---
name: "srand"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/rand/3"
---

# srand(3)

bad random number generator

Part of [rand(3)](/rand/3.md).

## NAME

**rand**, **rand\_r**, **srand** – bad random number generator

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<stdlib.h\>**

*int*\

**rand**(*void*);

*int*\

**rand\_r**(*unsigned \*ctx*);

*void*\

**srand**(*unsigned seed*);

## DESCRIPTION

The functions described in this manual page are not cryptographically secure. Applications which require unpredictable random numbers should use [arc4random(3)](/arc4random/3.md) instead.

The **rand**() function computes a sequence of pseudo-random integers in the range of 0 to RAND\_MAX, inclusive.

The **srand**() function seeds the algorithm with the *seed* parameter. Repeatable sequences of **rand**() output may be obtained by calling **srand**() with the same *seed*. **rand**() is implicitly initialized as if **srand**(*1*) had been invoked explicitly.

In FreeBSD 13, **rand**() is implemented using the same 128-byte state LFSR generator algorithm as [random(3)](/random/3.md). However, the legacy **rand\_r**() function is not (and can not be, because of its limited *\*ctx* size). **rand\_r**() implements the historical, poor-quality Park-Miller 32-bit LCG and should not be used in new designs.

## IMPLEMENTATION NOTES

Since FreeBSD 13, **rand**() is implemented with the same generator as [random(3)](/random/3.md), so the low-order bits should no longer be significantly worse than the high-order bits.

## SEE ALSO

[arc4random(3)](/arc4random/3.md), [random(3)](/random/3.md), [random(4)](/random/4.md)

## STANDARDS

The **rand**() and **srand**() functions conform to ISO/IEC 9899:1990 (“ISO C90”).

The **rand\_r**() function is not part of ISO/IEC 9899:1990 (“ISO C90”) and is marked obsolescent in IEEE Std 1003.1-2008 (“POSIX.1”). It may be removed in a future revision of POSIX.

## CAVEATS

Prior to FreeBSD 13, **rand**() used the historical Park-Miller generator with 32 bits of state and produced poor quality output, especially in the lower bits. **rand**() in earlier versions of FreeBSD, as well as other standards-conforming implementations, may continue to produce poor quality output.

*These* *functions* *should* *not* *be* *used* *in* *portable* *applications* *that* *want* *a* *high* *quality* *or* *high* *performance* *pseudorandom* *number* *generator*. One possible replacement, [random(3)](/random/3.md), is portable to Linux — but it is not especially fast, nor standardized.

If broader portability or better performance is desired, any of the widely available and permissively licensed SFC64/32, JSF64/32, PCG64/32, or SplitMix64 algorithm implementations may be embedded in your application. These algorithms have the benefit of requiring less space than [random(3)](/random/3.md) and being quite fast (in header inline implementations).