More GCC link time issues: undefined reference to main - c

Question

I'm writing software for a Cortex-A8 processor and I have to write some ARM assembly code to access specific registers. I'm making use of the GNU compilers and related toolchains; these tools are installed on the processor board (Freescale i.MX515) with Ubuntu. I make a connection to it from my host PC (Windows) using WinSCP and the PuTTY terminal.
As usual I started with a simple C project having main.c and functions.s. I compile the main.c using GCC, assemble the functions.s using as and link the generated object files using once again GCC, but I get strange errors during this process.
An important finding -
Meanwhile, I found out that my assembly code may have some issues because when I individually assemble it using the command as -o functions.o functions.s and try running the generated functions.o using ./functions.o command, the Bash shell is failing to recognize this file as an executable (on pressing tab functions.o is not getting selected/PuTTY is not highlighting the file).
Can anyone suggest what's happening here? Are there any specific options I have to send, to GCC during the linking process? The errors I see are strange and beyond my understanding, I don't understand to what the GCC is referring.
I'm pasting here the contents of main.c, functions.s, the Makefile and the list of errors.
Help, please!!!
**Latest errors included after the makefile was edited as suggested here **
ubuntu#ubuntu-desktop:~/Documents/Project/Others/helloworld$ make
gcc -c -mcpu=cortex-a8 main.c
as -mcpu=cortex-a8 -o functions.o functions.s
gcc -o hello main.o functions.o
functions.o: In function `_start':
(.text+0x0): multiple definition of `_start'
/usr/lib/gcc/arm-linux-gnueabi/4.3.3/../../../crt1.o:init.c:(.text+0x0): first defined here
collect2: ld returned 1 exit status
make: *** [hello] Error 1
main.c
#include <stdio.h>
#include <stdlib.h>
int main(void) {
puts("!!!Hello World!!!"); /* prints !!!Hello World!!! */
return EXIT_SUCCESS;
}
functions.s
* Main program */
.equ STACK_TOP, 0x20000800
.text
.global _start
.syntax unified
_start:
.word STACK_TOP, start
.type start, function
start:
movs r0, #10
movs r1, #0
.end
Makefile
all: hello
hello: main.o functions.o
gcc hello -o main.o functions.o
-- hello was included here after suggested here by guys at Stack Overflow, but the problem still persists; I still get the same errors.
main.o: main.c
gcc -c -mcpu=cortex-a8 main.c
functions.o: functions.s
as -mcpu=cortex-a8 -o functions.o functions.s
Errors
ubuntu#ubuntu-desktop:~/Documents/Project/Others/helloworld$ make
gcc -c -mcpu=cortex-a8 main.c
as -mcpu=cortex-a8 -o functions.o functions.s
gcc -o main.o functions.o
functions.o: In function `_start':
(.text+0x0): multiple definition of `_start'
/usr/lib/gcc/arm-linux-gnueabi/4.3.3/../../../crt1.o:init.c:(.text+0x0): first defined here
/usr/lib/gcc/arm-linux-gnueabi/4.3.3/../../../crt1.o: In function `_start':
init.c:(.text+0x30): undefined reference to `main'
collect2: ld returned 1 exit status
make: *** [hello] Error 1

Answer 1

In the makefile:
hello: main.o functions.o
gcc -o main.o functions.o
should be:
hello: main.o functions.o
gcc -o hello main.o functions.o
As it stands, you are linking functions.o, but not main.o, and producing an output executable called main.o, which is overwriting your existing main.o.

Answer 2

Shouldn't
hello: main.o functions.o
gcc -o main.o functions.o
be
hello: main.o functions.o
gcc -o hello main.o functions.o

Answer 3

As Bigbohne suggests, gcc is trying to link in the standard runtime library. Try adding the -nostdlib option to your gcc call:
gcc -nostdlib -o hello main.o functions.o

Answer 4

I think that has something to do with the Runtime library the gcc is linking at the end.
And in this library there already is a "_start".
I think you have to compile without "std library". but than you wont have printf,getchar and all the other useful stuff.

Answer 5

Normally execution of a program compiled with gcc starts in the function called _start (in a startup file provided by gcc), which calls main.
If you are supplying your own _start, you should pass -nostdlib to gcc when linking. This disables all default startup files and all default libraries. You should therefore also explicitly provide any libraries you need such as libc and libgcc.
Your _start however does not call main, so it is unclear why you bother to provide main and the .c file at all.
gcc -o main.o functions.o is a mistake, it should be gcc -o main main.o functions.o, but this will not fix the problem described above.