zend_string

In C, strings are represented as sequential lists of characters, char* or char[]. The end of the string is usually indicated by the special NUL character, '\0'. This comes with a few significant downsides:

Calculating the length of the string is expensive, as it requires walking the entire string to look for the terminating NUL character.
The string may not contain the NUL character itself.
It is easy to run into buffer overflows if the NUL byte is accidentally missing.

php-src uses the zend_string struct as an abstraction over char*, which explicitly stores the strings length, along with some other fields. It looks as follows:

struct _zend_string {
    zend_refcounted_h gc;
    zend_ulong        h; /* hash value */
    size_t            len;
    char              val[1];
};

The gc field is used for Reference counting. The h field contains a hash value, which is used for hash table lookups. The len field stores the length of the string in bytes, and the val field contains the actual string data.

You may wonder why the val field is declared as char val[1]. This is called the struct hack in C. It is used to create structs with a flexible size, namely by allowing the last element to be expanded arbitrarily. In this case, the size of zend_string depends on the strings length, which is determined at runtime (see _ZSTR_STRUCT_SIZE). When allocating the string, we append enough bytes to the allocation to hold the strings content.

Here’s a basic example of how to use zend_string:

// Allocate the string.
zend_string *string = ZSTR_INIT_LITERAL("Hello world!", /* persistent */ false);
// Write it to the output buffer.
zend_write(ZSTR_VAL(string), ZSTR_LEN(string));
// Decrease the reference count and free it if necessary.
zend_string_release(string);

ZSTR_INIT_LITERAL creates a zend_string from a string literal. It is just a wrapper around zend_string_init(char *string, size_t length, bool persistent) that provides the length of the string at compile time. The persistent parameter indicates whether the string is allocated using malloc (persistent == true) or emalloc, PHPs custom allocator (persistent == false) that is emptied after each request.

When you’re done using the string, you must call zend_string_release, or the memory will leak. zend_string_release will automatically call malloc or emalloc, depending on how the string was allocated. After releasing the string, you must not access any of its fields anymore, as it may have been freed if you were its last user.

API

The string API is defined in Zend/zend_string.h. It provides a number of functions for creating new strings.

`zend_string` creation
Function/Macro [1]	Description
`ZSTR_INIT(s, p)`	Creates a new string from a string literal.
`zend_string_init(s, l, p)`	Creates a new string from a character buffer.
`zend_string_alloc(l, p)`	Creates a new string of a given length without initializing its content.
`zend_string_concat2(s1, l1, s2, l2)`	Creates a non-persistent string by concatenating two character buffers.
`zend_string_concat3(...)`	Same as `zend_string_concat2`, but for three character buffers.
`ZSTR_EMPTY_ALLOC()`	Gets an immutable, empty string. This does not allocate memory.
`ZSTR_CHAR(char)`	Gets an immutable, single-character string. This does not allocate memory.
`ZSTR_KNOWN(ZEND_STR_const)`	Gets an immutable, predefined string. Used for string common within PHP itself, e.g. `"class"`. See `ZEND_KNOWN_STRINGS` in `Zend/zend_string.h`. This does not allocate memory.

As per php-src fashion, you are not supposed to access the zend_string fields directly. Instead, use the following macros. There are macros for both zend_string and zvals known to contain strings.

Accessor macros
`zend_string`	`zval`	Description
`ZSTR_LEN`	`Z_STRLEN[_P]`	Returns the length of the string in bytes.
`ZSTR_VAL`	`Z_STRVAL[_P]`	Returns the string data as a `char*`.
`ZSTR_HASH`	`Z_STRHASH[_P]`	Computes the string has if it hasn’t already been, and returns it.
`ZSTR_H`	-	Returns the string hash. This macro assumes that the hash has already been computed.

Reference counting macros
Macro	Description
`zend_string_copy(s)`	Increases the reference count and returns the same string. The reference count is not increased if the string is interned.
`zend_string_release(s)`	Decreases the reference count and frees the string if it goes to 0.
`zend_string_dup(s, p)`	Creates a true copy of the string in a new allocation, except if the string is interned.
`zend_string_separate(s)`	Duplicates the string if the reference count is greater than 1. See Reference counting for details.
`zend_string_realloc(s, l, p)`	Changes the size of the string. If the string has a reference count greater than 1 or if the string is interned, a new string is created. You must always use the return value of this function, as the original array may have been moved to a new location in memory.

There are various functions to compare strings. The zend_string_equals function compares two strings in full, while zend_string_starts_with checks whether the first argument starts with the second. There are variations for _ci and _literal, i.e. case-insensitive comparison and literal strings, respectively. We won’t go over all variations here, as they are straightforward to use.

Interned strings

Programs use some strings many times. For example, if your program declares a class called MyClass, it would be wasteful to allocate a new string "MyClass" every time it is referenced within your program. Instead, when repeated strings are expected, php-src uses a technique called string interning. Essentially, this is just a simple HashTable where existing interned strings are stored. When creating a new interned string, php-src first checks the interned string buffer. If it finds it there, it can return a pointer to the existing string. If it doesn’t, it allocates a new string and adds it to the buffer.

zend_string *str1 = zend_new_interned_string(
    ZSTR_INIT_LITERAL("MyClass", /* persistent */ false));

// In some other place entirely.
zend_string *str2 = zend_new_interned_string(
    ZSTR_INIT_LITERAL("MyClass", /* persistent */ false));

assert(ZSTR_IS_INTERNED(str1));
assert(ZSTR_IS_INTERNED(str2));
assert(str1 == str2);

Interned strings are not reference counted, as they are expected to live for the entire request, or longer.

With opcache, this goes one step further by sharing strings across different processes. For example, if you’re using php-fpm with 8 workers, all workers will share the same interned strings buffer. It gets a bit more complicated. During requests, no interned strings are actually created. Instead, this is delayed until the script is persisted to shared memory. This means that zend_new_interned_string may not actually return an interned string if opcache is enabled. Usually you don’t have to worry about this.

Reference counting