GLib Reference Manual: Random Numbers

## Random Numbers Random Numbers — pseudo-random number generator |

## Synopsis

    #include <glib.h>

                        GRand;
    GRand *             g_rand_new_with_seed                (guint32 seed);
    GRand *             g_rand_new_with_seed_array          (const guint32 *seed,
                                                             guint seed_length);
    GRand *             g_rand_new                          (void);
    GRand *             g_rand_copy                         (GRand *rand_);
    void                g_rand_free                         (GRand *rand_);
    void                g_rand_set_seed                     (GRand *rand_,
                                                             guint32 seed);
    void                g_rand_set_seed_array               (GRand *rand_,
                                                             const guint32 *seed,
                                                             guint seed_length);
    #define             g_rand_boolean                      (rand_)
    guint32             g_rand_int                          (GRand *rand_);
    gint32              g_rand_int_range                    (GRand *rand_,
                                                             gint32 begin,
                                                             gint32 end);
    gdouble             g_rand_double                       (GRand *rand_);
    gdouble             g_rand_double_range                 (GRand *rand_,
                                                             gdouble begin,
                                                             gdouble end);
    void                g_random_set_seed                   (guint32 seed);
    #define             g_random_boolean
    guint32             g_random_int                        (void);
    gint32              g_random_int_range                  (gint32 begin,
                                                             gint32 end);
    gdouble             g_random_double                     (void);
    gdouble             g_random_double_range               (gdouble begin,
                                                             gdouble end);

## Description

The following functions allow you to use a portable, fast and good pseudo-random number generator (PRNG).

### Warning

Do not use this API for cryptographic purposes such as key generation, nonces, salts or one-time pads.

This PRNG is suitable for non-cryptographic use such as in games (shuffling a card deck, generating levels), generating data for a test suite, etc. If you need random data for cryptographic purposes, it is recommended to use platform-specific APIs such as `/dev/random` on Unix, or `CryptGenRandom()` on Windows.

GRand uses the Mersenne Twister PRNG, which was originally developed by Makoto Matsumoto and Takuji Nishimura. Further information can be found at http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html.

If you just need a random number, you simply call the `g_random_*` functions, which will create a globally used GRand and use the according `g_rand_*` functions internally. Whenever you need a stream of reproducible random numbers, you better create a GRand yourself and use the `g_rand_*` functions directly, which will also be slightly faster. Initializing a GRand with a certain seed will produce exactly the same series of random numbers on all platforms. This can thus be used as a seed for e.g. games.

The `g_rand*_range` functions will return high quality equally distributed random numbers, whereas for example the `(`g_random_int()`%max)` approach often doesn't yield equally distributed numbers.

GLib changed the seeding algorithm for the pseudo-random number generator Mersenne Twister, as used by GRand and GRandom. This was necessary, because some seeds would yield very bad pseudo-random streams. Also the pseudo-random integers generated by `g_rand*_int_range()` will have a slightly better equal distribution with the new version of GLib.

The original seeding and generation algorithms, as found in GLib 2.0.x, can be used instead of the new ones by setting the environment variable `G_RANDOM_VERSION` to the value of '2.0'. Use the GLib-2.0 algorithms only if you have sequences of numbers generated with Glib-2.0 that you need to reproduce exactly.

## Details

### GRand

    typedef struct _GRand GRand;

The GRand struct is an opaque data structure. It should only be accessed through the `g_rand_*` functions.

### g_rand_new_with_seed ()

    GRand *             g_rand_new_with_seed                (guint32 seed);

Creates a new random number generator initialized with `seed`.

`seed` : | a value to initialize the random number generator.
Returns : | the new GRand.

### g_rand_new_with_seed_array ()

    GRand *             g_rand_new_with_seed_array          (const guint32 *seed,
                                                             guint seed_length);

Creates a new random number generator initialized with `seed`.

`seed` : | an array of seeds to initialize the random number generator.
`seed_length` : | an array of seeds to initialize the random number generator.
Returns : | the new GRand.

Since 2.4

### g_rand_new ()

    GRand *             g_rand_new                          (void);

Creates a new random number generator initialized with a seed taken either from `/dev/urandom` (if existing) or from the current time (as a fallback). On Windows, the seed is taken from `rand_s()`.

Returns : | the new GRand.

### g_rand_copy ()

    GRand *             g_rand_copy                         (GRand *rand_);

Copies a GRand into a new one with the same exact state as before. This way you can take a snapshot of the random number generator for replaying later.

`rand_` : | a GRand.
Returns : | the new GRand.

Since 2.4

### g_rand_free ()

    void                g_rand_free                         (GRand *rand_);

Frees the memory allocated for the GRand.

`rand_` : | a GRand.

### g_rand_set_seed ()

    void                g_rand_set_seed                     (GRand *rand_,
                                                             guint32 seed);

Sets the seed for the random number generator GRand to `seed`.

`rand_` : | a GRand.
`seed` : | a value to reinitialize the random number generator.

### g_rand_set_seed_array ()

    void                g_rand_set_seed_array               (GRand *rand_,
                                                             const guint32 *seed,
                                                             guint seed_length);

Initializes the random number generator by an array of longs. Array can be of arbitrary size, though only the first 624 values are taken. This function is useful if you have many low entropy seeds, or if you require more then 32bits of actual entropy for your application.

`rand_` : | a GRand.
`seed` : | array to initialize with
`seed_length` : | length of array

Since 2.4

### g_rand_boolean()

    #define             g_rand_boolean(rand_)

Returns a random gboolean from `rand_`. This corresponds to a unbiased coin toss.

`rand_` : | a GRand.
Returns : | a random gboolean.

### g_rand_int ()

    guint32             g_rand_int                          (GRand *rand_);

Returns the next random guint32 from `rand_` equally distributed over the range [0..2^32-1].

`rand_` : | a GRand.
Returns : | A random number.

### g_rand_int_range ()

    gint32              g_rand_int_range                    (GRand *rand_,
                                                             gint32 begin,
                                                             gint32 end);

Returns the next random gint32 from `rand_` equally distributed over the range [`begin`..`end`-1].

`rand_` : | a GRand.
`begin` : | lower closed bound of the interval.
`end` : | upper open bound of the interval.
Returns : | A random number.

### g_rand_double ()

    gdouble             g_rand_double                       (GRand *rand_);

Returns the next random gdouble from `rand_` equally distributed over the range [0..1).

`rand_` : | a GRand.
Returns : | A random number.

### g_rand_double_range ()

    gdouble             g_rand_double_range                 (GRand *rand_,
                                                             gdouble begin,
                                                             gdouble end);

Returns the next random gdouble from `rand_` equally distributed over the range [`begin`..`end`).

`rand_` : | a GRand.
`begin` : | lower closed bound of the interval.
`end` : | upper open bound of the interval.
Returns : | A random number.

### g_random_set_seed ()

    void                g_random_set_seed                   (guint32 seed);

Sets the seed for the global random number generator, which is used by the `g_random_*` functions, to `seed`.

`seed` : | a value to reinitialize the global random number generator.

### g_random_boolean

    #define             g_random_boolean()

Returns a random gboolean. This corresponds to a unbiased coin toss.

Returns : | a random gboolean.

### g_random_int ()

    guint32             g_random_int                        (void);

Return a random guint32 equally distributed over the range [0..2^32-1].

Returns : | A random number.

### g_random_int_range ()

    gint32              g_random_int_range                  (gint32 begin,
                                                             gint32 end);

Returns a random gint32 equally distributed over the range [`begin`..`end`-1].

`begin` : | lower closed bound of the interval.
`end` : | upper open bound of the interval.
Returns : | A random number.

### g_random_double ()

    gdouble             g_random_double                     (void);

Returns a random gdouble equally distributed over the range [0..1).

Returns : | A random number.

### g_random_double_range ()

    gdouble             g_random_double_range               (gdouble begin,
                                                             gdouble end);

Returns a random gdouble equally distributed over the range [`begin`..`end`).

`begin` : | lower closed bound of the interval.
`end` : | upper open bound of the interval.
Returns : | A random number.