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Although not specified by POSIX, arc4random() is an option on systems that support it. From the arc4random(3) manual page:

arc4random() fits into a middle ground not covered by other subsystems such as the strong, slow, and resource expensive random devices described in random(4) versus the fast but poor quality interfaces described in rand(3), random(3), and drand48(3).

To achieve the best random numbers possible, an implementation-specific function must be used. When unpredictability really matters and speed is not an issue,

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as in the creation of strong cryptographic keys, use a true entropy source, such as /dev/random or a hardware device capable of generating random numbers. Note that the /dev/random device may block for a long time if there are not enough events going on to generate sufficient entropy.

Compliant Solution (Windows)

On Windows platforms, the [{{CryptGenRandom()}}|http://msdn2.microsoft.com/en-us/library/aa379942.aspx] function may be used to generate cryptographically strong random numbers.  Note that the exact details of the implementation are unknown, including, for example,  what source of entropy {{CryptGenRandom()}} uses.  From the Microsoft Developer Network {{CryptGenRandom()}} reference \[[MSDN|AA. Bibliography#MSDN]\]:

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If an application has access to a good random source, it can fill the {{pbBuffer}} buffer with some random data before calling {{CryptGenRandom()}}. The CSP \[cryptographic service provider\] then uses this data to further randomize its internal seed. It is acceptable to omit the step of initializing the {{pbBuffer}} buffer before calling {{CryptGenRandom()}}.

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Automated Detection

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Tool	Version	Checker	Description
Section LDRA tool suite

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	Include Page c:LDRA_V c:LDRA_V
Section Fortify SCA	Section V. 5.0
Section Compass/ROSE

Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this rule.

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

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Related Guidelines

This rule appears in the C++ Secure Coding Standard as : MSC30-CPP. Do not use the rand() function for generating pseudorandom numbers.

This rule appears in the Java Secure Coding Standard as MSC30: MSC02-J. Generate truly strong random numbers.

Bibliography

\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 7.20.2.1, "The rand function"
\[[MITRE 072007|AA. Bibliography#MITRE 07]\] [CWE ID 327|http://cwe.mitre.org/data/definitions/327.html], "Use of a Broken or Risky Cryptographic Algorithm," [CWE ID 330|http://cwe.mitre.org/data/definitions/330.html], "Use of Insufficiently Random Values"
\[[MSDN|AA. Bibliography#MSDN]\] "[CryptGenRandom Function|http://msdn.microsoft.com/en-us/library/aa379942.aspx]"

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