Link error between C and assembly - c

Question

I'm compiling a c file foo.c:
#include <stdlib.h>
extern void *memcpy_optimized(void* __restrict, void* __restrict, size_t);
void foo() {
[blah blah blah]
memcpy_optimized((void *)a, (void *)b, 123);
}
then I have the assembly file memcpy_optimized.S:
.text
.fpu neon
.global memcpy_optimized
.type memcpy_optimized, %function
.align 4
memcpy_optimized:
.fnstart
mov ip, r0
cmp r2, #16
blt 4f # Have less than 16 bytes to copy
# First ensure 16 byte alignment for the destination buffer
tst r0, #0xF
beq 2f
tst r0, #1
ldrneb r3, [r1], #1
[blah blah blah]
.fnend
Both files compile fine with: gcc $< -o $# -c
but when I link the application with both resulting objects, I get the following error:
foo.c:(.text+0x380): undefined reference to `memcpy_optimized(void*, void *, unsigned int)'
Any idea what I'm doing wrong?
readelf -a obj/memcpy_optimized.o
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: ARM
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 436 (bytes into file)
Flags: 0x5000000, Version5 EABI
Size of this header: 52 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 40 (bytes)
Number of section headers: 11
Section header string table index: 8
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 00000000 000040 0000f0 00 AX 0 0 16
[ 2] .data PROGBITS 00000000 000130 000000 00 WA 0 0 1
[ 3] .bss NOBITS 00000000 000130 000000 00 WA 0 0 1
[ 4] .ARM.extab PROGBITS 00000000 000130 000000 00 A 0 0 1
[ 5] .ARM.exidx ARM_EXIDX 00000000 000130 000008 00 AL 1 0 4
[ 6] .rel.ARM.exidx REL 00000000 00044c 000010 08 9 5 4
[ 7] .ARM.attributes ARM_ATTRIBUTES 00000000 000138 000023 00 0 0 1
[ 8] .shstrtab STRTAB 00000000 00015b 000056 00 0 0 1
[ 9] .symtab SYMTAB 00000000 00036c 0000b0 10 10 9 4
[10] .strtab STRTAB 00000000 00041c 00002f 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
Relocation section '.rel.ARM.exidx' at offset 0x44c contains 2 entries:
Offset Info Type Sym.Value Sym. Name
00000000 0000012a R_ARM_PREL31 00000000 .text
00000000 00000a00 R_ARM_NONE 00000000 __aeabi_unwind_cpp_pr0
Unwind table index '.ARM.exidx' at offset 0x130 contains 1 entries:
0x0 <memcpy_optimized>: 0x80b0b0b0
Compact model 0
0xb0 finish
0xb0 finish
0xb0 finish
Symbol table '.symtab' contains 11 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 SECTION LOCAL DEFAULT 1
2: 00000000 0 SECTION LOCAL DEFAULT 2
3: 00000000 0 SECTION LOCAL DEFAULT 3
4: 00000000 0 NOTYPE LOCAL DEFAULT 1 $a
5: 00000000 0 SECTION LOCAL DEFAULT 4
6: 00000000 0 SECTION LOCAL DEFAULT 5
7: 00000000 0 NOTYPE LOCAL DEFAULT 5 $d
8: 00000000 0 SECTION LOCAL DEFAULT 7
9: 00000000 0 FUNC GLOBAL DEFAULT 1 memcpy_optimized
10: 00000000 0 NOTYPE GLOBAL DEFAULT UND __aeabi_unwind_cpp_pr0
No version information found in this file.
Attribute Section: aeabi
File Attributes
Tag_CPU_name: "7-A"
Tag_CPU_arch: v7
Tag_CPU_arch_profile: Application
Tag_ARM_ISA_use: Yes
Tag_THUMB_ISA_use: Thumb-2
Tag_FP_arch: VFPv3
Tag_Advanced_SIMD_arch: NEONv1
Tag_DIV_use: Not allowed

Answer 1

It seems to me that you compiled your foo.c as C++, hence the linking error. What made me say that is that the linker reported the full prototype of the missing function. C functions do not have their full prototype as their symbol (just the name of function), however the C++ mangled names represent the full prototype of the function.

Answer 2

In many Unix and GCC C implementations, names in C are decorated with an initial underscore in object code. So, to call memcpy_optimized in C, you must use the name _memcpy_optimized in assembly.