floor - calculate the floor of a number

floor, floorf, floorl - largest integral value not greater than argument


Header File

#include <math.h>


double floor(double x);
#include <math.h>

double floor(double x);
float floorf(float x);
long double floorl(long double x);

Link with -lm.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

floorf(), floorl():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE


This function returns the floor of x.

These functions return the largest integral value that is not greater than x.

For example, floor(0.5) is 0.0, and floor(-0.5) is -1.0.


This function returns, as a double, the largest integer that is not greater than x. You may safely cast that value to a long (or an int if it fits).

These functions return the floor of x.

If x is integral, +0, -0, NaN, or an infinity, x itself is returned.


#include <math.h>
int main(void) { printf("This is CS%i\n", (int) floor(50.1)); }


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
floor(), floorf(), floorl() 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).)


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


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