CON33-C. Avoid race conditions when using library functions

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.

Library Functions Subject to Data Races

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, 7.24.6.2 [ISO/IEC 9899:2011], specifically states that strerror() is not required to avoid data races. 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);
  }
}

Note that this code first sets errno to 0 to comply with ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only after the function returns a value indicating failure. 

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);
  }
}

Note that 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>

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 because of %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). Thestrerror_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 denial-of-service attack.

Related Vulnerabilities

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

Automated Detection

Related Guidelines

Bibliography