ceil, ceilf, ceill — ceiling function: smallest integral value not less than argument


#include <math.h>
double ceil( double x);
float ceilf( float x);
long double ceill( long double x);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
ceilf(), ceill():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19:
*/ _DEFAULT_SOURCE || /* Glibc versions <= 2.19:
[Note] Note

Link with −lm.


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.


These functions return the ceiling of x.

If x is integral, +0, −0, NaN, or infinite, x itself is returned.


No errors occur. POSIX.1-2001 documents a range error for overflows, but see NOTES.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
ceil(), ceilf(), ceill() Thread safety MT-Safe


C99, POSIX.1-2001, POSIX.1-2008.

The variant returning double also conforms to SVr4, 4.3BSD, C89.


SUSv2 and POSIX.1-2001 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 error-handling 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 IEEE-754 standard 32-bit and 64-bit floating-point 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.


floor(3), lrint(3), nearbyint(3), rint(3), round(3), trunc(3)


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  Copyright 2001 Andries Brouwer <>.
and Copyright 2008, Linux Foundation, written by Michael Kerrisk

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