Only free memory allocated dynamically

Freeing memory that is not allocated dynamically can lead to serious errors. The specific consequences of this error depends on the compiler, but ranges from nothing to abnormal program termination. Regardless of the compiler, avoid calling free() on non-dynamic memory.

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.

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.

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;
}

Compliant Solution 1

In the compliant solution, theprogram has been changed to eliminate the possibility of str referencing non-dynamic memory and when it is supplied to free().

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 {    
    printf("usage: $>a.exe [string]\n");
    return 1;
  }
  /* ... */
  free(str);
  return 0;
}

Non-compliant Code Example 2

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.

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.

/* 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);
}

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.

j1This violates recommendation 1 (sort of)