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table of contents

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 corresponding 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 complexz); float complex cprojf(float complexz); long double complex cprojl(long double complexz);

# 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.1‐2017 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 complex 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.1‐2017,
**<complex.h>**

# COPYRIGHT¶

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/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 .

2017 | IEEE/The Open Group |