ceil (3)
Leading comments
Copyright 2001 Andries Brouwer <aeb@cwi.nl>. and Copyright 2008, Linux Foundation, written by Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived wor...
NAME
ceil, ceilf, ceill  ceiling function: smallest integral value not less than argumentSYNOPSIS
#include <math.h> double ceil(double x);
float ceilf(float x);
long double ceill(long double x);
Link with lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
ceilf(), ceill():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE >= 600  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
DESCRIPTION
These functions return the smallest integral value that is not less than x.For example, ceil(0.5) is 1.0, and ceil(0.5) is 0.0.
RETURN VALUE
These functions return the ceiling of x.If x is integral, +0, 0, NaN, or infinite, x itself is returned.
ERRORS
No errors occur. POSIX.12001 documents a range error for overflows, but see NOTES.ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).Interface  Attribute  Value 
ceil(), ceilf(), ceill()  Thread safety  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.The variant returning double also conforms to SVr4, 4.3BSD, C89.
NOTES
SUSv2 and POSIX.12001 contain text about overflow (which might set errno to ERANGE, or raise an FE_OVERFLOW exception). In practice, the result cannot overflow on any current machine, so this errorhandling stuff is just nonsense. (More precisely, overflow can happen only when the maximum value of the exponent is smaller than the number of mantissa bits. For the IEEE754 standard 32bit and 64bit floatingpoint numbers the maximum value of the exponent is 128 (respectively, 1024), and the number of mantissa bits is 24 (respectively, 53).)The integral value returned by these functions may be too large to store in an integer type (int, long, etc.). To avoid an overflow, which will produce undefined results, an application should perform a range check on the returned value before assigning it to an integer type.