Inconsistent results from AC_CHECK_FUNCS

I am hav开发者_JS百科ing a problem with a configure script verifying the presence of some basic functions,

AC_CHECK_FUNCS([floor gettimeofday memset pow sqrt sin exp])

Under certain criteria, the results are expected (checking for x... yes). Otherwise the math functions above are claimed to be not defined. The difference is a call to verify some lapack routines first. These are checked by, AC_CHECK_LIB([lapack],[dsyev_],.... If these checks are not done first, then the above math functions fail to check (final compilation works).

I've reduced the configure script to the following that recreates the issue,

AC_INIT([TEST], [0.0], [none@none.com])
#AC_CHECK_LIB([lapack],[dsyev_], , AC_MSG_FAILURE([Missing lapack]))
AC_CHECK_FUNCS([floor gettimeofday memset pow sqrt sin exp])

Uncommenting the second line results in proper visual results. I imagine that the math library isn't included or checked for some reason. In fact, AC_CHECK_LIB([m],[exp]) works fine.

What's the cause of this, and what is the proper way to use this directive? What kind of x-platform considerations should I observe?

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The math functions are in the math library, so if you want them, you need to pull the math library into the link arguments that configure uses. AC_CHECK_LIB([m],[exp]) is one way to do that. The reason that AC_CHECK_LIB([lapack],...) also has that effect is that it quite probably pulls in -lm itself (not knowing the library, but I read it's for linear algebra).

I suggest that you avoid AC_CHECK_LIB and instead use AC_SEARCH_LIBS for functions that are potentially not in the standard C library. So

AC_SEARCH_LIBS(exp, m)
AC_SEARCH_LIBS(dsyev_, lapack)

(But using AC_CHECK_LIB is not actually wrong.)