2.7: C Library

So far I have used three Standard C library functions for output: printf(), putchar() and puts(). It’s time to bundle them as a library. This will give me more flexibility as I will not have to give a full list of the modules to link, the linker will handle the missing dependencies by looking into the libraries.

puts()

I have already packaged printf() and putchar() in stand alone modules. As I have removed my previous implementation of puts() from the system, I need to create puts.c.

/* puts.c -- write a string to stdout   */

#include <stdio.h>
#include "system.h" /* kputc(), kputs() */

int puts( const char *s) {
    kputs( s) ;
    kputc( '\n') ;
    return 0 ;
}

Updating Makefile

I need to tell GNU make how to manage and use the library, which means updating Makefile.

What’s the name, the content and a rule to maintain the library.

AR      = $(BINPFX)ar

LIBOBJS = printf.o putchar.o puts.o
LIBSTEM = stm32

lib$(LIBSTEM).a: $(LIBOBJS)
    $(AR) rc $@ $?

Where to look for and which libraries to use in the link phase.

LIB_PATHS = -L. -L$(LIBDIR)
LIBS = -l$(LIBSTEM) -lgcc

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

Library modules are implicitly part of the composition, so it’s not necessary to list them anymore.

#SRCS = startup.c uplow.2.c uptime.c printf.c putchar.c
SRCS = startup.c uplow.2.c uptime.c

I include libraries in the list of files to delete when doing a make clean.

clean:
    @echo CLEAN
    @rm -f *.o *.elf *.map *.lst *.bin *.hex *.a

Building uptime

Build terminates successfully producing the same executable as before.

$ make
f030f4.elf
   text    data     bss     dec     hex filename
   1792       0      12    1804     70c f030f4.elf
f030f4.hex
f030f4.bin

Checking the map produced by the linker I can see that it fetched the necessary modules for printf() and putchar() from the newly created library.

Archive member included to satisfy reference by file (symbol)

.\libstm32.a(printf.o)        uptime.o (printf)
.\libstm32.a(putchar.o)       uptime.o (putchar)
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(_thumb1_case_sqi.o)
                              .\libstm32.a(printf.o) (__gnu_thumb1_case_sqi)
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.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(_divsi3.o)
                              uptime.o (__aeabi_idiv)
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)

Building hello

I can rebuild my hello application using the latest system implementation and the newly made library.

SRCS = startup.c uplow.2.c hello.c

Build terminates successfully, the changes in size are due to the difference in the system implementation.

$ make
f030f4.elf
   text    data     bss     dec     hex filename
    445       0       8     453     1c5 f030f4.elf
f030f4.hex
f030f4.bin

Checking the map file produced in the link phase, I can see that only puts.o has been fetched from my local library.

Archive member included to satisfy reference by file (symbol)

.\libstm32.a(puts.o)          hello.o (puts)

Checkpoint

I had to deal with linking with gcc library before, so introducing my own library implementation of the standard C library output functions is a simple step.

Next I will continue on the topic of asynchronous serial transmission and look into baud rate and clock configuration.