MSC30-C. Do not use the rand function

The C Standard function rand (available in stdlib.h) does not have good random number properties. The numbers generated by rand have a comparatively short cycle, and the numbers may be predictable. To achieve the best random numbers possible, an implementation-specific function needs to be used.

Non-Compliant Code Example

The following code generates an ID with a numeric part produced by calling the rand() function. The IDs produced will be predictable and have limited randomness.

enum {len = 12};
char id[len];  /* id will hold the ID, starting with the characters "ID" */
               /* followed by a random integer */
int r;
int num;
/* ... */
r = rand();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

Compliant Solution (BSD)

A better pseudo random number generator is the BSD function random().

enum {len = 12};
char id[len];  /* id will hold the ID, starting with the characters "ID" */
               /* followed by a random integer */
int r;
int num;
/* ... */
srandom(time(0));  /* seed the PRNG with the current time */
/* ... */
r = random();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

The rand48 family of functions provides another alternative.

Note. These pseudo random number generators use mathematical algorithms to produce a sequence of numbers with good statistical properties, but the numbers produced are not genuinely random. For true randomness, Linux users can use the character devices /dev/random or /dev/urandom, but it is advisable to retrieve only a small number of characters from these devices. (The device /dev/random may block for a long time if there are not enough events going on to generate sufficient randomness; /dev/urandom does not block.)

Risk Assessment

Using the rand function may lead to programming problems (for example, non-unique unique IDs) or weak cryptography.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.20.2.1, "The rand function"