summaryrefslogtreecommitdiff
path: root/source4/lib/talloc/talloc_guide.txt
diff options
context:
space:
mode:
Diffstat (limited to 'source4/lib/talloc/talloc_guide.txt')
-rw-r--r--source4/lib/talloc/talloc_guide.txt492
1 files changed, 492 insertions, 0 deletions
diff --git a/source4/lib/talloc/talloc_guide.txt b/source4/lib/talloc/talloc_guide.txt
new file mode 100644
index 0000000000..b3b148d476
--- /dev/null
+++ b/source4/lib/talloc/talloc_guide.txt
@@ -0,0 +1,492 @@
+Using talloc in Samba4
+----------------------
+
+Andrew Tridgell
+September 2004
+
+The most current version of this document is available at
+ http://samba.org/ftp/unpacked/samba4/talloc_guide.txt
+
+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 mounthful 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_p(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 LOCAL-TALLOC
+smbtorture test to watch talloc in action. You may also like to add
+your own tests to source/torture/local/talloc.c to clarify how some
+particular situation is handled.
+
+
+Performance
+-----------
+
+All the additional features of talloc() over malloc() do come at a
+price. We have a simple performance test in Samba4 that measures
+talloc() versus malloc() performance, and it seems that talloc() is
+about 10% slower than malloc() on my x86 Debian Linux box. 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.
+
+
+talloc API
+----------
+
+The following is a complete guide to the talloc API. Read it all at
+least twice.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc(const void *context, size_t size);
+
+The talloc() function is the core of the talloc library. It takes a
+memory context, and returns a pointer to a new area of memory of the
+given size.
+
+The returned pointer is itself a talloc context, so you can use it as
+the context 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 context then the new child disappears as
+well. Alternatively you can free just the child.
+
+The context argument to talloc() can be NULL, in which case a new top
+level context is created.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_p(const void *context, type);
+
+The talloc_p() macro is the equivalent of
+
+ (type *)talloc(ctx, sizeof(type))
+
+You should use it in preference to talloc() whenever possible, as it
+provides additional type safety. It also automatically calls the
+talloc_set_name_const() function with the name being a string holding
+the name of 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 the pointer 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.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_reference(const void *context, const void *ptr);
+
+The talloc_reference() function makes "context" 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. That will destroy the
+ most recently established parent to the pointer and leave the
+ pointer as a child of its current parent.
+
+For more control on which parent to remove, see talloc_unlink()
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+int talloc_unlink(const void *context, const void *ptr);
+
+The talloc_unlink() function removes a specific parent from ptr. The
+context 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 the pointer 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.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void 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.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+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() 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 the ptr is freed with talloc_free().
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_named(const void *context, size_t size, const char *fmt, ...);
+
+The talloc_named() function creates a named talloc pointer. It is
+equivalent to:
+
+ ptr = talloc(context, size);
+ talloc_set_name(ptr, fmt, ....);
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_named_const(const void *context, size_t size, const char *name);
+
+This is equivalent to:
+
+ ptr = talloc(context, 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. 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_realloc(const void *context, void *ptr, size_t size);
+
+The talloc_realloc() function changes the size of a talloc
+pointer. It has the following equivalences:
+
+ talloc_realloc(context, NULL, size) ==> talloc(context, size);
+ talloc_realloc(context, ptr, 0) ==> talloc_free(ptr);
+
+The "context" 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_steal(const void *new_ctx, const void *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.
+
+NOTE: 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.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+off_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.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+off_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 ensire 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_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)
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_zero(const void *ctx, size_t size);
+
+The talloc_zero() function is equivalent to:
+
+ ptr = talloc(ctx, size);
+ if (ptr) memset(ptr, 0, size);
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_memdup(const void *ctx, const void *p, size_t size);
+
+The talloc_memdup() function is equivalent to:
+
+ ptr = talloc(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(ctx, strlen(p)+1);
+ if (ptr) memcpy(ptr, p, strlen(p)+1);
+
+This functions 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()
+
+This functions 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()
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+char *talloc_asprintf(const void *t, const char *fmt, ...);
+
+The talloc_asprintf() function is the talloc equivalent of the C
+library function asprintf()
+
+This functions 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.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_array_p(const void *ctx, type, uint_t count);
+
+The talloc_array_p() macro is equivalent to:
+
+ (type *)talloc(ctx, sizeof(type) * count);
+
+except that it provides integer overflow protection for the multiply,
+returning NULL if the multiply overflows.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+void *talloc_realloc_p(const void *ctx, void *ptr, type, uint_t count);
+
+The talloc_realloc_p() macro is equivalent to:
+
+ (type *)talloc_realloc(ctx, ptr, sizeof(type) * count);
+
+except that it provides integer overflow protection for the multiply,
+returning NULL if the multiply overflows.
+
+
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+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 ralloc function pointer. A realloc()
+implementation encapsulates the functionality of malloc(), free() and
+realloc() in one call, which is why it is useful to be able to pass
+around a single function pointer.
+