title: Failed to link math library with intel mkl using g++
body: <p>I tried to compile with intel mkl 11.1 using g++:  </p>
  
  <pre><code>g++ -m32 test.c -lmkl_intel -lmkl_intel_thread -lmkl_core -liomp5 -lpthread -lm
  </code></pre>
  
  <p>The error says:</p>
  
  <pre><code>/opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `logf'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `atan2'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `sin'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `fabs'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `exp'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `cos'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `sqrt'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_intel_thread.so: undefined reference to `log'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `pow'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `log10'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `ceil'
  /opt/intel/composer_xe_2013_sp1.0.080/mkl/lib/ia32/libmkl_core.so: undefined reference to `expf'
  </code></pre>
  
  <p>But, if I compile with</p>
  
  <pre><code>g++ -m32 test.c -lmkl_intel -lmkl_intel_thread -lmkl_core -lm -liomp5 -lpthread -lm
  </code></pre>
  
  <p>There will be no error. I don't understand why added a -lm after the -lmkl_core will help, by the way</p>
  
  <pre><code>gcc -m32 test.c -lmkl_intel -lmkl_intel_thread -lmkl_core -liomp5 -lpthread -lm
  </code></pre>
  
  <p>Turns out to be ok, I also don't understand what's the difference here.<br>
  Below is my test code.</p>
  
  <pre><code>/* C source code is found in dgemm_example.c */
  
  #define min(x,y) (((x) &lt; (y)) ? (x) : (y))
  
  #include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  #include "mkl.h"
  
  int main()
  {
      double *A, *B, *C;
      int m, n, k, i, j;
      double alpha, beta;
  
      printf ("\n This example computes real matrix C=alpha*A*B+beta*C using \n"
              " Intel(R) MKL function dgemm, where A, B, and  C are matrices and \n"
              " alpha and beta are double precision scalars\n\n");
  
      m = 2000, k = 200, n = 1000;
      printf (" Initializing data for matrix multiplication C=A*B for matrix \n"
              " A(%ix%i) and matrix B(%ix%i)\n\n", m, k, k, n);
      alpha = 1.0; beta = 0.0;
  
      printf (" Allocating memory for matrices aligned on 64-byte boundary for better \n"
              " performance \n\n");
      A = (double *)mkl_malloc( m*k*sizeof( double ), 64 );
      B = (double *)mkl_malloc( k*n*sizeof( double ), 64 );
      C = (double *)mkl_malloc( m*n*sizeof( double ), 64 );
      if (A == NULL || B == NULL || C == NULL) {
          printf( "\n ERROR: Can't allocate memory for matrices. Aborting... \n\n");
          mkl_free(A);
          mkl_free(B);
          mkl_free(C);
          return 1;
      }
  
      printf (" Intializing matrix data \n\n");
      for (i = 0; i &lt; (m*k); i++) {
          A[i] = (double)(i+1);
      }
  
      for (i = 0; i &lt; (k*n); i++) {
          B[i] = (double)(-i-1);
      }
  
      for (i = 0; i &lt; (m*n); i++) {
          C[i] = 0.0;
      }
  
      printf (" Computing matrix product using Intel(R) MKL dgemm function via CBLAS interface \n\n");
      cblas_dgemm(CblasRowMajor, CblasNoTrans, CblasNoTrans, 
              m, n, k, alpha, A, k, B, n, beta, C, n);
      printf ("\n Computations completed.\n\n");
  
      printf (" Top left corner of matrix A: \n");
      for (i=0; i&lt;min(m,6); i++) {
          for (j=0; j&lt;min(k,6); j++) {
              printf ("%12.0f", A[j+i*k]);
          }
          printf ("\n");
      }
  
      printf ("\n Top left corner of matrix B: \n");
      for (i=0; i&lt;min(k,6); i++) {
          for (j=0; j&lt;min(n,6); j++) {
              printf ("%12.0f", B[j+i*n]);
          }
          printf ("\n");
      }
  
      printf ("\n Top left corner of matrix C: \n");
      for (i=0; i&lt;min(m,6); i++) {
          for (j=0; j&lt;min(n,6); j++) {
              printf ("%12.5G", C[j+i*n]);
          }
          printf ("\n");
      }
  
      printf ("\n Deallocating memory \n\n");
      mkl_free(A);
      mkl_free(B);
      mkl_free(C);
  
      printf (" Example completed. \n\n");
      return 0;
  }
  </code></pre>