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About: About this document Memory Management

A useful idiom common to many GNU projects is to wrap the memory management functions to localise out of memory handling, naming them with an `x' prefix. By doing this, the rest of the project is relieved of having to remember to check for `NULL' returns from the various memory functions. These wrappers use the error API to report memory exhaustion and abort the program. I have placed the implementation code in `xmalloc.c':

#  include <config.h>

#include "common.h"
#include "error.h"

void *
xmalloc (size_t num)
  void *new = malloc (num);
  if (!new)
    sic_fatal ("Memory exhausted");
  return new;

void *
xrealloc (void *p, size_t num)
  void *new;

  if (!p)
    return xmalloc (num);

  new = realloc (p, num);
  if (!new)
    sic_fatal ("Memory exhausted");

  return new;

void *
xcalloc (size_t num, size_t size)
  void *new = xmalloc (num * size);
  bzero (new, num * size);
  return new;

Notice in the code above, that xcalloc is implemented in terms of xmalloc, since calloc itself is not available in some older C libraries. Also, the bzero function is actually deprecated in favour of memset in modern C libraries -- I'll explain how to take this into account later in 9.2.3 Beginnings of a `'.

Rather than create a separate `xmalloc.h' file, which would need to be #included from almost everywhere else, the logical place to declare these functions is in `common.h', since the wrappers will be called from most everywhere else in the code:

#ifdef __cplusplus
#  define BEGIN_C_DECLS         extern "C" {
#  define END_C_DECLS           }
#  define BEGIN_C_DECLS
#  define END_C_DECLS

#define XCALLOC(type, num)                                  \
        ((type *) xcalloc ((num), sizeof(type)))
#define XMALLOC(type, num)                                  \
        ((type *) xmalloc ((num) * sizeof(type)))
#define XREALLOC(type, p, num)                              \
        ((type *) xrealloc ((p), (num) * sizeof(type)))
#define XFREE(stale)                            do {        \
        if (stale) { free (stale);  stale = 0; }            \
                                                } while (0)


extern void *xcalloc    (size_t num, size_t size);
extern void *xmalloc    (size_t num);
extern void *xrealloc   (void *p, size_t num);
extern char *xstrdup    (const char *string);
extern char *xstrerror  (int errnum);


By using the macros defined here, allocating and freeing heap memory is reduced from:

char **argv = (char **) xmalloc (sizeof (char *) * 3);
do_stuff (argv);
if (argv)
  free (argv);

to the simpler and more readable:

char **argv = XMALLOC (char *, 3);
do_stuff (argv);
XFREE (argv);

In the same spirit, I have borrowed `xstrdup.c' and `xstrerror.c' from project GNU's libiberty. See section 9.1.5 Fallback Function Implementations.

This document was generated by Gary V. Vaughan on February, 8 2006 using texi2html