CPROJ(3P) POSIX Programmer's Manual CPROJ(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the correspond-
ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
NAME
cproj, cprojf, cprojl -- complex projection functions
SYNOPSIS
#include <complex.h>
double complex cproj(double complex z);
float complex cprojf(float complex z);
long double complex cprojl(long double complex z);
DESCRIPTION
The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here
and the ISO C standard is unintentional. This volume of POSIX.1-2008 defers to the ISO C standard.
These functions shall compute a projection of z onto the Riemann sphere: z projects to z, except that all complex infinities (even those
with one infinite part and one NaN part) project to positive infinity on the real axis. If z has an infinite part, then cproj(z) shall be
equivalent to:
INFINITY + I * copysign(0.0, cimag(z))
RETURN VALUE
These functions shall return the value of the projection onto the Riemann sphere.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
Two topologies are commonly used in complex mathematics: the complex plane with its continuum of infinities, and the Riemann sphere with
its single infinity. The complex plane is better suited for transcendental functions, the Riemann sphere for algebraic functions. The com-
plex types with their multiplicity of infinities provide a useful (though imperfect) model for the complex plane. The cproj() function
helps model the Riemann sphere by mapping all infinities to one, and should be used just before any operation, especially comparisons, that
might give spurious results for any of the other infinities. Note that a complex value with one infinite part and one NaN part is regarded
as an infinity, not a NaN, because if one part is infinite, the complex value is infinite independent of the value of the other part. For
the same reason, cabs() returns an infinity if its argument has an infinite part and a NaN part.
FUTURE DIRECTIONS
None.
SEE ALSO
carg(), cimag(), conj(), creal()
The Base Definitions volume of POSIX.1-2008, <complex.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technol-
ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Stan-
dard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE
/The Open Group 2013 CPROJ(3P)