diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/README.md.sds | 905 |
1 files changed, 905 insertions, 0 deletions
diff --git a/docs/README.md.sds b/docs/README.md.sds new file mode 100644 index 0000000..cac916c --- /dev/null +++ b/docs/README.md.sds @@ -0,0 +1,905 @@ +# Simple Dynamic Strings + +SDS is a string library for C designed to augment the limited libc string +handling functionalities by adding heap allocated strings which are: + +* Simpler to use. +* Binary safe. +* More computationally efficient. +* Still compatible with normal C string functions. + +This is achieved using an alternative design in which instead of using a C +structure to represent a string, we use a binary prefix that is stored +before the char buffer visible to the user. + + +--------+-------------------------------+-----------+ + | Header | Binary safe C alike string... | Null term | + +--------+-------------------------------+-----------+ + | + `-> Pointer returned to the user. + +Because of meta data stored before the actual returned pointer as a prefix, +and because every SDS string implicitly adds a ``NUL`` term at the end of +the string regardless of the actual contents, SDS strings work +well together with C strings and the user is free to use them interchangeably +with read-only functions that access the string. + +SDS was a C string library originally developed for everyday C +programming needs; later it was moved into Redis where it is used extensively +and where it was modified in order to be suitable for high performance +operations. Now it has been extracted from Redis and forked as a stand alone +project again. + +Because of its many years of life inside Redis, SDS provides both higher level +functions for easy strings manipulation in C, but also a set of low level +functions that make it possible to write high performance code without paying +a penalty for using a higher level string library. + + +## Advantages and disadvantages of SDS + +Normally, dynamic string libraries for C are implemented using a structure +that defines the string. The structure has a pointer field that is managed +by the string function, so it looks like this: + +```c +struct yourAverageStringLibrary { + char *buf; + size_t len; + ... possibly more fields here ... +}; +``` + +SDS strings, as mentioned in the [previous section](#simple-dynamic-strings), +do not follow this scheme, and are instead a single memory region with a prefix +that lives *before* the address of the char array itself. + +There are advantages and disadvantages with this approach over the traditional +one: + + +#### Disadvantages + +1. Many functions return the new string as value. + + Since SDS sometimes needs to create a new string with more space many + functions return the newly created string as the return value, so most SDS + API calls look something like this: + + ```c + s = sdscat(s, "Some more data"); + ``` + + As you can see `s` is used as input for `sdscat` but is also set to the + value returned by the SDS API call, since we are not sure if the call + modified the SDS string we passed or allocated a new one. Not remembering to + assign back the return value of `sdscat` or similar functions to the + variable holding the SDS string will result in a bug. + +2. If the same SDS string is used in different places in your program you have + to modify all the references when you modify it. + + Most of the times when you need to share a SDS string it is often better to + encapsulate it in a reference counted structure instead, because otherwise + it is too easy to incur memory leaks. + + +#### Advantages + +1. You can pass SDS strings to functions designed for C functions without + accessing a struct member or calling a function, like this: + + ```c + printf("%s\n", sds_string); + ``` + + In most other libraries this will be something like: + + ```c + printf("%s\n", string->buf); + ``` + + Or: + + ```c + printf("%s\n", getStringPointer(string)); + ``` + +2. Accessing individual chars is straightforward. + + C is a low level language so this is an important operation in many + programs. With SDS strings accessing individual chars is very natural: + + ```c + printf("%c %c\n", s[0], s[1]); + ``` + + With other libraries your best chance is to assign `string->buf` (or call + the function to get the string pointer) to a `char` pointer and work with + this. However since the other libraries may reallocate the buffer implicitly + every time you call a function that may modify the string you have to get a + reference to the buffer again. + +3. A single memory allocation has better cache locality. + + Usually when you access a string created by a string library using a + structure, you have two different memory allocations for the structure + representing the string, and the actual buffer holding the string. Over + time the buffer will be reallocated, and it is likely that it ends up in a + completely different part of memory compared to the structure itself. + + Since performance in modern programs is often dominated by cache misses, SDS + may perform better under many workloads. + + +## SDS basics + +The type of SDS strings is just the char pointer `char *`. However SDS defines +an `sds` type as alias of `char *` in its header file: you should use the +`sds` type in order to make sure you remember that a given variable in your +program holds an SDS string and not a C string, however this is not mandatory. + +This is the simplest SDS program you can write that does something: + +```c +sds mystring = sdsauto("Hello World!"); +printf("%s\n", mystring); +sdsfree(mystring); + +output> Hello World! +``` + +The above small program already shows a few important things about SDS: + +* SDS strings are created, and heap allocated, via the `sdsnew()` function, or other similar functions that we'll see in a moment. +* SDS strings can be passed to `printf()` like any other C string. +* SDS strings must be freed with `sdsfree()`, since they are heap allocated. + + +### Creating SDS strings + +```c +sds sdsnew(const void *init, size_t initlen); +sds sdsauto(const char *init); +sds sdsempty(void); +sds sdsdup(const sds s); +``` + +There are many ways to create SDS strings: + +* The `sdsauto` function creates an SDS string from a C null terminated string. We already saw how it works in the above example. +* The `sdsnew` function is similar to `sdsauto` but instead of creating the string assuming that the input string is null terminated, it gets an additional length parameter. This way you can create a string using binary data: + + ```c + char buf[3]; + sds mystring; + + buf[0] = 'A'; + buf[1] = 'B'; + buf[2] = 'C'; + mystring = sdsnew(buf, 3); + printf("%s of len %zu\n", mystring, sdslen(mystring)); + + output> ABC of len 3 + ``` + +* The `sdsempty()` function creates an empty zero-length string: + + ```c + sds mystring = sdsempty(); + printf("%zu\n", sdslen(mystring)); + + output> 0 + ``` + +* The `sdsdup()` function duplicates an already existing SDS string: + + ```c + sds s1, s2; + + s1 = sdsauto("Hello"); + s2 = sdsdup(s1); + printf("%s %s\n", s1, s2); + + output> Hello Hello + ``` + + +### Obtaining the string length + +```c +size_t sdslen(const sds s); +``` + +In the examples above we already used the `sdslen` function in order to get +the length of the string. This function works like `strlen` of the libc +except that: + +* It runs in constant time since the length is stored in the prefix of SDS strings, so calling `sdslen` is not expensive even when called with very large strings. +* The function is binary safe like any other SDS string function, so the length is the true length of the string regardless of the content, there is no problem if the string includes null term characters in the middle. + +As an example of the binary safeness of SDS strings, we can run the following +code: + +```c +sds s = sdsnew("A\0\0B", 4); +printf("%zu\n", sdslen(s)); + +output> 4 +``` + +Note that SDS strings are always null terminated at the end, so even in that +case `s[4]` will be a null term, however printing the string with `printf` +would result in just `"A"` to be printed since libc will treat the SDS string +like a normal C string. + + +### Destroying strings + +```c +void sdsfree(sds s); +``` + +To destroy an SDS string just call `sdsfree` with the string +pointer. Note, however, that empty strings created with `sdsempty` need to be +destroyed as well otherwise they'll result in a memory leak. + +The function `sdsfree` does not perform any operation if instead of an SDS +string pointer, `NULL` is passed, so you don't need to check for `NULL` explicitly before calling it: + +```c +if (string) sdsfree(string); /* Not needed. */ +sdsfree(string); /* Same effect but simpler. */ +``` + + +### Concatenating strings + +Concatenating strings to other strings is likely the operation you will end +using the most with a dynamic C string library. SDS provides different +functions to concatenate strings to existing strings. + +```c +sds sdscatlen(sds s, const void *t, size_t len); +sds sdscat(sds s, const char *t); +``` + +The main string concatenation functions are `sdscatlen` and `sdscat` that are +identical, the only difference being that `sdscat` does not have an explicit +length argument since it expects a null terminated string. + +```c +sds s = sdsempty(); +s = sdscat(s, "Hello "); +s = sdscat(s, "World!"); +printf("%s\n", s); + +output> Hello World! +``` + +Sometimes you want to cat an SDS string to another SDS string, so you don't +need to specify the length, but at the same time the string does not need to +be null terminated but can contain any binary data. For this there is a +special function: + +```c +sds sdscatsds(sds s, const sds t); +``` + +Usage is straightforward: + +```c +sds s1 = sdsauto("aaa"); +sds s2 = sdsauto("bbb"); +s1 = sdscatsds(s1, s2); +sdsfree(s2); +printf("%s\n", s1); + +output> aaabbb +``` + +Sometimes you don't want to append any special data to the string, but you want +to make sure that there are at least a given number of bytes composing the +whole string. + +```c +sds sdsgrowzero(sds s, size_t len); +``` + +The `sdsgrowzero` function will do nothing if the current string length is +already `len` bytes, otherwise it will enlarge the string to `len` just padding +it with zero bytes. + +```c +sds s = sdsauto("Hello"); +s = sdsgrowzero(s, 6); +s[5] = '!'; /* We are sure this is safe because of sdsgrowzero() */ +printf("%s\n", s); + +output> Hello! +``` + + +### Formatting strings + +There is a special string concatenation function that accepts a `printf` alike +format specifier and cats the formatted string to the specified string. + +```c +sds sdscatprintf(sds s, const char *fmt, ...) { +``` + +Example: + +```c +sds s; +int a = 10, b = 20; +s = sdsauto("The sum is: "); +s = sdscatprintf(s, "%d+%d = %d", a, b, a + b); +``` + +Often you need to create SDS string directly from `printf` format specifiers. +Because `sdscatprintf` is actually a function that concatenates strings, all +you need to do is to concatenate your string to an empty string: + + +```c +char *name = "Anna"; +int loc = 2500; +sds s; +s = sdscatprintf(sdsempty(), "%s wrote %d lines of LISP\n", name, loc); +``` + +You can use `sdscatprintf` in order to convert numbers into SDS strings: + +```c +int some_integer = 100; +sds num = sdscatprintf(sdsempty(), "%d\n", some_integer); +``` + +However this is slow and we have a special function to make it efficient. + + +### Fast number to string operations + +Creating an SDS string from an integer may be a common operation in certain +kind of programs, and while you may do this with `sdscatprintf` the performance +hit is big, so SDS provides a specialized function. + +```c +sds sdsfromlonglong(long long value); +``` + +Use it like this: + +```c +sds s = sdsfromlonglong(10000); +printf("%zu\n", sdslen(s)); + +output> 5 +``` + + +### Trimming strings and getting ranges + +String trimming is a common operation where a set of characters are +removed from the left and the right of the string. Another useful operation +on strings is to just take a range out of a larger string. + +```c +void sdstrim(sds s, const char *cset); +void sdsrange(sds s, ptrdiff_t start, ptrdiff_t end); +``` + +SDS provides both the operations with the `sdstrim` and `sdsrange` functions. +However note that both functions work differently than most functions modifying +SDS strings since the return value is null: basically those functions always +destructively modify the passed SDS string, never allocating a new one, because +both trimming and ranges will never need more room: the operations can only +remove characters from the original strings. + +Because of this behavior, both functions are fast and don't involve reallocation. + +This is an example of string trimming where newlines and spaces are removed +from an SDS strings: + +```c +sds s = sdsauto(" my string\n\n "); +sdstrim(s, " \n"); +printf("-%s-\n", s); + +output> -my string- +``` + +Basically `sdstrim` takes the SDS string to trim as first argument, and a +null terminated set of characters to remove from left and right of the string. +The characters are removed as long as they are not interrupted by a character +that is not in the list of characters to trim: this is why the space between +`"my"` and `"string"` was preserved in the above example. + +Taking ranges is similar, but instead to take a set of characters, it takes +two indexes, representing the start and the end as specified by zero-based +indexes inside the string, to obtain the range that will be retained. + +```c +sds s = sdsauto("Hello World!"); +sdsrange(s, 1, 4); +printf("-%s-\n", s); + +output> -ello- +``` + +Indexes can be negative to specify a position starting from the end of the +string, so that `-1` means the last character, `-2` the penultimate, and so forth: + +```c +sds s = sdsauto("Hello World!"); +sdsrange(s, 6, -1); +printf("-%s-\n", s); +sdsrange(s, 0, -2); +printf("-%s-\n", s); + +output> -World!- +output> -World- +``` + +`sdsrange` is very useful when implementing networking servers processing +a protocol or sending messages. For example the following code is used +implementing the write handler of the Redis Cluster message bus between +nodes: + +```c +void clusterWriteHandler(..., int fd, void *privdata, ...) { + clusterLink *link = (clusterLink*) privdata; + ssize_t nwritten = write(fd, link->sndbuf, sdslen(link->sndbuf)); + if (nwritten <= 0) { + /* Error handling... */ + } + sdsrange(link->sndbuf, nwritten, -1); + ... more code here ... +} +``` + +Every time the socket of the node we want to send the message to is writable +we attempt to write as much bytes as possible, and we use `sdsrange` in order +to remove from the buffer what was already sent. + +The function to queue new messages to send to some node in the cluster will +simply use `sdscatlen` in order to put more data in the send buffer. + +Note that the Redis Cluster bus implements a binary protocol, but since SDS +is binary safe this is not a problem, so the goal of SDS is not just to provide +a high level string API for the C programmer but also dynamically allocated +buffers that are easy to manage. + + +### String copying + +The most dangerous and infamous function of the standard C library is probably +`strcpy`, so perhaps it is funny how in the context of better designed dynamic +string libraries the concept of copying strings is almost irrelevant. Usually +what you do is to create strings with the content you want, or concatenating +more content as needed. + +However SDS features a string copy function that is useful in performance +critical code sections, however I guess its practical usefulness is limited +as the function never managed to get called in the context of the 50k +lines of code composing the Redis code base. + +```c +sds sdscpylen(sds s, const char *t, size_t len); +sds sdscpy(sds s, const char *t); +``` + +The string copy function of SDS is called `sdscpylen` and works like this: + +```c +s = sdsauto("Hello World!"); +s = sdscpylen(s, "Hello Superman!", 15); +``` + +As you can see the function receives as input the SDS string `s`, but also +returns an SDS string. This is common to many SDS functions that modify the +string: this way the returned SDS string may be the original one modified +or a newly allocated one (for example if there was not enough room in the +old SDS string). + +The `sdscpylen` will simply replace what was in the old SDS string with the +new data you pass using the pointer and length arguments. There is a similar +function called `sdscpy` that does not need a length but expects a null +terminated string instead. + +You may wonder why it makes sense to have a string copy function in the +SDS library, since you can simply create a new SDS string from scratch +with the new value instead of copying the value in an existing SDS string. +The reason is efficiency: `sdsnew` will always allocate a new string +while `sdscpylen` will try to reuse the existing string if there is enough +room to hold the new content specified by the user, and will allocate a new +one only if needed. + + +### Quoting strings + +In order to provide consistent output to the program user, or for debugging +purposes, it is often important to turn a string that may contain binary +data or special characters into a quoted string. Here by quoted string +we mean the common format for String literals in programming source code. +However today this format is also part of the well known serialization formats +like JSON and CSV, so it definitely escaped the simple goal of representing +literal strings in the source code of programs. + +An example of quoted string literal is the following: + +```c +"\x00Hello World\n" +``` + +The first byte is a zero byte while the last byte is a newline, so there are +two non alphanumerical characters inside the string. + +SDS uses a concatenation function for this goal, that concatenates to an +existing string the quoted string representation of the input string. + +```c +sds sdscatrepr(sds s, const char *p, size_t len); +``` + +The `scscatrepr` (where `repr` stands for *representation*) follows the usually +SDS string function rules accepting a char pointer and a length, so you can use +it with SDS strings, normal C strings by using strlen() as `len` argument, or +binary data. The following is an example usage: + +```c +sds s1 = sdsauto("abcd"); +sds s2 = sdsempty(); +s1[1] = 1; +s1[2] = 2; +s1[3] = '\n'; +s2 = sdscatrepr(s2, s1, sdslen(s1)); +printf("%s\n", s2); + +output> "a\x01\x02\n" +``` + +These are the rules `sdscatrepr` uses for conversion: + +* `\` and `"` are quoted with a backslash. +* It quotes special characters `'\n'`, `'\r'`, `'\t'`, `'\a'` and `'\b'`. +* All the other non printable characters not passing the `isprint` test are quoted in `\x..` form, that is: backslash followed by `x` followed by two digit hex number representing the character byte value. +* The function always adds initial and final double quotes characters. + +There is an SDS function that is able to perform the reverse conversion and is +documented in the [Tokenization](#tokenization) section below. + + +### Tokenization + +Tokenization is the process of splitting a larger string into smaller strings. +In this specific case, the split is performed specifying another string that +acts as separator. For example in the following string there are two substrings +that are separated by the `|-|` separator: + +``` +foo|-|bar|-|zap +``` + +A more common separator that consists of a single character is the comma: + +``` +foo,bar,zap +``` + +In many programs it is useful to process a line in order to obtain the sub +strings it is composed of, so SDS provides a function that returns an +array of SDS strings given a string and a separator. + +```c +sds *sdssplitlen(const char *s, size_t len, const char *sep, size_t seplen, size_t *count); +void sdsfreesplitres(sds *tokens, size_t count); +``` + +As usual the function can work with either SDS strings or normal C strings. +The first two arguments `s` and `len` specify the string to tokenize, and the +other two arguments `sep` and `seplen` the separator to use during the +tokenization. The final argument `count` is a pointer to an integer that will +be set to the number of tokens (sub strings) returned. + +The return value is a heap allocated array of SDS strings. + +```c +sds *tokens; +size_t count; + +sds line = sdsauto("Hello World!"); +tokens = sdssplitlen(line, sdslen(line), " ", 1, &count); + +for (size_t j = 0; j < count; j++) + printf("%s\n", tokens[j]); +sdsfreesplitres(tokens, count); + +output> Hello +output> World! +``` + +The returned array is heap allocated, and the single elements of the array +are normal SDS strings. You can free everything calling `sdsfreesplitres` +as in the example. Alternatively you are free to release the array yourself +using the `free` function and use and/or free the individual SDS strings +as usual. + +A valid approach is to set the array elements you reused in some way to +`NULL`, and use `sdsfreesplitres` to free all the rest. + + +### Command line oriented tokenization + +Splitting by a separator is a useful operation, but usually it is not enough +to perform one of the most common tasks involving some non trivial string +manipulation, that is, implementing a **Command Line Interface** for a program. + +This is why SDS provides an additional function that allows you to split +arguments provided by the user via the keyboard in an interactive manner, or +via a file, network, or any other means, into tokens. + +```c +sds *sdssplitargs(const char *line, int *argc); +``` + +The `sdssplitargs` function returns an array of SDS strings exactly like +`sdssplitlen`. The function to free the result is also identical, and is +`sdsfreesplitres`. The difference is in the way the tokenization is performed. + +For example if the input is the following line: + +``` +call "Sabrina" and "Mark Smith\n" +``` + +The function will return the following tokens: + +* "call" +* "Sabrina" +* "and" +* "Mark Smith\n" + +Basically different tokens need to be separated by one or more spaces, and +every single token can also be a quoted string in the same format that +`sdscatrepr` is able to emit. + + +### String joining + +There are two functions doing the reverse of tokenization by joining strings +into a single one. + +```c +sds sdsjoin(char **argv, int argc, char *sep, size_t seplen); +sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen); +``` + +The two functions take as input an array of strings of length `argc` and +a separator and its length, and produce as output an SDS string consisting +of all the specified strings separated by the specified separator. + +The difference between `sdsjoin` and `sdsjoinsds` is that the former accept +C null terminated strings as input while the latter requires all the strings +in the array to be SDS strings. Because of this only `sdsjoinsds` is +able to deal with binary data. + +```c +char *tokens[3] = {"foo", "bar", "zap"}; +sds s = sdsjoin(tokens, 3, "|", 1); +printf("%s\n", s); + +output> foo|bar|zap +``` + + +### Error handling + +All the SDS functions that return an SDS pointer may also return `NULL` on +out of memory, this is basically the only check you need to perform. + +However many modern C programs handle out of memory by simply aborting the program +so you may want to do this as well by wrapping `malloc` and other related +memory allocation calls directly. + + +## SDS internals and advanced usage + +At the very beginning of this documentation it was explained how SDS strings +are allocated, however the prefix stored before the pointer returned to the +user was classified as a *header* without further details. For an advanced +usage it is better to dig more into the internals of SDS and show the +structure implementing it: + +```c +struct sdshdr { + size_t len; + size_t free; + char buf[]; +}; +``` + +As you can see, the structure may resemble the one of a conventional string +library, however the `buf` field of the structure is different since it is +not a pointer but an array without any length declared, so `buf` actually +points at the first byte just after the `free` size_t. So in order to create +an SDS string we just allocate a piece of memory that is as large as the +`sdshdr` structure plus the length of our string, plus an additional byte +for the mandatory null term that every SDS string has. + +The `len` field of the structure is quite obvious, and is the current length +of the SDS string, always computed every time the string is modified via +SDS function calls. The `free` field instead represents the amount of free +memory in the current allocation that can be used to store more characters. + +So the actual SDS layout is this one: + + +------------+------------------------+-----------+---------------\ + | Len | Free | H E L L O W O R L D \n | Null term | Free space \ + +------------+------------------------+-----------+---------------\ + | + `-> Pointer returned to the user. + +You may wonder why there is some free space at the end of the string, it +looks like a waste. Actually after a new SDS string is created, there is no +free space at the end at all: the allocation will be as small as possible to +just hold the header, string, and null term. However other access patterns +will create extra free space at the end, like in the following program: + +```c +s = sdsempty(); +s = sdscat(s, "foo"); +s = sdscat(s, "bar"); +s = sdscat(s, "123"); +``` + +Since SDS tries to be efficient it can't afford to reallocate the string every +time new data is appended, since this would be very inefficient, so it uses +the **preallocation of some free space** every time you enlarge the string. + +The preallocation algorithm used is the following: every time the string +is reallocated in order to hold more bytes, the actual allocation size performed +is two times the minimum required. So for instance if the string currently +is holding 30 bytes, and we concatenate 2 more bytes, instead of allocating 32 +bytes in total SDS will allocate 64 bytes. + +However there is a hard limit to the allocation it can perform ahead, and is +defined by `SDS_MAX_PREALLOC`. SDS will never allocate more than 1MB of +additional space (by default, you can change this default). + + +### Shrinking strings + +```c +sds sdsRemoveFreeSpace(sds s); +size_t sdsAllocSize(sds s); +``` + +Sometimes there are class of programs that require to use very little memory. +After strings concatenations, trimming, ranges, the string may end having +a non trivial amount of additional space at the end. + +It is possible to resize a string back to its minimal size in order to hold +the current content by using the function `sdsRemoveFreeSpace`. + +```c +s = sdsRemoveFreeSpace(s); +``` + +There is also a function that can be used in order to get the size of the +total allocation for a given string, and is called `sdsAllocSize`. + +```c +sds s = sdsauto("Ladies and gentlemen"); +s = sdscat(s, "... welcome to the C language."); +printf("%zu\n", sdsAllocSize(s)); +s = sdsRemoveFreeSpace(s); +printf("%zu\n", sdsAllocSize(s)); + +output> 109 +output> 59 +``` + +NOTE: SDS Low level API uses camelCase in order to warn you that you are playing with fire. + + +### Manual modifications of SDS strings + + void sdsupdatelen(sds s); + +Sometimes you may want to hack with an SDS string manually, without using +SDS functions. In the following example we implicitly change the length +of the string, however we want the logical length to reflect the null terminated +C string. + +The function `sdsupdatelen` does just that, updating the internal length +information for the specified string to the length obtained via `strlen`. + +```c +sds s = sdsauto("foobar"); +s[2] = '\0'; +printf("%zu\n", sdslen(s)); +sdsupdatelen(s); +printf("%zu\n", sdslen(s)); + +output> 6 +output> 2 +``` + + +### Sharing SDS strings + +If you are writing a program in which it is advantageous to share the same +SDS string across different data structures, it is absolutely advised to +encapsulate SDS strings into structures that remember the number of references +of the string, with functions to increment and decrement the number of references. + +This approach is a memory management technique called *reference counting* and +in the context of SDS has two advantages: + +* It is less likely that you'll create memory leaks or bugs due to non freeing SDS strings or freeing already freed strings. +* You'll not need to update every reference to an SDS string when you modify it (since the new SDS string may point to a different memory location). + +While this is definitely a very common programming technique I'll outline +the basic ideas here. You create a structure like this: + +```c +struct mySharedStrings { + int refcount; + sds string; +} +``` + +When new strings are created, the structure is allocated and returned with +`refcount` set to 1. Then you have two functions to change the reference count +of the shared string: + +* `incrementStringRefCount` will simply increment `refcount` of 1 in the structure. It will be called every time you add a reference to the string on some new data structure, variable, or whatever. +* `decrementStringRefCount` is used when you remove a reference. This function is however special since when the `refcount` drops to zero, it automatically frees the SDS string, and the `mySharedString` structure as well. + + +### Interactions with heap checkers + +Because SDS returns pointers into the middle of memory chunks allocated with +`malloc`, heap checkers may have issues, however: + +* The popular Valgrind program will detect SDS strings are *possibly lost* memory and never as *definitely lost*, so it is easy to tell if there is a leak or not. I used Valgrind with Redis for years and every real leak was consistently detected as "definitely lost". +* OSX instrumentation tools don't detect SDS strings as leaks but are able to correctly handle pointers pointing to the middle of memory chunks. + + +### Zero copy append from syscalls + +At this point you should have all the tools to dig more inside the SDS +library by reading the source code, however there is an interesting pattern +you can implement using the low level API exported, that is used inside Redis +in order to improve performances of the networking code. + +Using `sdsIncrLen()` and `sdsMakeRoomFor()` it is possible to implement the +following scheme, to cat bytes coming from the kernel to the end of an +sds string without copying into an intermediate buffer: + +```c +oldlen = sdslen(s); +s = sdsMakeRoomFor(s, BUFFER_SIZE); +nread = read(fd, s + oldlen, BUFFER_SIZE); +... check for nread <= 0 and handle it ... +sdsIncrLen(s, nread); +``` + +`sdsIncrLen` is documented inside the source code of `sds.c`. + + +## Embedding SDS into your project + +This is as simple as copying the `sds.c` and `sds.h` files inside your +project. The source code is small and every C99 compiler should deal with +it without issues. + + +## Credits and license + +SDS was created by Salvatore Sanfilippo and is released under the BSD two +clause license. See the LICENSE file in this source distribution for more +information. |