Undefined reference to 'printf' in ARM assembler for Android

Question

I am trying to follow an example that I found on a blog.

It doesn't seem that androideabi can call printf directly on assembler code level (maybe I am missing a flag when I compile?).

I am running with these commands:

arm-linux-androideabi-as -o fib.o fibonacci.s
arm-linux-androideabi-ld --sysroot $env:SYSROOT -s -o fib fib.o
fib.o(.text+0x8): error: undefined reference to 'printf'

I get an undefined reference from the assembly code.

This would be neat if it actually pulled off. I am of course open to alternative solution if anyone wants to share their awesome knowledge of low level programming!

Any tips for this problem? Code is below:

.syntax unified

.equ maxfib,4000000

previous .req r4
current  .req r5
next     .req r6
sum      .req r7
max      .req r8
tmp      .req r9

.section .rodata
 .align 2
fibstring:
 .asciz "fib is %d\n"
sumstring:
 .asciz "%d\n"

len = . - sumstring
.text
 .align 2
 .global main
 .extern printf
 .type main, %function
main:
 stmfd sp!, {r4-r9, lr}
 ldr max,   =maxfib
 mov previous, 1
 mov current, 1
 mov sum, 0

loop:
 cmp current, max
 bgt last

 add next, current, previous

 movs tmp, current, lsr 1      @ set carry flag from lsr - for the odd-valued terms
                               @ we discard the result of the movs and are only interested
                               @ in the side effect of the lsr which pushes the lower bit
                               @ of current (1 for odd; 0 for even) into the carry flag
                               @ movs will update the status register (c.f. mov which will not)
 addcc sum, sum, current       @ we add current to the sum ONLY when cc (carry clear) is true
                               @ these are even-valued fibonacci terms

 mov previous, current
 mov current, next
 b loop

last:
 mov r1, sum
 ldr r0, =sumstring            @ store address of start of string to r0
 bl         printf

 mov r0, 0
 ldmfd sp!, {r4-r9, pc}
 mov r7, 1                     @ set r7 to 1 - the syscall for exit
 swi 0                         @ then invoke the syscall from linux

Answer 1

You need to link to the C runtime library aka libc, in order to get printf. Either you can add -lc at the end of your linking command, or use the gcc frontend instead of using ld directly.

I.e., either do this:

arm-linux-androideabi-ld --sysroot $env:SYSROOT -s -o fib fib.o -lc

Or this:

arm-linux-androideabi-gcc --sysroot $env:SYSROOT -Wl,-s -o fib fib.o