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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. + |