# expm1(3) — Linux manual page

```EXPM1(3)                Linux Programmer's Manual               EXPM1(3)
```

## NAME         top

```       expm1, expm1f, expm1l - exponential minus 1
```

## SYNOPSIS         top

```       #include <math.h>

double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);

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

expm1():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

expm1f(), expm1l():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION         top

```       These functions return a value equivalent to

exp(x) - 1

The result is computed in a way that is accurate even if the
value of x is near zero—a case where exp(x) - 1 would be
inaccurate due to subtraction of two numbers that are nearly
equal.
```

## RETURN VALUE         top

```       On success, these functions return exp(x) - 1.

If x is a NaN, a NaN is returned.

If x is +0 (-0), +0 (-0) is returned.

If x is positive infinity, positive infinity is returned.

If x is negative infinity, -1 is returned.

If the result overflows, a range error occurs, and the functions
return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.
```

## ERRORS         top

```       See math_error(7) for information on how to determine whether an
error has occurred when calling these functions.

The following errors can occur:

Range error, overflow
errno is set to ERANGE (but see BUGS).  An overflow
floating-point exception (FE_OVERFLOW) is raised.
```

## ATTRIBUTES         top

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

┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface                             │ Attribute     │ Value   │
├──────────────────────────────────────┼───────────────┼─────────┤
│expm1(), expm1f(), expm1l()           │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
```

## CONFORMING TO         top

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

## BUGS         top

```       Before glibc 2.17, on certain architectures (e.g., x86, but not
x86_64) expm1() raised a bogus underflow floating-point exception
for some large negative x values (where the function result
approaches -1).

Before approximately glibc version 2.11, expm1() raised a bogus
invalid floating-point exception in addition to the expected
overflow exception, and returned a NaN instead of positive
infinity, for some large positive x values.

Before version 2.11, the glibc implementation did not set errno
to ERANGE when a range error occurred.
```

```       exp(3), log(3), log1p(3)
```

## COLOPHON         top

```       This page is part of release 5.13 of the Linux man-pages project.
A description of the project, information about reporting bugs,