TALLOC(3)

NAME
talloc - hierarchical reference counted memory pool system with destructors

SYNOPSIS

#include <talloc.h>

DESCRIPTION

If you are used to talloc from Samba3 then please read this carefully, as talloc has changed a lot.

The new talloc is a hierarchical, reference counted memory pool system with destructors. Quite a mouthful really, but not too bad once you get used to it.

Perhaps the biggest change from Samba3 is that there is no distinction between a "talloc context" and a "talloc pointer". Any pointer returned from talloc() is itself a valid talloc context. This means you can do this:

        struct foo *X = talloc(mem_ctx, struct foo);
        X->name = talloc_strdup(X, "foo");

and the pointer X->name would be a "child" of the talloc context X which is itself a child of mem_ctx. So if you do talloc_free(mem_ctx) then it is all destroyed, whereas if you do talloc_free(X) then just X and X->name are destroyed, and if you do talloc_free(X->name) then just the name element of X is destroyed.

If you think about this, then what this effectively gives you is an n-ary tree, where you can free any part of the tree with talloc_free().

If you find this confusing, then I suggest you run the testsuite program to watch talloc in action. You may also like to add your own tests to testsuite.c to clarify how some particular situation is handled.

TALLOC API

The following is a complete guide to the talloc API. Read it all at least twice.

  (type *)talloc(const void *ctx, type);

The talloc() macro is the core of the talloc library. It takes a memory ctx and a type, and returns a pointer to a new area of memory of the given type.

The returned pointer is itself a talloc context, so you can use it as the ctx argument to more calls to talloc() if you wish.

The returned pointer is a "child" of the supplied context. This means that if you talloc_free() the ctx then the new child disappears as well. Alternatively you can free just the child.

The ctx argument to talloc() can be NULL, in which case a new top level context is created.

  void *talloc_size(const void *ctx, size_t size);

The function talloc_size() should be used when you don't have a convenient type to pass to talloc(). Unlike talloc(), it is not type safe (as it returns a void *), so you are on your own for type checking.

  (typeof(ptr)) talloc_ptrtype(const void *ctx, ptr);

The talloc_ptrtype() macro should be used when you have a pointer and want to allocate memory to point at with this pointer. When compiling with gcc >= 3 it is typesafe. Note this is a wrapper of talloc_size() and talloc_get_name() will return the current location in the source file. and not the type.

  int talloc_free(void *ptr);

The talloc_free() function frees a piece of talloc memory, and all its children. You can call talloc_free() on any pointer returned by talloc().

The return value of talloc_free() indicates success or failure, with 0 returned for success and -1 for failure. The only possible failure condition is if ptr had a destructor attached to it and the destructor returned -1. See “talloc_set_destructor()” for details on destructors.

If this pointer has an additional parent when talloc_free() is called then the memory is not actually released, but instead the most recently established parent is destroyed. See “talloc_reference()” for details on establishing additional parents.

For more control on which parent is removed, see “talloc_unlink()”.

talloc_free() operates recursively on its children.

From the 2.0 version of talloc, as a special case, talloc_free() is refused on pointers that have more than one parent, as talloc would have no way of knowing which parent should be removed. To free a pointer that has more than one parent please use talloc_unlink().

To help you find problems in your code caused by this behaviour, if you do try and free a pointer with more than one parent then the talloc logging function will be called to give output like this:

    	    ERROR: talloc_free with references at some_dir/source/foo.c:123
    		reference at some_dir/source/other.c:325
    		reference at some_dir/source/third.c:121

Please see the documentation for talloc_set_log_fn() and talloc_set_log_stderr() for more information on talloc logging functions.

  void *talloc_reference(const void *ctx, const void *ptr);

The talloc_reference() function makes ctx an additional parent of ptr.

The return value of talloc_reference() is always the original pointer ptr, unless talloc ran out of memory in creating the reference in which case it will return NULL (each additional reference consumes around 48 bytes of memory on intel x86 platforms).

If ptr is NULL, then the function is a no-op, and simply returns NULL.

After creating a reference you can free it in one of the following ways:

    • you can talloc_free() any parent of the original pointer. That will reduce the number of parents of this pointer by 1, and will cause this pointer to be freed if it runs out of parents.

    • you can talloc_free() the pointer itself if it has at maximum one parent. This behaviour has been changed since the release of version 2.0. Further information in the description of "talloc_free".

For more control on which parent to remove, see “talloc_unlink()”.

  int talloc_unlink(const void *ctx, void *ptr);

The talloc_unlink() function removes a specific parent from ptr. The ctx passed must either be a context used in talloc_reference() with this pointer, or must be a direct parent of ptr.

Note that if the parent has already been removed using talloc_free() then this function will fail and will return -1. Likewise, if ptr is NULL, then the function will make no modifications and return -1.

Usually you can just use talloc_free() instead of talloc_unlink(), but sometimes it is useful to have the additional control on which parent is removed.

  void talloc_set_destructor(const void *ptr, int (*destructor)(void *));

The function talloc_set_destructor() sets the destructor for the pointer ptr. A destructor is a function that is called when the memory used by a pointer is about to be released. The destructor receives ptr as an argument, and should return 0 for success and -1 for failure.

The destructor can do anything it wants to, including freeing other pieces of memory. A common use for destructors is to clean up operating system resources (such as open file descriptors) contained in the structure the destructor is placed on.

You can only place one destructor on a pointer. If you need more than one destructor then you can create a zero-length child of the pointer and place an additional destructor on that.

To remove a destructor call talloc_set_destructor() with NULL for the destructor.

If your destructor attempts to talloc_free() the pointer that it is the destructor for then talloc_free() will return -1 and the free will be ignored. This would be a pointless operation anyway, as the destructor is only called when the memory is just about to go away.

  int talloc_increase_ref_count(const void *ptr);

The talloc_increase_ref_count(ptr) function is exactly equivalent to:

    talloc_reference(NULL, ptr);

You can use either syntax, depending on which you think is clearer in your code.

It returns 0 on success and -1 on failure.

  size_t talloc_reference_count(const void *ptr);

Return the number of references to the pointer.

  void talloc_set_name(const void *ptr, const char *fmt, ...);

Each talloc pointer has a "name". The name is used principally for debugging purposes, although it is also possible to set and get the name on a pointer in as a way of "marking" pointers in your code.

The main use for names on pointer is for "talloc reports". See “talloc_report_depth_cb()”, “talloc_report_depth_file()”, “talloc_report()” “talloc_report()” and “talloc_report_full()” for details. Also see “talloc_enable_leak_report()” and “talloc_enable_leak_report_full()”.

The talloc_set_name() function allocates memory as a child of the pointer. It is logically equivalent to:

    talloc_set_name_const(ptr, talloc_asprintf(ptr, fmt, ...));

Note that multiple calls to talloc_set_name() will allocate more memory without releasing the name. All of the memory is released when the ptr is freed using talloc_free().

  void talloc_set_name_const(const void *ptr, const char *name);

The function talloc_set_name_const() is just like talloc_set_name(), but it takes a string constant, and is much faster. It is extensively used by the "auto naming" macros, such as talloc_p().

This function does not allocate any memory. It just copies the supplied pointer into the internal representation of the talloc ptr. This means you must not pass a name pointer to memory that will disappear before ptr is freed with talloc_free().

  void *talloc_named(const void *ctx, size_t size, const char *fmt, ...);

The talloc_named() function creates a named talloc pointer. It is equivalent to:

    ptr = talloc_size(ctx, size);
    talloc_set_name(ptr, fmt, ....);

  void *talloc_named_const(const void *ctx, size_t size, const char *name);

This is equivalent to:

    ptr = talloc_size(ctx, size);
    talloc_set_name_const(ptr, name);

  const char *talloc_get_name(const void *ptr);

This returns the current name for the given talloc pointer, ptr. See “talloc_set_name()” for details.

  void *talloc_init(const char *fmt, ...);

This function creates a zero length named talloc context as a top level context. It is equivalent to:

    talloc_named(NULL, 0, fmt, ...);

  void *talloc_new(void *ctx);

This is a utility macro that creates a new memory context hanging off an existing context, automatically naming it "talloc_new: __location__" where __location__ is the source line it is called from. It is particularly useful for creating a new temporary working context.

  (type *)talloc_realloc(const void *ctx, void *ptr, type, count);

The talloc_realloc() macro changes the size of a talloc pointer. It has the following equivalences:

    talloc_realloc(ctx, NULL, type, 1) ==> talloc(ctx, type);
    talloc_realloc(ctx, ptr, type, 0)  ==> talloc_free(ptr);

The ctx argument is only used if ptr is not NULL, otherwise it is ignored.

talloc_realloc() returns the new pointer, or NULL on failure. The call will fail either due to a lack of memory, or because the pointer has more than one parent (see “talloc_reference()”).

  void *talloc_realloc_size(const void *ctx, void *ptr, size_t size);

the talloc_realloc_size() function is useful when the type is not known so the type-safe talloc_realloc() cannot be used.

  TYPE *talloc_steal(const void *new_ctx, const TYPE *ptr);

The talloc_steal() function changes the parent context of a talloc pointer. It is typically used when the context that the pointer is currently a child of is going to be freed and you wish to keep the memory for a longer time.

The talloc_steal() function returns the pointer that you pass it. It does not have any failure modes.

It is possible to produce loops in the parent/child relationship if you are not careful with talloc_steal(). No guarantees are provided as to your sanity or the safety of your data if you do this.

Note that if you try and call talloc_steal() on a pointer that has more than one parent then the result is ambiguous. Talloc will choose to remove the parent that is currently indicated by talloc_parent() and replace it with the chosen parent. You will also get a message like this via the talloc logging functions:

    	  WARNING: talloc_steal with references at some_dir/source/foo.c:123
    		reference at some_dir/source/other.c:325
    		reference at some_dir/source/third.c:121

To unambiguously change the parent of a pointer please see the function “talloc_reparent()”. See the talloc_set_log_fn() documentation for more information on talloc logging.

  TYPE *talloc_reparent(const void *old_parent, const void *new_parent, const TYPE *ptr);

The talloc_reparent() function changes the parent context of a talloc pointer. It is typically used when the context that the pointer is currently a child of is going to be freed and you wish to keep the memory for a longer time.

The talloc_reparent() function returns the pointer that you pass it. It does not have any failure modes.

The difference between talloc_reparent() and talloc_steal() is that talloc_reparent() can specify which parent you wish to change. This is useful when a pointer has multiple parents via references.

  TYPE *talloc_move(const void *new_ctx, TYPE **ptr);

The talloc_move() function is a wrapper around talloc_steal() which zeros the source pointer after the move. This avoids a potential source of bugs where a programmer leaves a pointer in two structures, and uses the pointer from the old structure after it has been moved to a new one.

  size_t talloc_total_size(const void *ptr);

The talloc_total_size() function returns the total size in bytes used by this pointer and all child pointers. Mostly useful for debugging.

Passing NULL is allowed, but it will only give a meaningful result if talloc_enable_leak_report() or talloc_enable_leak_report_full() has been called.

  size_t talloc_total_blocks(const void *ptr);

The talloc_total_blocks() function returns the total memory block count used by this pointer and all child pointers. Mostly useful for debugging.

Passing NULL is allowed, but it will only give a meaningful result if talloc_enable_leak_report() or talloc_enable_leak_report_full() has been called.

  void talloc_report(const void *ptr, FILE *f);

The talloc_report() function prints a summary report of all memory used by ptr. One line of report is printed for each immediate child of ptr, showing the total memory and number of blocks used by that child.

You can pass NULL for the pointer, in which case a report is printed for the top level memory context, but only if talloc_enable_leak_report() or talloc_enable_leak_report_full() has been called.

  void talloc_report_full(const void *ptr, FILE *f);

This provides a more detailed report than talloc_report(). It will recursively print the entire tree of memory referenced by the pointer. References in the tree are shown by giving the name of the pointer that is referenced.

You can pass NULL for the pointer, in which case a report is printed for the top level memory context, but only if talloc_enable_leak_report() or talloc_enable_leak_report_full() has been called.

void talloc_report_depth_cb(const void *ptr, int depth, int max_depth, void (*callback)(const void *ptr, int depth, int max_depth, int is_ref, void *priv), void *priv);

This provides a more flexible reports than talloc_report(). It will recursively call the callback for the entire tree of memory referenced by the pointer. References in the tree are passed with is_ref = 1 and the pointer that is referenced.

You can pass NULL for the pointer, in which case a report is printed for the top level memory context, but only if talloc_enable_leak_report() or talloc_enable_leak_report_full() has been called.

The recursion is stopped when depth >= max_depth. max_depth = -1 means only stop at leaf nodes.

void talloc_report_depth_file(const void *ptr, int depth, int max_depth, FILE *f);

This provides a more flexible reports than talloc_report(). It will let you specify the depth and max_depth.

  void talloc_enable_leak_report(void);

This enables calling of talloc_report(NULL, stderr) when the program exits. In Samba4 this is enabled by using the --leak-report command line option.

For it to be useful, this function must be called before any other talloc function as it establishes a "null context" that acts as the top of the tree. If you don't call this function first then passing NULL to talloc_report() or talloc_report_full() won't give you the full tree printout.

Here is a typical talloc report:

    talloc report on 'null_context' (total 267 bytes in 15 blocks)
    libcli/auth/spnego_parse.c:55  contains   31 bytes in   2 blocks
    libcli/auth/spnego_parse.c:55  contains   31 bytes in   2 blocks
    iconv(UTF8,CP850)              contains   42 bytes in   2 blocks
    libcli/auth/spnego_parse.c:55  contains   31 bytes in   2 blocks
    iconv(CP850,UTF8)              contains   42 bytes in   2 blocks
    iconv(UTF8,UTF-16LE)           contains   45 bytes in   2 blocks
    iconv(UTF-16LE,UTF8)           contains   45 bytes in   2 blocks

  void talloc_enable_leak_report_full(void);

This enables calling of talloc_report_full(NULL, stderr) when the program exits. In Samba4 this is enabled by using the --leak-report-full command line option.

For it to be useful, this function must be called before any other talloc function as it establishes a "null context" that acts as the top of the tree. If you don't call this function first then passing NULL to talloc_report() or talloc_report_full() won't give you the full tree printout.

Here is a typical full report:

    full talloc report on 'root' (total 18 bytes in 8 blocks)
    p1               contains     18 bytes in   7 blocks (ref 0)
        r1               contains     13 bytes in   2 blocks (ref 0)
            reference to: p2
        p2               contains      1 bytes in   1 blocks (ref 1)
        x3               contains      1 bytes in   1 blocks (ref 0)
        x2               contains      1 bytes in   1 blocks (ref 0)
        x1               contains      1 bytes in   1 blocks (ref 0)

  (type *)talloc_zero(const void *ctx, type);

The talloc_zero() macro is equivalent to:

    ptr = talloc(ctx, type);
    if (ptr) memset(ptr, 0, sizeof(type));

  void *talloc_zero_size(const void *ctx, size_t size)

The talloc_zero_size() function is useful when you don't have a known type.

  void *talloc_memdup(const void *ctx, const void *p, size_t size);

The talloc_memdup() function is equivalent to:

    ptr = talloc_size(ctx, size);
    if (ptr) memcpy(ptr, p, size);

  char *talloc_strdup(const void *ctx, const char *p);

The talloc_strdup() function is equivalent to:

    ptr = talloc_size(ctx, strlen(p)+1);
    if (ptr) memcpy(ptr, p, strlen(p)+1);

This function sets the name of the new pointer to the passed string. This is equivalent to:

    talloc_set_name_const(ptr, ptr)

  char *talloc_strndup(const void *t, const char *p, size_t n);

The talloc_strndup() function is the talloc equivalent of the C library function strndup(3).

This function sets the name of the new pointer to the passed string. This is equivalent to:

    talloc_set_name_const(ptr, ptr)

  char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);

The talloc_vasprintf() function is the talloc equivalent of the C library function vasprintf(3).

This function sets the name of the new pointer to the new string. This is equivalent to:

    talloc_set_name_const(ptr, ptr)

  char *talloc_asprintf(const void *t, const char *fmt, ...);

The talloc_asprintf() function is the talloc equivalent of the C library function asprintf(3).

This function sets the name of the new pointer to the passed string. This is equivalent to:

    talloc_set_name_const(ptr, ptr)

  char *talloc_asprintf_append(char *s, const char *fmt, ...);

The talloc_asprintf_append() function appends the given formatted string to the given string.

This function sets the name of the new pointer to the new string. This is equivalent to:

    talloc_set_name_const(ptr, ptr)

  (type *)talloc_array(const void *ctx, type, unsigned int count);

The talloc_array() macro is equivalent to:

    (type *)talloc_size(ctx, sizeof(type) * count);

except that it provides integer overflow protection for the multiply, returning NULL if the multiply overflows.

  void *talloc_array_size(const void *ctx, size_t size, unsigned int count);

The talloc_array_size() function is useful when the type is not known. It operates in the same way as talloc_array(), but takes a size instead of a type.

  (typeof(ptr)) talloc_array_ptrtype(const void *ctx, ptr, unsigned int count);

The talloc_ptrtype() macro should be used when you have a pointer to an array and want to allocate memory of an array to point at with this pointer. When compiling with gcc >= 3 it is typesafe. Note this is a wrapper of talloc_array_size() and talloc_get_name() will return the current location in the source file. and not the type.

  void *talloc_realloc_fn(const void *ctx, void *ptr, size_t size)

This is a non-macro version of talloc_realloc(), which is useful as libraries sometimes want a realloc function pointer. A realloc(3) implementation encapsulates the functionality of malloc(3), free(3) and realloc(3) in one call, which is why it is useful to be able to pass around a single function pointer.

  void *talloc_autofree_context(void);

This is a handy utility function that returns a talloc context which will be automatically freed on program exit. This can be used to reduce the noise in memory leak reports.

  void *talloc_check_name(const void *ptr, const char *name);

This function checks if a pointer has the specified name. If it does then the pointer is returned. It it doesn't then NULL is returned.

  (type *)talloc_get_type(const void *ptr, type);

This macro allows you to do type checking on talloc pointers. It is particularly useful for void* private pointers. It is equivalent to this:

    (type *)talloc_check_name(ptr, #type)

  talloc_set_type(const void *ptr, type);

This macro allows you to force the name of a pointer to be a particular type. This can be used in conjunction with talloc_get_type() to do type checking on void* pointers.

It is equivalent to this:

    talloc_set_name_const(ptr, #type)

  talloc_set_log_fn(void (*log_fn)(const char *message));

This function sets a logging function that talloc will use for warnings and errors. By default talloc will not print any warnings or errors.

  talloc_set_log_stderr(void);

This sets the talloc log function to write log messages to stderr

PERFORMANCE

All the additional features of talloc(3) over malloc(3) do come at a price. We have a performance test in Samba that measures talloc() versus malloc() performance, and it seems that talloc() is about 50% slower than malloc() (AMD Ryzen 9 3900X). For Samba, the great reduction in code complexity that we get by using talloc makes this worthwhile, especially as the total overhead of talloc/malloc in Samba is already quite small.

SEE ALSO

malloc(3), strndup(3), vasprintf(3), asprintf(3), http://talloc.samba.org/

AUTHOR

The original Samba software and related utilities were created by Andrew Tridgell. Samba is now developed by the Samba Team as an Open Source project similar to the way the Linux kernel is developed.

COPYRIGHT/LICENSE

Copyright (C) Andrew Tridgell 2004

This program is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, see http://www.gnu.org/licenses/.