2

I have a problem with lapack on my new ubuntu machine (edit: Ubuntu 20.04.5 LTS). I installed liblapack-dev and libblas-dev using apt-get. I used a following code which works normally on different machine.

#include <stdlib.h>
#include <stdio.h>

/* DGESV prototype /
extern void dgesv( int n, int* nrhs, double* a, int* lda, int* ipiv,
                double* b, int* ldb, int* info );
/* Auxiliary routines prototypes /
extern void print_matrix( char desc, int m, int n, double* a, int lda );
extern void print_int_vector( char* desc, int n, int* a );


/* Parameters */
#define N 5
#define NRHS 3
#define LDA N
#define LDB N


/* Main program /
int main() {
        / Locals /
        int n = N, nrhs = NRHS, lda = LDA, ldb = LDB, info;
        / Local arrays /
        int ipiv[N];
        double a[LDAN] = {
            6.80, -2.11,  5.66,  5.97,  8.23,
           -6.05, -3.30,  5.36, -4.44,  1.08,
           -0.45,  2.58, -2.70,  0.27,  9.04,
            8.32,  2.71,  4.35, -7.17,  2.14,
           -9.67, -5.14, -7.26,  6.08, -6.87
        };
        double b[LDB*NRHS] = {
            4.02,  6.19, -8.22, -7.57, -3.03,
           -1.56,  4.00, -8.67,  1.75,  2.86,
            9.81, -4.09, -4.57, -8.61,  8.99
        };
        /* Executable statements /
        printf( " DGESV Example Program Results\n" );
        / Solve the equations AX = B /
        dgesv( &n, &nrhs, a, &lda, ipiv, b, &ldb, &info );
        /* Check for the exact singularity /
        if( info > 0 ) {
                printf( "The diagonal element of the triangular factor of A,\n" );
                printf( "U(%i,%i) is zero, so that A is singular;\n", info, info );
                printf( "the solution could not be computed.\n" );
                exit( 1 );
        }
        / Print solution /
        print_matrix( "Solution", n, nrhs, b, ldb );
        / Print details of LU factorization /
        print_matrix( "Details of LU factorization", n, n, a, lda );
        / Print pivot indices /
        print_int_vector( "Pivot indices", n, ipiv );
        exit( 0 );
} / End of DGESV Example */


/* Auxiliary routine: printing a matrix /
void print_matrix( char desc, int m, int n, double* a, int lda ) {
        int i, j;
        printf( "\n %s\n", desc );
        for( i = 0; i < m; i++ ) {
                for( j = 0; j < n; j++ ) printf( " %6.2f", a[i+j*lda] );
                printf( "\n" );
        }
}


/* Auxiliary routine: printing a vector of integers /
void print_int_vector( char desc, int n, int* a ) {
        int j;
        printf( "\n %s\n", desc );
        for( j = 0; j < n; j++ ) printf( " %6i", a[j] );
        printf( "\n" );
}

but compiling with the following command:

gcc -Wall test.c -lm -lblas -llapack

leads to the following problem:

/usr/bin/ld: /tmp/ccTfUyXO.o: in function `main':
test.c:(.text+0x2e3): undefined reference to `dgesv'
collect2: error: ld returned 1 exit status

I have exactly the same problem with other, more complex code which uses different lapack routines.

Any idea what may cause the problem? I also tried to add -L flag with the directory containing lapack files after -llapack but it did not help.

T. Skora
  • 23

1 Answers1

1

The issue seems to be that, although dgesv is the name of the Fortran function, it is exposed in the C/C++ API as dgesv_. So whereas

$ gcc -Wall test.c -lm -lblas -llapack
/usr/bin/ld: /tmp/ccp4Lcye.o: in function `main':
test.c:(.text+0x2e3): undefined reference to `dgesv'
collect2: error: ld returned 1 exit status

fails;

$ sed -i 's/dgesv/&_/' test.c
$ 
$ gcc -Wall test.c -lm -lblas -llapack; echo $?
0

succeeds, and the resulting executable runs successfully:

$ ./a.out
 DGESV Example Program Results

Solution
  -0.80  -0.39   0.96
  -0.70  -0.55   0.22
   0.59   0.84   1.90
   1.32  -0.10   5.36
   0.57   0.11   4.04


Details of LU factorization
   8.23   1.08   9.04   2.14  -6.87
   0.83  -6.94  -7.92   6.55  -3.99
   0.69  -0.67 -14.18   7.24  -5.19
   0.73   0.75   0.02 -13.82  14.19
  -0.26   0.44  -0.59  -0.34  -3.43


Pivot indices
      5      5      3      4      5

Alternatively, you might consider installing the lapacke-dev package and using LAPACKE_dgesv instead.

For further reading see Using BLAS and LAPACK from C/C++ .

steeldriver
  • 142,475