed since boot */

#ifdef LED_ON
static void userLEDtoggle( void) {
    GPIO( LED_IOP)[ ODR] ^= 1 << LED_PIN ;   /* Toggle User LED */

void SysTick_Handler( void) {
    uptime += 1 ;
#ifdef LED_ON
    userLEDtoggle() ;

The global variable uptime is marked volatile, the compiler needs this information to avoid optimization as the value changes concurrently when an interrupt is triggered.

I move the user LED toggling code to a dedicated local function userLEDtoggle() as this is not the only task of SysTick_Handler() anymore and a call to toggle the LED is needed during initialization. I adjust the initialization code accordingly.

I write a first uptime.1.c to print the count of seconds every time the uptime counter value changes.

/* uptime.1.c -- tells how long the system has been running */
#include <stdio.h>

extern volatile unsigned uptime ;
extern void kputc( unsigned char c) ;

void kputu( unsigned u) {
    unsigned r = u % 10 ;
    u /= 10 ;
    if( u)
        kputu( u) ;

    kputc( '0' + r) ;

int main( void) {
    static unsigned last ;

    for( ;;)
        if( last != uptime) {
            last = uptime ;
            kputu( last) ;
            puts( " sec") ;
        } else
            __asm( "WFI") ; /* Wait for System Tick Interrupt */

As before for kputc(), the implementation of kputu() to print an unsigned integer in decimal format is not optimal but it is functional.


I update Makefile with the composition.

SRCS = startup.c uplow.1.c uptime.1.c

Unfortunately, when I try to build an executable, the link phase fails.

$ make
D:\Program Files (x86)\GNU Arm Embedded Toolchain\9 2020-q2-update\bin\arm-none-
eabi-ld.exe: uptime.1.o: in function `kputu':
D:\Projects\stm32bringup/uptime.1.c:8: undefined reference to `
D:\Program Files (x86)\GNU Arm Embedded Toolchain\9 2020-q2-update\bin\arm-none-
eabi-ld.exe: D:\Projects\stm32bringup/uptime.1.c:9: undefined r
eference to `__aeabi_uidiv'
make: *** [Makefile:55: f030f4.elf] Error 1

The compiler has generated code that references two functions __aeabi_uidivmod and __aeabi_uidiv when compiling the lines 8 and 9 of uptime.1.c.

    unsigned r = u % 10 ;
    u /= 10 ;

This happens because the compiler generates code for Cortex-M0 which has no integer division support. So integer division needs to be implemented by code as it is not supported by hardware.

I need to pass the linker a reference to GNU Arm Embedded Toolchain library for Cortex-M0. The library file is libggc.a, the option -l and -L of the linker tell what the library name is (-lgcc => libgcc.a) and where to look for it.

LIBDIR  = $(GCCDIR)/lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp
LIBS = -lgcc

$(PROJECT).elf: $(OBJS)
    @echo $@
    $(LD) -T$(LD_SCRIPT) $(LIB_PATHS) -Map=$(PROJECT).map -cref -o $@ $^ $(LIBS)
    $(SIZE) $@
    $(OBJDUMP) -hS $@ > $(PROJECT).lst

Once the Makefile has been updated, the build finish successfully.

$ make
   text    data     bss     dec     hex filename
    777       0      12     789     315 f030f4.elf

Checking the linker produced map file,, I can see which library (libgcc.a) but also which modules in the library (_udivsi3.o and _dvmd_tls.o) have been used to resolve the symbols (__aeabi_uidiv and __aeabi_idiv0).

Archive member included to satisfy reference by file (symbol)

D:/Program Files (x86)/GNU Arm Embedded Toolchain/9 2020-q2-update/lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp\libgcc.a(_udivsi3.o)
                              uptime.1.o (__aeabi_uidiv)
D:/Program Files (x86)/GNU Arm Embedded Toolchain/9 2020-q2-update/lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp\libgcc.a(_dvmd_tls.o)
                              D:/Program Files (x86)/GNU Arm Embedded Toolchain/9 2020-q2-update/lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp\libgcc.a(_udivsi3.o) (__aeabi_idiv0)


I flash the board and start execution, the output works as expected, the first line “1 sec” appears one second after reset with a new line following every second after that.

uptime v1 output

Publish and Retest on Linux

I push the changes to the git server then pull them back on my Linux machine for retesting. The build fails with an error reported by the linker.

arm-none-eabi-ld: cannot find -lgcc

The location of the Cortex-M0 libgcc.a library is in the same subfolder as in the Windows distribution. Only the reference to the location of the installation differs, I use absolute path for Windows and relative path (~/…) for Linux. The issue seems to be that ld doesn’t expand ~. So I let gmake handle the expansion.

#GCCDIR = ~/Packages/gcc-arm-none-eabi-9-2020-q2-update
 GCCDIR = $(HOME)/Packages/gcc-arm-none-eabi-9-2020-q2-update

Library management

With Cortex-M0 version of libgcc.a available I have some extra flexibility in handling usage of the library.

1 Work with a local copy of the gcc library.

  • copy libgcc.a locally
  • LIB_PATHS = -L.
  • LIBS = -lgcc

2 Work with a local copy of the modules extracted from the gcc library.

  • ar x libgcc.a _udivsi3.o _dvmd_tls.o
  • LIB_PATHS = -L.
  • LIBS = _udivsi3.o _dvmd_tls.o

3 Work with my own library made from the needed modules extracted from the gcc library.

  • ar qc libstm32.a _udivsi3.o _dvmd_tls.o
  • LIB_PATHS = -L.
  • LIBS = -lstm32

The ar command distributed by the GNU Arm embedded toolchain is the same GNU ar as the Linux distribution or Cygwin and MSYS2 on Windows. So I use my native environment implementation for convenience. This is true for the utility commands (ar, objcopy, objdump, size) but not for gcc and ld.


I have hacked a quick prototype of uptime and found an extra dependency to Gnu Arm Embedded Toolchain: some modules included in libgcc.a have to be included at link time as the chipset I am using has no support for integer division. At this stage I will reuse the library as it is, but I know where to look in the map file generated by the linker to find which modules are included. If I ever need a better control of the link phase, I can use ar to extract locally those modules from the library.

Next I will write uptime with a better structure.

Back to top