Page History

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 also terminate abnormally.

h2. Non-compliant Code Example 1

The following piece of code validates the number of command line arguemnts. If the correct number of commmand line arguements have been specified, memory is allocated with {{malloc()}} and referenced by {{str}}. Next, the second command line argument is copied into {{str}} for further processing. Once this processing is complete, {{str}} is freed.However, if the incorrect number of arguments have been specified, {{str}} is set to a string literal and printed. Because {{str}} now references memory that was not dynamically allocated, an error will occur when {{str}} memory is freed.

{code

}

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

This example attempts to resize the string referenced by {{buf}} to make enough room to append the string {{line}}. However, once in the function {{append()}}, there is no way to determine how {{buf}} was allocated. When {{realloc()}} is called on {{buf}}, since {{buf}} does not point to dynamic memory, an error may occur.

{code

}
void append(char *buf, size_t count, size_t size) {
  char *line = " <- THIS IS A LINE";
  int line_len = strlen(line);

  if ((count + line_len) > size) 
    buf = realloc(buf,count+line_len);
  strcat(buf,line);
}

h2. Compliant Solution 2

Correcting the above example is an exercise in documentation. Since realloc is used to resize the memory pointed to by buf, the function append has the precondition that buf must point to dynamically allocated memory.
{code}
/\* NOTE: buf must point to dynamically allocated memory \*/

void append(char \*buf, size_t count, size_t size)
{

   char *line = " <- THIS IS A LINE";

   int line_len = strlen(line);

    if ((count + line_len) > size)

      buf = realloc(buf,count+line_len);

   strncat(buf,line);

 }
{code}

{code}

h2. Compliant Code Example 2A

Alternatively, the function append could be rewritten not to use {{realloc()}} to resize buf. This solution goes beyond the scope of this document, but is nonetheless viable and, depending on the context of the program, may be preferred.

----
[j1]This violates recommendation 1 (sort of)