ldexp(), ldexpf(), ldexpl()

QNX SDP8.0C Library ReferenceAPIDeveloper

Load exponent: multiply a floating-point number by an integral power of 2

Synopsis:

#include <math.h>

double ldexp( double x, 
              int exp );

float ldexp( float x, 
             int exp );

long double ldexpl( long double x,
                    int exp );

Arguments:

x: A floating-point number.
exp: The exponent of 2 to multiply x by.

Library:

libm: The general-purpose math library.
libm-sve: A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

If you want only selected processes to run with the SVE version, you can include both libraries in your OS image and use the -l m or -l m-sve option to qcc to link explicitly against the appropriate one.
If you want all processes to use the SVE version, include libm-sve.so in your OS image and set up a symbolic link from libm.so to libm-sve.so. Use the -l m option to qcc to link against the library.

Note:

Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

Description:

These functions multiply the floating-point number x by 2^exp.

To check for error situations, use feclearexcept() and fetestexcept(). For example:

Call feclearexcept(FE_ALL_EXCEPT) before calling ldexp(), ldexpf(), or ldexpl().
On return, if fetestexcept(FE_ALL_EXCEPT) is nonzero, then an error has occurred.

Returns:

x * 2^exp


If:	These functions return:	Errors:
`x` is ±0.0	`x`	—
`x` is ±Inf	`x`	—
`x` is NaN	`x`	—
The result would cause overflow	Inf	FE_OVERFLOW
The correct value would cause underflow	The correct value, after rounding	—

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

Examples:

#include <stdio.h>
#include <math.h>
#include <fenv.h>
#include <stdlib.h>

int main( void )
{
    int except_flags;
    double value;

    feclearexcept(FE_ALL_EXCEPT);

    value = ldexp( 4.7072345, 5 );

    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    printf( "%f\n", value );

    return EXIT_SUCCESS;
}

produces the output:

150.631504

Classification:

C11, POSIX 1003.1


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025