Some C standard library functions are not guaranteed to be reentrant with respect to threads. 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.

According to the C Standard, the library functions listed in the following table may contain data races when invoked by multiple threads.

Section 2.9.1 of the Portable Operating System Interface (POSIX^®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013] extends the list of functions that are not required to be thread-safe.

Noncompliant Code Example

In this noncompliant code example, the function f() is called from within a multithreaded application but encounters an error while calling a system function. The strerror() function returns a human-readable error string given an error number.

The C Standard, 7.26.6.3 paragraph 3 [ISO/IEC 9899:2024], specifically states that strerror() is not required to avoid data races.

The strerror function is not required to avoid data races with other calls to the strerror function.

An implementation could write the error string into a static array and return a pointer to it, and that array might be accessible and modifiable by other threads.

#include <errno.h>
#include <stdio.h>
#include <string.h>
 
void f(FILE *fp) {
  fpos_t pos;
  errno = 0;

  if (0 != fgetpos(fp, &pos)) {
    char *errmsg = strerror(errno);
    printf("Could not get the file position: %s\n", errmsg);
  }
}

This code first sets errno to 0 to comply with ERR30-C. Take care when reading errno. 

Compliant Solution (Annex K, strerror_s()) 

This compliant solution uses the strerror_s() function from Annex K of the C Standard, which has the same functionality as strerror() but guarantees thread-safety:

#define __STDC_WANT_LIB_EXT1__ 1
#include <errno.h>
#include <stdio.h>
#include <string.h>
 
enum { BUFFERSIZE = 64 };
void f(FILE *fp) {
  fpos_t pos;
  errno = 0;

  if (0 != fgetpos(fp, &pos)) {
    char errmsg[BUFFERSIZE];
    if (strerror_s(errmsg, BUFFERSIZE, errno) != 0) {
      /* Handle error */
    }
    printf("Could not get the file position: %s\n", errmsg);
  }
}

Because Annex K is optional, strerror_s() may not be available in all implementations. 

Compliant Solution (POSIX, strerror_r())

This compliant solution uses the POSIX strerror_r() function, which has the same functionality as strerror() but guarantees thread safety:

#include <errno.h>
#include <stdio.h>
#include <string.h>

enum { BUFFERSIZE = 64 };
 
void f(FILE *fp) {
  fpos_t pos;
  errno = 0;

  if (0 != fgetpos(fp, &pos)) {
    char errmsg[BUFFERSIZE];
    if (strerror_r(errno, errmsg, BUFFERSIZE) != 0) {
      /* Handle error */
    }
    printf("Could not get the file position: %s\n", errmsg);
  }
}

Linux provides two versions of strerror_r(), known as the XSI-compliant version and the GNU-specific version. This compliant solution assumes the XSI-compliant version, which is the default when an application is compiled as required by POSIX (that is, by defining _POSIX_C_SOURCE or _XOPEN_SOURCE appropriately). The strerror_r() manual page lists versions that are available on a particular system.

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 a denial-of-service attack.

Related Vulnerabilities

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

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported, but no explicit checker
CodeSonar		BADFUNC.RANDOM.RAND BADFUNC.TEMP.TMPNAM BADFUNC.TTYNAME	Use of rand (includes check for uses of srand()) Use of tmpnam (includes check for uses of tmpnam_r()) Use of ttyname
Compass/ROSE			A module written in Compass/ROSE can detect violations of this rule
Cppcheck Premium		premium-cert-con33-c
Helix QAC		C5037 C++5021 DF4976, DF4977
Klocwork		CERT.CONC.LIB_FUNC_USE
LDRA tool suite		44 S	Partially Implemented
Parasoft C/C++test		CERT_C-CON33-a	Avoid using thread-unsafe functions
PC-lint Plus		586	Fully supported
Polyspace Bug Finder		CERT C: Rule CON33-C	Checks for data race through standard library function call (rule fully covered)

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-330 and CON33-C

Independent( MSC30-C, MSC32-C, CON33-C)

Intersection( CWE-330, CON33-C) =

• Use of rand() or srand() from multiple threads, introducing a race condition.

CWE-330 – CON33-C =

• Use of rand() or srand() without introducing race conditions

• Use of other dangerous functions

CON33-C – CWE-330 =

• Use of other global functions (besides rand() and srand()) introducing race conditions

CWE-377 and CON33-C

Intersection( CWE-377, CON33-C) =

• Use of tmpnam() from multiple threads, introducing a race condition.

CWE-377 – CON33-C =

• Insecure usage of tmpnam() without introducing race conditions

• Insecure usage of other functions for creating temporary files (see CERT recommendation FIO21-C for details)

CON33-C – CWE-377 =

• Use of other global functions (besides tmpnam()) introducing race conditions

CWE-676 and CON33-C

• Independent( ENV33-C, CON33-C, STR31-C, EXP33-C, MSC30-C, ERR34-C)

• CON33-C lists standard C library functions that manipulate global data (e.g., locale()), that can be dangerous to use in a multithreaded context.

• CWE-676 = Union( CON33-C, list) where list =

• Invocation of the following functions without introducing a race condition:

• rand(), srand(, getenv(), getenv_s(), strtok(), strerror(), asctime(), ctime(), localtime(), gmtime(), setlocale(), ATOMIC_VAR_INIT, atomic_init(), tmpnam(), mbrtoc16(), c16rtomb(), mbrtoc32(), c32rtomb()

• Invocation of other dangerous functions

Bibliography