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As noted under undefined behavior 179 in Annex J of the C Standard [ISO/IEC 9899:2011], the behavior of a program is undefined when

the pointer argument to the free or realloc function does not match a pointer earlier returned by a memory management function, or the space has been deallocated by a call to free or realloc.

Freeing memory that is not allocated dynamically can lead to serious errors similar to those discussed in MEM31-C. Free dynamically allocated memory when no longer needed. The consequences of this error depend on the implementation, but they range from nothing to abnormal program termination. Regardless of the implementation, do not call free() on anything other than a pointer returned by a dynamic memory allocation function, such as malloc(), calloc(), realloc(), or aligned_alloc().

A similar situation arises when realloc() is supplied a pointer to non-dynamically allocated memory. The realloc() function is used to resize a block of dynamic memory. If realloc() is supplied a pointer to memory not allocated by a memory allocation function, such as malloc(), the program may terminate abnormally.

This rule does not apply to null pointers. The standard officially declares that when given a null pointer, free() does nothing.

Noncompliant Code Example

This noncompliant code example sets str to reference either dynamically allocated memory or a statically allocated string literal depending on the value of argc. In either case, str is passed as an argument to free(). If anything other than dynamically allocated memory is referenced by str, the call to free(str) is erroneous.

#include <stdlib.h>
#include <string.h>
 
enum { MAX_ALLOCATION = 1000 };

int main(int argc, const char *argv[]) {
  char *str = NULL;
  size_t len;

  if (argc == 2) {
    len = strlen(argv[1]) + 1;
    if (len > MAX_ALLOCATION) {
      /* Handle error */
    }
    str = (char *)malloc(len);
    if (str == NULL) {
      /* Handle error */
    }
    strcpy(str, argv[1]);
  } else {
    str = "usage: $>a.exe [string]";
    printf("%s\n", str);
  }
  free(str);
  return 0;
}

Compliant Solution

This compliant solution eliminates the possibility of str, referencing nondynamic memory when it is supplied to free():

#include <stdlib.h>
#include <string.h>
 
enum { MAX_ALLOCATION = 1000 };

int main(int argc, const char *argv[]) {
  char *str = NULL;
  size_t len;

  if (argc == 2) {
    len = strlen(argv[1]) + 1;
    if (len > MAX_ALLOCATION) {
      /* Handle error */
    }
    str = (char *)malloc(len);
    if (str == NULL) {
      /* Handle error */
    }
    strcpy(str, argv[1]);
  } else {
    printf("%s\n", "usage: $>a.exe [string]");
    return -1;
  }
  free(str);
  return 0;
}

Noncompliant Code Example (realloc())

In this noncompliant example, the pointer parameter to realloc(), buf, does not refer to dynamically allocated memory:

#include <stdlib.h>
 
enum { BUFSIZE = 256 };
 
void f(void) {
  char buf[BUFSIZE];
  char *p = (char *)realloc(buf, 2 * BUFSIZE);
  if (p == NULL) {
    /* handle error */
  }
}

Compliant Solution (realloc())

In this compliant solution, buf refers to dynamically allocated memory:

#include <stdlib.h>
 
enum { BUFSIZE = 256 };
 
void f(void) {
  char *buf = (char *)malloc(BUFSIZE * sizeof(char));
  char *p = (char *)realloc(buf, 2 * BUFSIZE);
  if (p == NULL) {
    /* handle error */
  }
}

Note that realloc() will behave properly even if malloc() failed, because when given a null pointer, realloc() behaves like a call to malloc().

Risk Assessment

Freeing or reallocating memory that was not dynamically allocated can lead to arbitrary code execution if that memory is reused by malloc().

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography