Some functions in the C standard library are not guaranteed to be reentrant with respect to threads. Some functions (such as strtok() and asctime()) return a pointer to the result stored in function-allocated memory on a per-process basis. Other functions (such as rand()) store state information in function-allocated memory on a per-process basis. Multiple threads invoking the same function can cause concurrency problems, which often result in abnormal behavior and can cause more serious vulnerabilities such as abnormal termination, denial-of-service attack, and data integrity violations.

As per the N1401-C1X document, the following library functions are not required to avoid data races:

• rand()
• getenv()
• strtok()
• strerror()
• asctime()
• ctime()

Section 2.9.1 of the System Interfaces volume of POSIX.1-2008 has a much longer list of functions that are not required to be thread safe.

Noncompliant Code Example

Consider a multithreaded application that encounters an error while calling a system function. The strerror() function returns a human-readable error string given an error number. According to C99, Section 7.22.6.2 specifically states that strerror() is not required to avoid data races. Conventionally it could rely on a static array that maps error numbers to error strings, and that array might be accessible and modifiable by other threads.

FILE* fd = fopen( filename, "r");
if (fd == NULL) {
  char* errmsg = strerror(errno);
  printf("Could not open file because of %s\n", errmsg);
}

Compliant Solution

The compliant solution uses strerror_r(), which has the same functionality as strerror() but guarantees thread safety.

FILE* fd = fopen( filename, "r");
if (fd == NULL) {
  char errmsg[BUFSIZ];
  if (strerror_r( errno, errmsg, BUFSIZ) != 0) {
    /* handle error */
  }
  printf("Could not open file because of %s\n", errmsg);
}

Risk Assessment

Race conditions caused by multiple threads invoking the same library function can lead to abnormal termination of the application, data integrity violations, or denial-of-service attack.

Automated Detection

A module written in Compass/ROSE can detect violations of this rule.

References

\[[N1401-C1X Draft|http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1401.pdf]\] Section 7.21.2.1 rand() function, Section 7.21.4.6 getenv() function, Section 7.22.5.8 strtok() function, Section 7.22.6.2 strerror() function, Section 7.25.3.1 asctime() function, Section 7.25.3.2 ctime() function
\[[Historical information about POSIX.1 Thread Safety|http://www.unix.org/whitepapers/reentrant.html]\]