---
name: "random_r"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/random_r"
---

# random\_r(3)

reentrant random number generator

## NAME

random\_r, srandom\_r, initstate\_r, setstate\_r - reentrant random number generator

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <stdlib.h>
```

**int random\_r(struct random\_data \*restrict** *buf***,**              **int32\_t \*restrict** *result***);** **int srandom\_r(unsigned int** *seed***, struct random\_data \****buf***);**

**int initstate\_r(unsigned int** *seed***,**              **char** *statebuf***\[restrict .***statelen***\], size\_t** *statelen***,**              **struct random\_data \*restrict** *buf***);** **int setstate\_r(char \*restrict** *statebuf***,**              **struct random\_data \*restrict** *buf***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**random\_r**(), **srandom\_r**(), **initstate\_r**(), **setstate\_r**():

```
    /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
```

## DESCRIPTION

These functions are the reentrant equivalents of the functions described in [**random**(3)](/random/3.md). They are suitable for use in multithreaded programs where each thread needs to obtain an independent, reproducible sequence of random numbers.

The **random\_r**() function is like [**random**(3)](/random/3.md), except that instead of using state information maintained in a global variable, it uses the state information in the argument pointed to by *buf*, which must have been previously initialized by **initstate\_r**(). The generated random number is returned in the argument *result*.

The **srandom\_r**() function is like [**srandom**(3)](/srandom/3.md), except that it initializes the seed for the random number generator whose state is maintained in the object pointed to by *buf*, which must have been previously initialized by **initstate\_r**(), instead of the seed associated with the global state variable.

The **initstate\_r**() function is like [**initstate**(3)](/initstate/3.md), except that it initializes the state in the object pointed to by *buf*, rather than initializing the global state variable. Before calling this function, the *buf.state* field must be initialized to NULL. The **initstate\_r**() function records a pointer to the *statebuf* argument inside the structure pointed to by *buf*. Thus, *statebuf* should not be deallocated so long as *buf* is still in use. (So, *statebuf* should typically be allocated as a static variable, or allocated on the heap using [**malloc**(3)](/malloc/3.md) or similar.)

The **setstate\_r**() function is like [**setstate**(3)](/setstate/3.md), except that it modifies the state in the object pointed to by *buf*, rather than modifying the global state variable. *state* must first have been initialized using **initstate\_r**() or be the result of a previous call of **setstate\_r**().

## RETURN VALUE

All of these functions return 0 on success. On error, -1 is returned, with *errno* set to indicate the error.

## ERRORS

- **EINVAL**

  A state array of less than 8 bytes was specified to **initstate\_r**().

- **EINVAL**

  The *statebuf* or *buf* argument to **setstate\_r**() was NULL.

- **EINVAL**

  The *buf* or *result* argument to **random\_r**() was NULL.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **random\_r**(), **srandom\_r**(), **initstate\_r**(), **setstate\_r**() | Thread safety | MT-Safe race:buf |

## STANDARDS

GNU.

## BUGS

The **initstate\_r**() interface is confusing. It appears that the *random\_data* type is intended to be opaque, but the implementation requires the user to either initialize the *buf.state* field to NULL or zero out the entire structure before the call.

## SEE ALSO

[**drand48**(3)](/drand48/3.md), [**rand**(3)](/rand/3.md), [**random**(3)](/random/3.md)