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The following code generates an ID with a numeric part produced by calling the rand() function. The IDs produced are 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 */
/* ... */

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A better pseudorandom number generator is the random() function. While the low dozen bits generated by rand() go through a cyclic pattern, all the bits generated by random() are usable.

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;
/* ... */
time_t now = time(NULL);
if (now == (time_t) -1) {
  /* handle error */
}
srandom(now);  /* seed the PRNG with the current time */
/* ... */
r = random();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

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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).

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Wiki Markup
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()}}.

#include<Wincrypt.h>

HCRYPTPROV hCryptProv;
union {
    BYTE bs[sizeof(long int)];
    long int li;
} rand_buf;

if (!CryptGenRandom(hCryptProv, sizeof(rand_buf), &rand_buf) {
    /* Handle error */
} else {
    printf("Random number: %ld\n", rand_buf.li);
}

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\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.2.1, "The rand function"
\[[MITRE 07|AA. C References#MITRE 07]\] [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"
\[[MSDN|AA. C References#MSDN]\] "[CryptGenRandom Function|http://msdn.microsoft.com/en-us/library/aa379942.aspx]"

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Image Modified      49. Miscellaneous (MSC)      MSC31-C. Ensure that return values are compared against the proper type