C code that uses blas, lapack and openmp working with gcc 4.9 but gives run time error with gcc 6

Question

I have written a code in C.

You can find the source code here.

It makes use of the libraries blas, lapack and openmp.

I have compiled the blas and lapack libraries following these instrucions.

I use these flags to tell the compiler the libraries that it should link: -lblas -llapack -fopenmp.

I was using gcc 4.9 and the program can run correctly.

Recently I have updated gcc to gcc 6 and it shows many warning msgs about the implicit declaration of the blas and lapack functions:

src/PSIRWLS-train.c:152:17: warning: implicit declaration of function 'dgemm_' [-Wimplicit-function-declaration]
                 dgemm_(&trans, &trans, &(dataset.l), &ncols, &size,&factorA, KSC, &(dataset.l), miZ, &size, &factor, miKSM, &(dataset.l));

And when I run the app a segmentation fault error appears.

I am completely lost about the differences of gcc 4.9 and gcc 6, do you know any explanation about this?

The link to your code leads to error 404... You may be interested by [cblas](http://www.netlib.org/blas/#_cblas) and [Lapacke](http://www.netlib.org/lapack/lapacke.html) which are the C interfaces to BLAS / LAPACK. Otherwise, you may have to decleare [`extern dgemm_(...)`](http://nicolas.limare.net/pro/notes/2014/10/31_cblas_clapack_lapacke/) . Notice that declaring fortran functions this way may lead to portability problems, because the naming convention changes between Linux and the W-thing... Using Cblas and Lapacke leaves this issue (and some others) to the developers of these interfaces. — francis, Sep 23 '16 at 21:15

score 1 · Accepted Answer · answered Sep 24 '16 at 00:59

Problem solved.

1 - Some default flags are different in gcc 4 and 6. gcc 6 shows warnings when you don't decleare blas and lapack functions this way:

extern void dgemm_(...)

2 - It scaped me the initialization of one variable that had to be initialized to 0. I coded int i,j = 0; when I had to code int i=0, j=0;

gcc 4 initialized i to 0 (and the program was running correctly) and gcc 6 didn't (creating a segmentation fault because these variables were to index)

score 0 · Answer 2 · edited May 23 '17 at 12:32

I had a similar problem once, and it seems very likely that there is some undefined behaviour in your code (e.g. a double free), which is handled in a compiler specific manner. Perhaps this changed between gcc 4.9 and 6.0.

It's hard to say without knowing your code, but you can use a number of tools yourself to track this issue down, for example GDB or especially valgrind's memcheck (worked brilliantly for me):

valgrind --tool memcheck <your binary here>

This will report locations of memory related errors and undefined behaviours in your code.

C code that uses blas, lapack and openmp working with gcc 4.9 but gives run time error with gcc 6

2 Answers2