Simple Dynamic Strings
===
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 real-only 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 are 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 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));
```
**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.
* 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. However note that 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);
```
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 = 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
to 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");
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");
sdsrange(s,0,-2);
printf("-%s-\n");
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
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 infamus 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
