SIGNBITSection: Linux Programmer's Manual (3)
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NAMEsignbit - test sign of a real floating-point number
Link with -lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
DESCRIPTIONsignbit() is a generic macro which can work on all real floating-point types. It returns a non-zero value if the value of x has its sign bit set.
This is not the same as x < 0.0, because IEEE 754 floating point allows zero to be signed. The comparison -0.0 < 0.0 is false, but signbit(-0.0) will return a non-zero value.
RETURN VALUEThe signbit() macro returns non-zero if the sign of x is negative; otherwise it returns zero.
ERRORSNo errors occur.
CONFORMING TOC99, POSIX.1-2001. This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).
COLOPHONThis page is part of release 3.21 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.