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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.


Copyright © 2008 Daniel Amelang

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