# libc.info: rand48

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# `rand48', `drand48', `erand48', `lrand48', `nrand48', `mrand48', `jrand48', `srand48', `seed48', `lcong48' --pseudo random number generators and initialization routines

```*Synopsis*
#include <stdlib.h>
double drand48(void);
double erand48(unsigned short XSEED[3]);
long lrand48(void);
long nrand48(unsigned short XSEED[3]);
long mrand48(void);
long jrand48(unsigned short XSEED[3]);
void srand48(long SEED);
unsigned short *seed48(unsigned short XSEED[3]);
void lcong48(unsigned short P[7]);
*Description*
The `rand48' family of functions generates pseudo-random numbers using
a linear congruential algorithm working on integers 48 bits in size.
The particular formula employed is r(n+1) = (a * r(n) + c) mod m where
the default values are for the multiplicand a = 0xfdeece66d =
25214903917 and the addend c = 0xb = 11. The modulo is always fixed at
m = 2 ** 48.  r(n) is called the seed of the random number generator.```
```   For all the six generator routines described next, the first
computational step is to perform a single iteration of the algorithm.```
```   `drand48' and `erand48' return values of type double. The full 48
bits of r(n+1) are loaded into the mantissa of the returned value, with
the exponent set such that the values produced lie in the interval
[0.0, 1.0].```
```   `lrand48' and `nrand48' return values of type long in the range [0,
2**31-1]. The high-order (31) bits of r(n+1) are loaded into the lower
bits of the returned value, with the topmost (sign) bit set to zero.```
```   `mrand48' and `jrand48' return values of type long in the range
[-2**31, 2**31-1]. The high-order (32) bits of r(n+1) are loaded into
the returned value.```
```   `drand48', `lrand48', and `mrand48' use an internal buffer to store
r(n). For these functions the initial value of r(0) = 0x1234abcd330e =
20017429951246.```
```   On the other hand, `erand48', `nrand48', and `jrand48' use a
user-supplied buffer to store the seed r(n), which consists of an array
of 3 shorts, where the zeroth member holds the least significant bits.```
`   All functions share the same multiplicand and addend.`
```   `srand48' is used to initialize the internal buffer r(n) of
`drand48', `lrand48', and `mrand48' such that the 32 bits of the seed
value are copied into the upper 32 bits of r(n), with the lower 16 bits
of r(n) arbitrarily being set to 0x330e.  Additionally, the constant
multiplicand and addend of the algorithm are reset to the default
values given above.```
```   `seed48' also initializes the internal buffer r(n) of `drand48',
`lrand48', and `mrand48', but here all 48 bits of the seed can be
specified in an array of 3 shorts, where the zeroth member specifies
the lowest bits. Again, the constant multiplicand and addend of the
algorithm are reset to the default values given above.  `seed48'
returns a pointer to an array of 3 shorts which contains the old seed.
This array is statically allocated, thus its contents are lost after
each new call to `seed48'.```
```   Finally, `lcong48' allows full control over the multiplicand and
addend used in `drand48', `erand48', `lrand48', `nrand48', `mrand48',
and `jrand48', and the seed used in `drand48', `lrand48', and `mrand48'.
An array of 7 shorts is passed as parameter; the first three shorts are
used to initialize the seed; the second three are used to initialize the
multiplicand; and the last short is used to initialize the addend.  It
is thus not possible to use values greater than 0xffff as the addend.```
```   Note that all three methods of seeding the random number generator
always also set the multiplicand and addend for any of the six
generator calls.```
`   For a more powerful random number generator, see `random'.`
```*Portability*
SUS requires these functions.```
`   No supporting OS subroutines are required.`