Next: , Previous: , Up: The Macros  


ax_c_float_words_bigendian

Synopsis

AX_C_FLOAT_WORDS_BIGENDIAN([ACTION-IF-TRUE], [ACTION-IF-FALSE], [ACTION-IF-UNKNOWN])

Description

Checks the ordering of words within a multi-word float. This check is necessary because on some systems (e.g. certain ARM systems), the float word ordering can be different from the byte ordering. In a multi-word float context, "big-endian" implies that the word containing the sign bit is found in the memory location with the lowest address. This implemenation was inspired by the AC_C_BIGENDIAN macro in autoconf.

The endianess is detected by first compiling C code that contains a special double float value, then grepping the resulting object file for certain strings of ASCII values. The double is specially crafted to have a binary representation that corresponds with a simple string. In this implementation, the string "noonsees" was selected because the individual word values ("noon" and "sees") are palindromes, thus making this test byte-order agnostic. If grep finds the string "noonsees" in the object file, the target platform stores float words in big-endian order. If grep finds "seesnoon", float words are in little-endian order. If neither value is found, the user is instructed to specify the ordering.

Source Code

Download the latest version of ax_c_float_words_bigendian.m4 or browse the macro’s revision history.

License

Copyright © 2008 Daniel Amelang dan@amelang.net

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.