**Q:**
The predefined constant `M_PI`
seems to be missing from
my machine's copy of
`<math.h>`.

**A:**
That constant
(which is apparently supposed to be the value of pi,
accurate to the machine's precision),
is not
standard;
in fact a standard-conforming copy of `<math.h>`
should
*not* `#define` a symbol `M_PI`.
[footnote]
If you need pi, you'll have to define it yourself,
or compute it with `4*atan(1.0)`
or
`acos(-1.0)`.
(You could use
a
construction
like

#ifndef M_PI #define M_PI 3.1415926535897932385 #endifto provide your own #definition only if some system header file has not.)

References:
PCS Sec. 13 p. 237

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