What distinguishes these LAPACK programmes? One compiles, the other does not - c

Question

I have two programmes using the LAPACK routine dgeev in C. One appears to be working, the other does not compile claiming undefined reference to dgeev. I seek to understand why.
The first code below - called mamapack.c - produces sensible results when compiled and run like this:
ludi#ludi-M17xR4:~/Desktop/tests$ gcc -o mamapack mamapack.c -L/usr/local/lib -llapack -lblas && ./mamapack
#include<stdio.h>
#include<math.h>
#include <stdlib.h>
//...........................................................................
void dgeTranspose( double *Transposed, double *M ,int n) {
int i,j;
for(i=0;i<n;i++)
for(j=0;j<n;j++)
Transposed[i+n*j] = M[i*n+j];
}
//...........................................................................
// MatrixComplexEigensystem: computes the eigenvectors and eigenValues of input matrix A
// The eigenvectors are stored in columns
//............................................................................
void MatrixComplexEigensystem( double *eigenvectorsVR, double *eigenvaluesW, double *A, int N) {
int i;
double *AT = (double *) malloc( N*N*sizeof(double ) );
dgeTranspose( AT, A , N);
char JOBVL ='N'; // Compute Right eigenvectors
char JOBVR ='V'; // Do not compute Left eigenvectors
double VL[1];
int LDVL = 1;
int LDVR = N;
int LWORK = 4*N;
double *WORK = (double *)malloc( LWORK*sizeof(double));
double *RWORK = (double *)malloc( 2*N*sizeof(double));
int INFO;
double *eigenvaluesWR =eigenvaluesW;
double *eigenvaluesWI = eigenvaluesW + N;
dgeev_( &JOBVL, &JOBVR, &N, AT , &N,
eigenvaluesWR, eigenvaluesWI,
VL, &LDVL,
eigenvectorsVR, &LDVR,
WORK, &LWORK, &INFO );
printf("\nping1\n");
dgeTranspose( AT, eigenvectorsVR , N);
for(i=0;i<N*N;i++) eigenvectorsVR[i]=AT[i];
free(WORK);
free(RWORK);
free(AT);
}
int main() {
int i,j;
const int N = 3;
double A[] = { 1. , 0. , 0. , 0. , 1., 0. , 0., 0., 1.};
double eigenVectors[N*N];
double eigenValues[2*N];
MatrixComplexEigensystem( eigenVectors, eigenValues, A, N);
printf("\nEigenvectors\n");
for(i=0;i<N;i++){
for(j=0;j<N;j++) printf("%e ", eigenVectors[i*N + j]);
printf("\n");
}
printf("\nEigenvalues \n");
for(i=0;i<N;i++) printf("%e ", eigenValues[i] );
printf("\n--------------------------------------------------------\n");
return 0;
}
Then I ran another code that I called lapack1.c, which is just the official "Example program in C" from this documentation: https://software.intel.com/sites/products/documentation/doclib/mkl_sa/11/mkl_lapack_examples/dgeev.htm
(I dare not post it separately due to potential copyright restrictions)
ludi#ludi-M17xR4:~/Desktop/tests$ gcc -o lapack1 lapack1.c
-L/usr/local/lib -llapack -lblas && ./lapack1
produces
tmp/cciRzQru.o: In function main': lapack1.c:(.text+0xf3): undefined
reference todgeev' lapack1.c:(.text+0x1b6): undefined reference to
`dgeev' collect2: error: ld returned 1 exit status

Answer 1

Check your function names carefully. The linker is complaining about an undefined reference to function dgeev(). The working code is calling a different function, named dgeev_().
Compiling with the option -D as follows:
ludi#ludi-M17xR4:~/Desktop/tests$ gcc -Ddgeev=dgeev_ -o lapack1
lapack1.c -L/usr/local/lib -llapack -lblas && ./lapack1
Will indeed work.