About UTF-8 string routines
UTF-8 string types
- ALLEGRO_USTR
- ALLEGRO_USTR_INFO
Creating and destroying strings
Predefined strings
- al_ustr_empty_string
Creating strings by referencing other data
Sizes and offsets
Getting code points
Inserting into strings
Appending to strings
Removing parts of strings
Assigning one string to another
Replacing parts of string
- al_ustr_set_chr
- al_ustr_replace_range
Searching
Comparing
UTF-16 conversion
Low-level UTF-8 routines
- al_utf8_width
- al_utf8_encode
Low-level UTF-16 routines
- al_utf16_width
- al_utf16_encode

These functions are declared in the main Allegro header file:

#include <allegro5/allegro.h>

About UTF-8 string routines

Some parts of the Allegro API, such as the font rountines, expect Unicode strings encoded in UTF-8. The following basic routines are provided to help you work with UTF-8 strings, however it does not mean you need to use them. You should consider another library (e.g. ICU) if you require more functionality.

Briefly, Unicode is a standard consisting of a large character set of over 100,000 characters, and rules, such as how to sort strings. A code point is the integer value of a character, but not all code points are characters, as some code points have other uses. Unlike legacy character sets, the set of code points is open ended and more are assigned with time.

Clearly it is impossible represent each code point with a 8-bit byte (limited to 256 code points) or even a 16-bit integer (limited to 65536 code points). It is possible to store code points in a 32-bit integers but it is space inefficient, and not actually that useful (at least, when handling the full complexity of Unicode; Allegro only does the very basics). There exist different Unicode Transformation Formats for encoding code points into smaller code units. The most important transformation formats are UTF-8 and UTF-16.

UTF-8 is a variable-length encoding which encodes each code point to between one and four 8-bit bytes each. UTF-8 has many nice properties, but the main advantages are that it is backwards compatible with C strings, and ASCII characters (code points in the range 0-127) are encoded in UTF-8 exactly as they would be in ASCII.

UTF-16 is another variable-length encoding, but encodes each code point to one or two 16-bit words each. It is, of course, not compatible with traditional C strings. Allegro does not generally use UTF-16 strings.

Here is a diagram of the representation of the word "ål", with a NUL terminator, in both UTF-8 and UTF-16.

                   ---------------- ---------------- --------------
           String         å                l              NUL
                   ---------------- ---------------- --------------
      Code points    U+00E5 (229)     U+006C (108)     U+0000 (0)
                   ---------------- ---------------- --------------
      UTF-8 bytes     0xC3, 0xA5          0x6C            0x00
                   ---------------- ---------------- --------------
   UTF-16LE bytes     0xE5, 0x00       0x6C, 0x00      0x00, 0x00
                   ---------------- ---------------- --------------

You can see the aforementioned properties of UTF-8. The first code point U+00E5 ("å") is outside of the ASCII range (0-127) so is encoded to multiple code units -- it requires two bytes. U+006C ("l") and U+0000 (NUL) both exist in the ASCII range so take exactly one byte each, as in a pure ASCII string. A zero byte never appears except to represent the NUL character, so many functions which expect C-style strings will work with UTF-8 strings without modification.

On the other hand, UTF-16 represents each code point by either one or two 16-bit code units (two or four bytes). The representation of each 16-bit code unit depends on the byte order; here we have demonstrated little endian.

Both UTF-8 and UTF-16 are self-synchronising. Starting from any offset within a string, it is efficient to find the beginning of the previous or next code point.

Not all sequences of bytes or 16-bit words are valid UTF-8 and UTF-16 strings respectively. UTF-8 also has an additional problem of overlong forms, where a code point value is encoded using more bytes than is strictly necessary. This is invalid and needs to be guarded against.

In the following "ustr" functions, be careful whether a function takes code unit (byte) or code point indices. In general, all position parameters are in code unit offsets. This may be surprising, but if you think about, is required for good performance. (It also means some functions will work even if they do not contain UTF-8, since they only care about storing bytes, so you may actually store arbitrary data in the ALLEGRO_USTRs.)

For actual text processing, where you want to specify positions with code point indices, you should use al_ustr_offset to find the code unit offset position. However, most of the time you would probably just work with byte offsets.

UTF-8 string types

ALLEGRO_USTR

typedef struct _al_tagbstring ALLEGRO_USTR;

An opaque type representing a string. ALLEGRO_USTRs normally contain UTF-8 encoded strings, but they may be used to hold any byte sequences, including NULs.

ALLEGRO_USTR_INFO

typedef struct _al_tagbstring ALLEGRO_USTR_INFO;

A type that holds additional information for an ALLEGRO_USTR that references an external memory buffer.

See also: al_ref_cstr, al_ref_buffer and al_ref_ustr.

Creating and destroying strings

al_ustr_new

ALLEGRO_USTR *al_ustr_new(const char *s)

Create a new string containing a copy of the C-style string s. The string must eventually be freed with al_ustr_free.

al_ustr_new_from_buffer

ALLEGRO_USTR *al_ustr_new_from_buffer(const char *s, size_t size)

Create a new string containing a copy of the buffer pointed to by s of the given size in bytes. The string must eventually be freed with al_ustr_free.

al_ustr_newf

ALLEGRO_USTR *al_ustr_newf(const char *fmt, ...)

Create a new string using a printf-style format string.

Notes:

The "%s" specifier takes C string arguments, not ALLEGRO_USTRs. Therefore to pass an ALLEGRO_USTR as a parameter you must use al_cstr, and it must be NUL terminated. If the string contains an embedded NUL byte everything from that byte onwards will be ignored.

The "%c" specifier outputs a single byte, not the UTF-8 encoding of a code point. Therefore it is only usable for ASCII characters (value <= 127) or if you really mean to output byte values from 128--255. To insert the UTF-8 encoding of a code point, encode it into a memory buffer using al_utf8_encode then use the "%s" specifier. Remember to NUL terminate the buffer.

al_ustr_free

void al_ustr_free(ALLEGRO_USTR *us)

Free a previously allocated string. Does nothing if the argument is NULL.

al_cstr

const char *al_cstr(const ALLEGRO_USTR *us)

Get a char * pointer to the data in a string. This pointer will only be valid while the ALLEGRO_USTR object is not modified and not destroyed. The pointer may be passed to functions expecting C-style strings, with the following caveats:

ALLEGRO_USTRs are allowed to contain embedded NUL ('') bytes. That means al_ustr_size(u) and strlen(al_cstr(u)) may not agree.
An ALLEGRO_USTR may be created in such a way that it is not NUL terminated. A string which is dynamically allocated will always be NUL terminated, but a string which references the middle of another string or region of memory will not be NUL terminated.
If the ALLEGRO_USTR references another string, the returned C string will point into the referenced string. Again, no NUL terminator will be added to the referenced string.

al_ustr_to_buffer

void al_ustr_to_buffer(const ALLEGRO_USTR *us, char *buffer, int size)

Write the contents of the string into a pre-allocated buffer of the given size in bytes. The result will always be NUL terminated, so a maximum of size - 1 bytes will be copied.

al_cstr_dup

char *al_cstr_dup(const ALLEGRO_USTR *us)

Create a NUL ('') terminated copy of the string. Any embedded NUL bytes will still be presented in the returned string. The new string must eventually be freed with al_free.

If an error occurs NULL is returned.

See also: al_cstr, al_ustr_to_buffer, al_free

al_ustr_dup

ALLEGRO_USTR *al_ustr_dup(const ALLEGRO_USTR *us)

Return a duplicate copy of a string. The new string will need to be freed with al_ustr_free.

al_ustr_dup_substr

ALLEGRO_USTR *al_ustr_dup_substr(const ALLEGRO_USTR *us, int start_pos,
   int end_pos)

Return a new copy of a string, containing its contents in the byte interval [start_pos, end_pos). The new string will be NUL terminated and will need to be freed with al_ustr_free.

If necessary, use al_ustr_offset to find the byte offsets for a given code point that you are interested in.

Predefined strings

al_ustr_empty_string

const ALLEGRO_USTR *al_ustr_empty_string(void)

Return a pointer to a static empty string. The string is read only and must not be freed.

Creating strings by referencing other data

al_ref_cstr

const ALLEGRO_USTR *al_ref_cstr(ALLEGRO_USTR_INFO *info, const char *s)

Create a string that references the storage of a C-style string. The information about the string (e.g. its size) is stored in the structure pointed to by the info parameter. The string will not have any other storage allocated of its own, so if you allocate the info structure on the stack then no explicit "free" operation is required.

The string is valid until the underlying C string disappears.

Example:

ALLEGRO_USTR_INFO info;
ALLEGRO_USTR *us = al_ref_cstr(&info, "my string");

al_ref_buffer

const ALLEGRO_USTR *al_ref_buffer(ALLEGRO_USTR_INFO *info, const char *s, size_t size)

Create a string that references the storage of an underlying buffer. The size of the buffer is given in bytes. You can use it to reference only part of a string or an arbitrary region of memory.

The string is valid while the underlying memory buffer is valid.

al_ref_ustr

const ALLEGRO_USTR *al_ref_ustr(ALLEGRO_USTR_INFO *info, const ALLEGRO_USTR *us,
   int start_pos, int end_pos)

Create a read-only string that references the storage of another ALLEGRO_USTR string. The information about the string (e.g. its size) is stored in the structure pointed to by the info parameter. The new string will not have any other storage allocated of its own, so if you allocate the info structure on the stack then no explicit "free" operation is required.

The referenced interval is [start_pos, end_pos). Both are byte offsets.

The string is valid until the underlying string is modified or destroyed.

If you need a range of code-points instead of bytes, use al_ustr_offset to find the byte offsets.

Sizes and offsets

al_ustr_size

size_t al_ustr_size(const ALLEGRO_USTR *us)

Return the size of the string in bytes. This is equal to the number of code points in the string if the string is empty or contains only 7-bit ASCII characters.

al_ustr_length

size_t al_ustr_length(const ALLEGRO_USTR *us)

Return the number of code points in the string.

al_ustr_offset

int al_ustr_offset(const ALLEGRO_USTR *us, int index)

Return the byte offset (from the start of the string) of the code point at the specified index in the string. A zero index parameter will return the first character of the string. If index is negative, it counts backward from the end of the string, so an index of -1 will return an offset to the last code point.

If the index is past the end of the string, returns the offset of the end of the string.

al_ustr_next

bool al_ustr_next(const ALLEGRO_USTR *us, int *pos)

Find the byte offset of the next code point in string, beginning at *pos. *pos does not have to be at the beginning of a code point.

Returns true on success, and the value pointed to by pos will be updated to the found offset. Otherwise returns false if *pos was already at the end of the string, and *pos is unmodified.

This function just looks for an appropriate byte; it doesn't check if found offset is the beginning of a valid code point. If you are working with possibly invalid UTF-8 strings then it could skip over some invalid bytes.

al_ustr_prev

bool al_ustr_prev(const ALLEGRO_USTR *us, int *pos)

Find the byte offset of the previous code point in string, before *pos. *pos does not have to be at the beginning of a code point. Returns true on success, then value pointed to by pos will be updated to the found offset. Otherwise returns false if *pos was already at the end of the string, then *pos is unmodified.

Getting code points

al_ustr_get

int32_t al_ustr_get(const ALLEGRO_USTR *ub, int pos)

Return the code point in us beginning at byte offset pos.

On success returns the code point value. If pos was out of bounds (e.g. past the end of the string), return -1. On an error, such as an invalid byte sequence, return -2.

al_ustr_get_next

int32_t al_ustr_get_next(const ALLEGRO_USTR *us, int *pos)

Find the code point in us beginning at byte offset *pos, then advance to the next code point.

On success return the code point value. If pos was out of bounds (e.g. past the end of the string), return -1. On an error, such as an invalid byte sequence, return -2. As with al_ustr_next, invalid byte sequences may be skipped while advancing.

al_ustr_prev_get

int32_t al_ustr_prev_get(const ALLEGRO_USTR *us, int *pos)

Find the beginning of a code point before byte offset *pos, then return it. Note this performs a pre-increment.

On success returns the code point value. If pos was out of bounds (e.g. past the end of the string), return -1. On an error, such as an invalid byte sequence, return -2. As with al_ustr_prev, invalid byte sequences may be skipped while advancing.

Inserting into strings

al_ustr_insert

bool al_ustr_insert(ALLEGRO_USTR *us1, int pos, const ALLEGRO_USTR *us2)

Insert us2 into us1 beginning at byte offset pos. pos cannot be less than 0. If pos is past the end of us1 then the space between the end of the string and pos will be padded with NUL ('') bytes.

If required, use al_ustr_offset to find the byte offset for a given code point index.

Returns true on success, false on error.

al_ustr_insert_cstr

bool al_ustr_insert_cstr(ALLEGRO_USTR *us, int pos, const char *s)

Like al_ustr_insert but inserts a C-style string at byte offset pos.

al_ustr_insert_chr

size_t al_ustr_insert_chr(ALLEGRO_USTR *us, int pos, int32_t c)

Insert a code point into us beginning at byte offset pos. pos cannot be less than 0. If pos is past the end of us then the space between the end of the string and pos will be padded with NUL ('') bytes.

Returns the number of bytes inserted, or 0 on error.

Appending to strings

al_ustr_append

bool al_ustr_append(ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2)

Append us2 to the end of us1.

Returns true on success, false on error.

This function can be used to append an arbitrary buffer:

ALLEGRO_USTR_INFO info;
al_ustr_append(us, al_ref_buffer(&info, buf, size));

al_ustr_append_cstr

bool al_ustr_append_cstr(ALLEGRO_USTR *us, const char *s)

Append C-style string s to the end of us.

Returns true on success, false on error.

al_ustr_append_chr

size_t al_ustr_append_chr(ALLEGRO_USTR *us, int32_t c)

Append a code point to the end of us.

Returns the number of bytes added, or 0 on error.

al_ustr_appendf

bool al_ustr_appendf(ALLEGRO_USTR *us, const char *fmt, ...)

This function appends formatted output to the string us. fmt is a printf-style format string. See al_ustr_newf about the "%s" and "%c" specifiers.

Returns true on success, false on error.

al_ustr_vappendf

bool al_ustr_vappendf(ALLEGRO_USTR *us, const char *fmt, va_list ap)

Like al_ustr_appendf but you pass the variable argument list directly, instead of the arguments themselves. See al_ustr_newf about the "%s" and "%c" specifiers.

Returns true on success, false on error.

Removing parts of strings

al_ustr_remove_chr

bool al_ustr_remove_chr(ALLEGRO_USTR *us, int pos)

Remove the code point beginning at byte offset pos. Returns true on success. If pos is out of range or pos is not the beginning of a valid code point, returns false leaving the string unmodified.

Use al_ustr_offset to find the byte offset for a code-points offset.

al_ustr_remove_range

bool al_ustr_remove_range(ALLEGRO_USTR *us, int start_pos, int end_pos)

Remove the interval [start_pos, end_pos) from a string. start_pos and end_pos are byte offsets. Both may be past the end of the string but cannot be less than 0 (the start of the string).

Returns true on success, false on error.

al_ustr_truncate

bool al_ustr_truncate(ALLEGRO_USTR *us, int start_pos)

Truncate a portion of a string at byte offset start_pos onwards. start_pos can be past the end of the string (has no effect) but cannot be less than 0.

Returns true on success, false on error.

al_ustr_ltrim_ws

bool al_ustr_ltrim_ws(ALLEGRO_USTR *us)

Remove leading whitespace characters from a string, as defined by the C function isspace().

Returns true on success, or false on error.

al_ustr_rtrim_ws

bool al_ustr_rtrim_ws(ALLEGRO_USTR *us)

Remove trailing ("right") whitespace characters from a string, as defined by the C function isspace().

Returns true on success, or false on error.

al_ustr_trim_ws

bool al_ustr_trim_ws(ALLEGRO_USTR *us)

Remove both leading and trailing whitespace characters from a string.

Returns true on success, or false on error.

Assigning one string to another

al_ustr_assign

bool al_ustr_assign(ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2)

Overwrite the string us1 with another string us2. Returns true on success, false on error.

al_ustr_assign_substr

bool al_ustr_assign_substr(ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2,
   int start_pos, int end_pos)

Overwrite the string us1 with the contents of us2 in the byte interval [start_pos, end_pos). The end points will be clamed to the bounds of us2.

Usually you will first have to use al_ustr_offset to find the byte offsets.

Returns true on success, false on error.

al_ustr_assign_cstr

bool al_ustr_assign_cstr(ALLEGRO_USTR *us1, const char *s)

Overwrite the string us with the contents of the C-style string s. Returns true on success, false on error.

Replacing parts of string

al_ustr_set_chr

size_t al_ustr_set_chr(ALLEGRO_USTR *us, int start_pos, int32_t c)

Replace the code point beginning at byte offset pos with c. pos cannot be less than 0. If pos is past the end of us1 then the space between the end of the string and pos will be padded with NUL ('') bytes. If pos is not the start of a valid code point, that is an error and the string will be unmodified.

On success, returns the number of bytes written, i.e. the offset to the following code point. On error, returns 0.

al_ustr_replace_range

bool al_ustr_replace_range(ALLEGRO_USTR *us1, int start_pos1, int end_pos1,
   const ALLEGRO_USTR *us2)

Replace the part of us1 in the byte interval [start_pos, end_pos) with the contents of us2. start_pos cannot be less than 0. If start_pos is past the end of us1 then the space between the end of the string and start_pos will be padded with NUL ('') bytes.

Use al_ustr_offset to find the byte offsets.

Returns true on success, false on error.

Searching

al_ustr_find_chr

int al_ustr_find_chr(const ALLEGRO_USTR *us, int start_pos, int32_t c)

Search for the encoding of code point c in us from byte offset start_pos (inclusive).

Returns the position where it is found or -1 if it is not found.

al_ustr_rfind_chr

int al_ustr_rfind_chr(const ALLEGRO_USTR *us, int end_pos, int32_t c)

Search for the encoding of code point c in us backwards from byte offset end_pos (exclusive). Returns the position where it is found or -1 if it is not found.

al_ustr_find_set

int al_ustr_find_set(const ALLEGRO_USTR *us, int start_pos,
   const ALLEGRO_USTR *accept)

This function finds the first code point in us, beginning from byte offset start_pos, that matches any code point in accept. Returns the position if a code point was found. Otherwise returns -1.

al_ustr_find_set_cstr

int al_ustr_find_set_cstr(const ALLEGRO_USTR *us, int start_pos,
   const char *accept)

Like al_ustr_find_set but takes a C-style string for accept.

al_ustr_find_cset

int al_ustr_find_cset(const ALLEGRO_USTR *us, int start_pos,
   const ALLEGRO_USTR *reject)

This function finds the first code point in us, beginning from byte offset start_pos, that does not match any code point in reject. In other words it finds a code point in the complementary set of reject. Returns the byte position of that code point, if any. Otherwise returns -1.

al_ustr_find_cset_cstr

int al_ustr_find_cset_cstr(const ALLEGRO_USTR *us, int start_pos,
   const char *reject)

Like al_ustr_find_cset but takes a C-style string for reject.

al_ustr_find_str

int al_ustr_find_str(const ALLEGRO_USTR *haystack, int start_pos,
   const ALLEGRO_USTR *needle)

Find the first occurrence of string needle in haystack, beginning from byte offset pos (inclusive). Return the byte offset of the occurrence if it is found, otherwise return -1.

al_ustr_find_cstr

int al_ustr_find_cstr(const ALLEGRO_USTR *haystack, int start_pos,
   const char *needle)

Like al_ustr_find_str but takes a C-style string for needle.

al_ustr_rfind_str

int al_ustr_rfind_str(const ALLEGRO_USTR *haystack, int end_pos,
   const ALLEGRO_USTR *needle)

Find the last occurrence of string needle in haystack before byte offset end_pos (exclusive). Return the byte offset of the occurrence if it is found, otherwise return -1.

al_ustr_rfind_cstr

int al_ustr_rfind_cstr(const ALLEGRO_USTR *haystack, int end_pos,
   const char *needle)

Like al_ustr_rfind_str but takes a C-style string for needle.

al_ustr_find_replace

bool al_ustr_find_replace(ALLEGRO_USTR *us, int start_pos,
   const ALLEGRO_USTR *find, const ALLEGRO_USTR *replace)

Replace all occurrences of find in us with replace, beginning at byte offset start_pos. The find string must be non-empty. Returns true on success, false on error.

al_ustr_find_replace_cstr

bool al_ustr_find_replace_cstr(ALLEGRO_USTR *us, int start_pos,
   const char *find, const char *replace)

Like al_ustr_find_replace but takes C-style strings for find and replace.

Comparing

al_ustr_equal

bool al_ustr_equal(const ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2)

Return true iff the two strings are equal. This function is more efficient than al_ustr_compare so is preferable if ordering is not important.

al_ustr_compare

int al_ustr_compare(const ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2)

This function compares us1 and us2 by code point values. Returns zero if the strings are equal, a positive number if us1 comes after us2, else a negative number.

This does not take into account locale-specific sorting rules. For that you will need to use another library.

al_ustr_ncompare

int al_ustr_ncompare(const ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2, int n)

Like al_ustr_compare but only compares up to the first n code points of both strings.

Returns zero if the strings are equal, a positive number if us1 comes after us2, else a negative number.

al_ustr_has_prefix

bool al_ustr_has_prefix(const ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2)

Returns true iff us1 begins with us2.

al_ustr_has_prefix_cstr

bool al_ustr_has_prefix_cstr(const ALLEGRO_USTR *us1, const char *s2)

Returns true iff us1 begins with s2.

al_ustr_has_suffix

bool al_ustr_has_suffix(const ALLEGRO_USTR *us1, const ALLEGRO_USTR *us2)

Returns true iff us1 ends with us2.

al_ustr_has_suffix_cstr

bool al_ustr_has_suffix_cstr(const ALLEGRO_USTR *us1, const char *s2)

Returns true iff us1 ends with s2.

UTF-16 conversion

al_ustr_new_from_utf16

ALLEGRO_USTR *al_ustr_new_from_utf16(uint16_t const *s)

Create a new string containing a copy of the 0-terminated string s which must be encoded as UTF-16. The string must eventually be freed with al_ustr_free.

al_ustr_size_utf16

size_t al_ustr_size_utf16(const ALLEGRO_USTR *us)

Returns the number of bytes required to encode the string in UTF-16 (including the terminating 0). Usually called before al_ustr_encode_utf16 to determine the size of the buffer to allocate.

al_ustr_encode_utf16

size_t al_ustr_encode_utf16(const ALLEGRO_USTR *us, uint16_t *s,
   size_t n)

Encode the string into the given buffer, in UTF-16. Returns the number of bytes written. There are never more than n bytes written. The minimum size to encode the complete string can be queried with al_ustr_size_utf16. If the n parameter is smaller than that, the string will be truncated but still always 0 terminated.

Low-level UTF-8 routines

al_utf8_width

size_t al_utf8_width(int c)

Returns the number of bytes that would be occupied by the specified code point when encoded in UTF-8. This is between 1 and 4 bytes for legal code point values. Otherwise returns 0.

al_utf8_encode

size_t al_utf8_encode(char s[], int32_t c)

Encode the specified code point to UTF-8 into the buffer s. The buffer must have enough space to hold the encoding, which takes between 1 and 4 bytes. This routine will refuse to encode code points above 0x10FFFF.

Returns the number of bytes written, which is the same as that returned by al_utf8_width.

Low-level UTF-16 routines

al_utf16_width

size_t al_utf16_width(int c)

Returns the number of bytes that would be occupied by the specified code point when encoded in UTF-16. This is either 2 or 4 bytes for legal code point values. Otherwise returns 0.

al_utf16_encode

size_t al_utf16_encode(uint16_t s[], int32_t c)

Encode the specified code point to UTF-16 into the buffer s. The buffer must have enough space to hold the encoding, which takes either 2 or 4 bytes. This routine will refuse to encode code points above 0x10FFFF.

Returns the number of bytes written, which is the same as that returned by al_utf16_width.

Allegro version 5.0.10 (GIT) - Last updated: 2013-06-16 03:02:20 UTC