gcc-linux-gnueabi-arm error undefined reference to `main' - c

Question

I am trying to generate assembly and executable of a simple neon-based c code. The code is,
#include <arm_neon.h>
void NeonTest(short int * __restrict a, short int * __restrict b, short int * __restrict z)
{
int i;
for (i = 0; i < 200; i++) {
z[i] = a[i] * b[i];
}
}
First, I am going for the assembly to calculate the neon instructions,
arm-linux-gnueabi-gcc -O2 -march=armv7-a -mtune=cortex-a9 -ftree-vectorize -mhard-float -mfloat-abi=softfp -mfpu=neon -S neon_test.c -o nt.s
Then I am converting the nt.s file to object file.
arm-linux-gnueabi-gcc -O2 -march=armv7-a -mtune=cortex-a9 -ftree-vectorize -mhard-float -mfloat-abi=softfp -mfpu=neon -c nt.s -o nt.o
Finally, for the executable I do,
arm-linux-gnueabi-gcc -O2 -march=armv7-a -mtune=cortex-a9 -ftree-vectorize -mhard-float -mfloat-abi=softfp -mfpu=neon nt.o -o nt
I get the error,
/usr/lib/gcc-cross/arm-linux-gnueabi/4.7/../../../../arm-linux-gnueabi/lib/crt1.o: In function `_start':
(.text+0x34): undefined reference to `main'
collect2: error: ld returned 1 exit status
I am using Ubuntu 14LTS on Intel system.

Answer 1

You're not including the C file that contains main() when compiling, so the linker isn't seeing it
You need to add it:
arm-linux-gnueabi-gcc -O2 -march=armv7-a -mtune=cortex-a9 -ftree-vectorize -mhard-float -mfloat-abi=softfp -mfpu=neon nt.o main.o -o nt
where main.o is also created following the same step as neon.o

Answer 2

Every program needs a starting point so the computer knows where to begin execution. In C/C++, the starting point is the beginning of the function int main. Give your program an int main by either linking your object file to an object file with int main, or adding one in this code.
To add main in your code, beneath your function definition, try
int main()
{
NeonTest(/* your parameters */);
}