Simple Dynamic Strings
===

## Notes about this fork

This fork merges commits from redis and pending pull requests from the upstream repository.

I will review and accept pull requests to enhance this library and add new features.

## Notes about version 2

This is an updated version of SDS in an attempt
to finally unify Redis, Disque, Hiredis, and the stand alone SDS versions.
This version is **NOT* binary compatible** with SDS version 1, but the API
is 99% compatible so switching to the new lib should be trivial.

Note that this version of SDS may be a slower with certain workloads, but
uses less memory compared to V1 since header size is dynamic and depends to
the string to alloc.

Moreover it includes a few more API functions, notably `sdscatfmt` which
is a faster version of `sdscatprintf` that can be used for the simpler
cases in order to avoid the libc `printf` family functions performance
penalty.

How SDS strings work
===

SDS is a string library for C designed to augment the limited libc string
handling functionalities by adding heap allocated strings that are:

* Simpler to use.
* Binary safe.
* Computationally more efficient.
* But yet... 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 actual pointer to the string that is returned by SDS 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 of every SDS string implicitly adding a null term at the end of
the string regardless of the actual content of the string, SDS strings work
well together with C strings and the user is free to use them interchangeably
with other std C string functions that access the string in read-only.

SDS was a C string I developed in the past for my 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 was
extracted from Redis and forked as a stand alone project.

Because of its many years 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 an 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 already mentioned don't follow this schema, and are instead
a single allocation with a prefix that lives *before* the address actually
returned for the string.

There are advantages and disadvantages with this approach over the traditional
approach:

**Disadvantage #1**: many functions return the new string as value, since sometimes SDS requires to create a new string with more space, so the most SDS API calls look 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.

**Disadvantage #2**: if an SDS string is shared in different places in your program you have to modify all the references when you modify the string. However most of the times when you need to share SDS strings it is much better to encapsulate them into structures with a `reference count` otherwise it is too easy to incur into memory leaks.

**Advantage #1**: you can pass SDS strings to functions designed for C strings 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));
```

**Advantage #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.

**Advantage #3**: single allocation has better cache locality. Usually when you access a string created by a string library using a structure, you have two different allocations for the structure representing the string, and the actual buffer holding the string. Over the time the buffer is reallocated, and it is likely that it ends in a totally different part of memory compared to the structure itself. Since modern programs performances are often dominated by cache misses, SDS may perform better in 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 = sdsnew("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 require to be freed with `sdsfree()`, since they are heap allocated.

Creating SDS strings
---

```c
sds sdsnewlen(const void *init, size_t initlen);
sds sdsnew(const char *init);
sds sdsempty(void);
sds sdsdup(const sds s);
```

There are many ways to create SDS strings:

* The `sdsnew` function creates an SDS string starting from a C null terminated string. We already saw how it works in the above example.
* The `sdsnewlen` function is similar to `sdsnew` 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 = sdsnewlen(buf,3);
    printf("%s of len %d\n", mystring, (int) sdslen(mystring));

    output> ABC of len 3
    ```

  Note: `sdslen` return value is casted to `int` because it returns a `size_t`
type. You can use the right `printf` specifier instead of casting.

  Note: If the first argument to `sdsnewlen` is `0`, it will zero-initialize the
string's memory. This way you can create an empty buffer of a certain length.

* The `sdsempty()` function creates an empty zero-length string:

    ```c
    sds mystring = sdsempty();
    printf("%d\n", (int) sdslen(mystring));

    output> 0
    ```

* The `sdsdup()` function duplicates an already existing SDS string:

    ```c
    sds s1, s2;

    s1 = sdsnew("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 = sdsnewlen("A\0\0B",4);
printf("%d\n", (int) 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);
```

The destroy an SDS string there is just to call `sdsfree` with the string
pointer. Note that even empty strings created with `sdsempty` need to be
destroyed as well otherwise they'll result into 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 = sdsnew("aaa");
sds s2 = sdsnew("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 = sdsnew("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 = sdsnew("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 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("%d\n", (int) 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
regarding strings is the ability to just take a range out of a larger
string.

```c
void sdstrim(sds s, const char *cset);
void sdsrange(sds s, int start, int end);
void sdssubstr(sds s, size_t start, size_t len);
```

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 void: 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 string.

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 = sdsnew("         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 = sdsnew("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 = sdsnew("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 and 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
an 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 that:

```c
s = sdsnew("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 argument. 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: `sdsnewlen` will always allocate a new string
while `sdscpylen` will try to reuse the existing string if there is enough
room to old 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 for 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
literals 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 `sdscatrepr` (where `repr` means *representation*) follows the usual
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 = sdsnew("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"
```

This is 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* 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 three 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, int len, const char *sep, int seplen, int *count);
void sdsfreesplitres(sds *tokens, int count);
```

As usual the function can work with both 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;
int count, j;

sds line = sdsnew("Hello World!");
tokens = sdssplitlen(line,sdslen(line)," ",1,&count);

for (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 usually.

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 also 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 mean, 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);
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. However because of this only `sdsjoinsds` is
able to deal with binary data.

```c
char *tokens[3] = {"foo","bar","zap"};
sds s = sdsjoin(tokens,3,"|");
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 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 an *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 {
    int len;
    int 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` integer. 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 34 bytes.

However there is an 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 = sdsnew("Ladies and gentlemen");
s = sdscat(s,"... welcome to the C language.");
printf("%d\n", (int) sdsAllocSize(s));
s = sdsRemoveFreeSpace(s);
printf("%d\n", (int) sdsAllocSize(s));

output> 109
output> 59
```

NOTE: SDS Low level API use camelCase in order to warn you that you are playing with the 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 = sdsnew("foobar");
s[2] = '\0';
printf("%d\n", (int)sdslen(s));
sdsupdatelen(s);
printf("%d\n", (int)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 that:

```c
struct mySharedString {
    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 mount 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 mount the
following schema, 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 following files inside your
project:

* sds.c
* sds.h
* sdsalloc.h

The source code is small and every C99 compiler should deal with
it without issues.

Using a different allocator for SDS
===

Internally sds.c uses the allocator defined into `sdsalloc.h`. This header
file just defines macros for malloc, realloc and free, and by default libc
`malloc()`, `realloc()` and `free()` are used. Just edit this file in order
to change the name of the allocation functions.

The program using SDS can call the SDS allocator in order to manipulate
SDS pointers (usually not needed but sometimes the program may want to
do advanced things) by using the API exported by SDS in order to call the
allocator used. This is especially useful when the program linked to SDS
is using a different allocator compared to what SDS is using.

The API to access the allocator used by SDS is composed of three functions: `sds_malloc()`, `sds_realloc()` and `sds_free()`.

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.

Oran Agra improved SDS version 2 by adding dynamic sized headers in order to
save memory for small strings and allow strings greater than 4GB.
