From the Posix man pagePOSIX specification of math.h<math.h>:
The <math.h> header shall provide for the following constants. The values are of type double and are accurate within the precision of the double type. M_PI Value of pi M_PI_2 Value of pi/2 M_PI_4 Value of pi/4 M_1_PI Value of 1/pi M_2_PI Value of 2/pi M_2_SQRTPI Value of 2/ sqrt pi The
<math.h>header shall provide for the following constants. The values are of typedoubleand are accurate within the precision of thedoubletype.
M_PIValue of πM_PI_2Value of π/2M_PI_4Value of π/4M_1_PIValue of 1/πM_2_PIValue of 2/πM_2_SQRTPIValue of 2/βπ
