__malloc_hook(3)

1__malloc_hook(3)           Library Functions Manual           __malloc_hook(3)
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NAME

6       __malloc_hook,  __malloc_initialize_hook, __memalign_hook, __free_hook,
7       __realloc_hook,  __after_morecore_hook  -  malloc  debugging  variables
8       (DEPRECATED)
9

LIBRARY

11       Standard C library (libc, -lc)
12

SYNOPSIS

14       #include <malloc.h>
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16       void *(*volatile __malloc_hook)(size_t size, const void *caller);
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18       void *(*volatile __realloc_hook)(void *ptr, size_t size,
19                                const void *caller);
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21       void *(*volatile __memalign_hook)(size_t alignment, size_t size,
22                                const void *caller);
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24       void (*volatile __free_hook)(void *ptr, const void *caller);
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26       void (*__malloc_initialize_hook)(void);
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28       void (*volatile __after_morecore_hook)(void);
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DESCRIPTION

31       The  GNU  C  library  lets  you modify the behavior of malloc(3), real‐
32       loc(3), and free(3) by specifying appropriate hook functions.  You  can
33       use  these hooks to help you debug programs that use dynamic memory al‐
34       location, for example.
35
36       The variable __malloc_initialize_hook points  at  a  function  that  is
37       called  once  when the malloc implementation is initialized.  This is a
38       weak variable, so it can be overridden in the application with a  defi‐
39       nition like the following:
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41           void (*__malloc_initialize_hook)(void) = my_init_hook;
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43       Now the function my_init_hook() can do the initialization of all hooks.
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45       The four functions pointed to by __malloc_hook, __realloc_hook, __mema‐
46       lign_hook, __free_hook have a prototype like the  functions  malloc(3),
47       realloc(3), memalign(3), free(3), respectively, except that they have a
48       final argument caller that gives the address  of  the  caller  of  mal‐
49       loc(3), etc.
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51       The  variable __after_morecore_hook points at a function that is called
52       each time after sbrk(2) was asked for more memory.
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STANDARDS

55       GNU.
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NOTES

58       The use of these hook functions is not safe in multithreaded  programs,
59       and  they  are  now  deprecated.   From  glibc 2.24 onwards, the __mal‐
60       loc_initialize_hook variable has been removed from the  API,  and  from
61       glibc  2.34  onwards, all the hook variables have been removed from the
62       API.  Programmers should instead preempt calls to  the  relevant  func‐
63       tions  by  defining and exporting malloc(), free(), realloc(), and cal‐
64       loc().
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EXAMPLES

67       Here is a short example of how to use these variables.
68
69       #include <stdio.h>
70       #include <malloc.h>
71
72       /* Prototypes for our hooks */
73       static void my_init_hook(void);
74       static void *my_malloc_hook(size_t, const void *);
75
76       /* Variables to save original hooks */
77       static void *(*old_malloc_hook)(size_t, const void *);
78
79       /* Override initializing hook from the C library */
80       void (*__malloc_initialize_hook)(void) = my_init_hook;
81
82       static void
83       my_init_hook(void)
84       {
85           old_malloc_hook = __malloc_hook;
86           __malloc_hook = my_malloc_hook;
87       }
88
89       static void *
90       my_malloc_hook(size_t size, const void *caller)
91       {
92           void *result;
93
94           /* Restore all old hooks */
95           __malloc_hook = old_malloc_hook;
96
97           /* Call recursively */
98           result = malloc(size);
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100           /* Save underlying hooks */
101           old_malloc_hook = __malloc_hook;
102
103           /* printf() might call malloc(), so protect it too */
104           printf("malloc(%zu) called from %p returns %p\n",
105                   size, caller, result);
106
107           /* Restore our own hooks */
108           __malloc_hook = my_malloc_hook;
109
110           return result;
111       }
112