Before the lifetime of the last pointer that stores the return value of a call to a standard memory allocation function has ended, it must be matched by a call to free() with that pointer value.

Noncompliant Code Example

In this noncompliant example, the object allocated by the call to malloc() is not freed before the end of the lifetime of the last pointer text_buffer referring to the object:

#include <stdlib.h>
 
enum { BUFFER_SIZE = 32 };

int f(void) {
  char *text_buffer = (char *)malloc(BUFFER_SIZE); 
  if (text_buffer == NULL) {
    return -1;
  }
  return 0;
}

Compliant Solution

In this compliant solution, the pointer is deallocated with a call to free():

#include <stdlib.h>

enum { BUFFER_SIZE = 32 };

int f(void) {
  char *text_buffer = (char *)malloc(BUFFER_SIZE); 
  if (text_buffer == NULL) {
    return -1;
  }
 
  free(text_buffer);
  return 0;
}

Exceptions

MEM31-C-EX1: Allocated memory does not need to be freed if it is assigned to a pointer with static storage duration whose lifetime is the entire execution of a program. The following code example illustrates a pointer that stores the return value from malloc() in a static variable:

#include <stdlib.h>
 
enum { BUFFER_SIZE = 32 };

int f(void) {
  static char *text_buffer = NULL;
  if (text_buffer == NULL) {
    text_buffer = (char *)malloc(BUFFER_SIZE); 
    if (text_buffer == NULL) {
      return -1;
    }
  }
  return 0;
}

Risk Assessment

Failing to free memory can result in the exhaustion of system memory resources, which can lead to a denial-of-service attack.

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported, but no explicit checker
CodeSonar		ALLOC.LEAK	Leak
Compass/ROSE
Coverity		RESOURCE_LEAK ALLOC_FREE_MISMATCH	Finds resource leaks from variables that go out of scope while owning a resource
Cppcheck		leakReturnValNotUsed	Doesn't use return value of memory allocation function
Klocwork		MLK.MIGHT MLK.MUST MLK.RET.MUST MLK.RET.MIGHT
LDRA tool suite		50 D	Partially implemented
Parasoft C/C++test	9.5	BD-RES-LEAK
Parasoft Insure++			Detects leaks at runtime
Polyspace Bug Finder	R2016a	Memory leak	Memory allocated dynamically not freed
PRQA QA-C	9.1	1771
PRQA QA-C++	4.2	3337, 3338
SonarQube C/C++ Plugin		S3584
Splint

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-404/CWE-459/CWE-771/CWE-772 and FIO42-C/MEM31-C

Intersection( FIO42-C, MEM31-C) = Ø CWE-404 = CWE-459 = CWE-771 = CWE-772

CWE-404 = Union( FIO42-C, MEM31-C list) where list =

• Failure to free resources besides files or memory chunks, such as mutexes)

Bibliography