Program InvertMultiply

implicit none
integer (kind=4),parameter :: nx=3
integer (kind=4) :: ipiv(nx)
integer (kind=4) :: info,i,j
real (kind=8) :: A(nx,nx), B(nx,nx), C(nx,nx), work(nx)
real (kind=8) :: alpha,beta

external DGEMM
external DGETRF
external DGETRI

A=reshape((/1,-1,-1,0,1,0,0,0,1/),(/3,3/));
B=A ! copy of A
call DGETRF(nx, nx, B, nx, ipiv, info)
call DGETRI(nx, B, nx, ipiv, work, nx, info)
alpha=1
beta=0
CALL DGEMM('N','N',nx,nx,nx,alpha,A,nx,B,nx,beta,C,nx)
print *,'A*Ainv'
do i=1,nx
  write(*,fmt="(6(1x,f4.1))")C(i,:)
end do
! should be diagonal
End Program InvertMultiply

It checks for the availability of LAPACK procedures. I am able to link and compile for:

gfortran  InvertMultiply.f90 -llapack -lblas

a.out is generated and provides the correct answer.

Nevertheless, if I divide the compilation link into 2 steps:

gfortran -c InvertMultiply.f90

gfortran  -o InvertMultiply.o -llapack -lblas
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In     function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

I don’t know where’s wrong and how to fix? Can someone help me?