NDK对{extern“ C”函数}的引用未定义？

Question

I try to develop an Android app. I already managed to implement a rather small c++ function using NDK. But now I want to use a pretty large c++ library which I found on Google Docs. When I now try to build it with ndk-build tool, I get this error in cygwin console:

Compile++ thumb  : ndkfoo <= subspace.cpp
Compile++ thumb  : ndkfoo <= classifier.cpp
Compile++ thumb  : ndkfoo <= eigen.cpp
Compile++ thumb  : ndkfoo <= image.cpp
Compile++ thumb  : ndkfoo <= imageio.cpp
Compile++ thumb  : ndkfoo <= local.cpp
Compile++ thumb  : ndkfoo <= matrix.cpp
Compile++ thumb  : ndkfoo <= sample.cpp
SharedLibrary  : libndkfoo.so
./obj/local/armeabi/objs/ndkfoo/eigen.o: In function `LibSubspace::geneigen(doub
le*, double*, int, double*, int, bool)':
U:\workspace\test/jni/eigen.cpp:91: undefined reference to `ilaenv_'
U:\workspace\test/jni/eigen.cpp:96: undefined reference to `dsygv_'
U:\workspace\test/jni/eigen.cpp:128: undefined reference to `dggev_'
./obj/local/armeabi/objs/ndkfoo/eigen.o: In function `LibSubspace::eigen(double*
, double*, double*, int, bool)':
U:\workspace\test/jni/eigen.cpp:55: undefined reference to `ilaenv_'
U:\workspace\test/jni/eigen.cpp:58: undefined reference to `dsyev_'
collect2: ld returned 1 exit status
make: *** [obj/local/armeabi/libndkfoo.so] Error 1

The Code of this function is as follow:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <float.h>
#include <assert.h>
#include <string.h>

#include "eigen.h"

extern "C" int ilaenv_(int *ispec, const char *name__, const char *opts,
    int *n1, int *n2, int *n3, int *n4, int name_len, int opts_len);

extern "C" int dsyev_(const char *jobz, const char *uplo, int *n, double *a,
    int *lda, double *w, double *work, int *lwork, 
    int *info);

extern "C" int dsygv_(int *itype, const char *jobz, const char *uplo, int *
    n, double *a, int *lda, double *b, int *ldb, 
    double *w, double *work, int *lwork, int *info);

extern "C" int dggev_(const char *jobvl, const char *jobvr, int *n, double *
    a, int *lda, double *b, int *ldb, double *alphar, 
    double *alphai, double *beta, double *vl, int *ldvl, 
    double *vr, int *ldvr, double *work, int *lwork, 
    int *info);

namespace LibSubspace {

int eigen(double* A, double* V, double* E, int n, bool verbose) {
    int info;
    int ispec = 1;
    int lwork;

    lwork = (ilaenv_(&ispec,"DSYEV","U",&n,&n,&n,&n,5,1)+2)*n;
    double *work = (double *)malloc(lwork*sizeof(double));
    memcpy(V,A,n*n*sizeof(double));
    dsyev_("V","U",&n,V,&n,E,work,&lwork,&info);
    free(work);

    //check the return value
    if(info!=0) {
        if(verbose) {
            printf("Error computing eigenvectors: ");
            if(info>0) {
                printf("Algorithm failed to converge\n");
            } else if(info<0) {
                printf("Illegal argument\n");
            } else {
                printf("Unknown error\n");
            }
        }
        return 0;
    }

    return 1;
}


int geneigen(double *A, double *B, int n, double *W, int algorithm,
             bool verbose) {
    //getting the optimal lwork
    int ispec = 1;
    int lwork;
    int info;

    switch(algorithm) {

        case EIGEN_CHOL: 
        {
            //computing eigenvectors
            lwork = (ilaenv_(&ispec,"DSYGV","U",&n,&n,&n,&n,5,1)+2)*n;
            double *work = (double *)malloc(lwork*sizeof(double));
            int problemType = 1;
            char job = 'V';
            char uplo = 'U';
            dsygv_(&problemType,&job,&uplo,&n,A,&n,B,&n,W,work,&lwork,&info);

            free(work);

            //check the return value
            if(info!=0) {
                if(verbose) {
                    printf("Error computing eigenvectors: ");
                    if(info>n) {
                        printf("Matrix B is not positive definite\n");
                    } else if(info<=n) {
                        printf("The problem failed to converge\n");
                    } else if(info<0) {
                        printf("Illegal argument\n");
                    } else {
                        printf("Unknown error\n");
                    }
                }
                return 0;
            }
        }
        break;

        case EIGEN_QZ:
        {
            //more general algorithm
            double *alphar = (double *)malloc(n*sizeof(double));
            double *alphai = (double *)malloc(n*sizeof(double));
            double *beta = (double *)malloc(n*sizeof(double));
            double *VR = (double *)malloc(n*n*sizeof(double));
            lwork = 8*n;
            double *work = (double *)malloc(lwork*sizeof(double));
            dggev_("N","V",&n,A,&n,B,&n,alphar,alphai,beta,
                                   NULL,&n,VR,&n,work,&lwork,&info);
            //eigenvalues
            for(long i=0;i<n;i++) {
                if(beta[i]!=0) {
                    W[i] = alphar[i]/beta[i];
                } else W[i] = 0;
            }
            //eigenvectors
            for(long i=0;i<n;i++) {
                for(long j=0;j<n;j++) {
                    A[i*n+j] = VR[i*n+j];
                }
            }
            free(alphar);
            free(alphai);
            free(beta);
            free(VR);   
            free(work);

            if(info!=0) {
                printf("Error computing eigenvectors: ");
                if(info<0) {
                    printf("Illegal argument\n");
                } else if(info<=n) {
                    printf("QZ iteration failed\n");
                } else {
                    printf("Unknown error\n");
                }
                return 0;
            }
        }
        break;

    }

    return 1;
}

} //namespace

Unfortunatly I absolutly DONT understand why these errors occur. I am not much familiar with C++, thats why I wanted to use NDK in the hope of not having to change anything of the existing library.

Answer 1

Google tells me that these functions are from the runtime of Fortran-to-C compiler. So you need to either link against a library called libf2c , or find the sources and make them a part of the NDK project too. The latter sounds like a more promising approach - I don't think a ready made NDK build exists.