2.6 Long double

In general, the algorithms in the library are written for double precision only. The long double type is not supported for actual computation.

One reason for this choice is that the precision of long double is platform dependent. The IEEE standard only specifies the minimum precision of extended precision numbers, while the precision of double is the same on all platforms.

However, it is sometimes necessary to interact with external data in long-double format, so the vector and matrix datatypes include long-double versions.

It should be noted that in some system libraries the stdio.h formatted input/output functions printf and scanf are not implemented correctly for long double. Undefined or incorrect results are avoided by testing these functions during the configure stage of library compilation and eliminating certain GSL functions which depend on them if necessary. The corresponding line in the configure output looks like this,

checking whether printf works with long double... no

Consequently when long double formatted input/output does not work on a given system it should be impossible to link a program which uses GSL functions dependent on this.

If it is necessary to work on a system which does not support formatted long double input/output then the options are to use binary formats or to convert long double results into double for reading and writing.