C string handling

A string is defined as a contiguous sequence of code units terminated by the first zero code unit (often called the NUL code unit). which use a code unit of type wchar_t, which is 16 or 32 bits on modern machines. This was intended for Unicode but it is increasingly common to use UTF-8 in normal strings for Unicode instead. Strings are passed to functions by passing a pointer to the first code unit. Since char* and wchar_t* are different types, the functions that process wide strings are different than the ones processing normal strings and have different names. String literals ("text" in the C source code) are converted to arrays (char[] in C, or const char[] in C++) during compilation. The result is an array of code units containing all the characters plus a trailing zero code unit. In C90 L"text" produces a wide string. A string literal can contain the zero code unit (one way is to put \0 into the source), but this will cause the string to end at that point. The rest of the literal will be placed in memory (with another zero code unit added to the end) but it is impossible to know those code units were translated from the string literal, therefore such source code is not a string literal. ==Character encodings==

Each string ends at the first occurrence of the zero code unit of the appropriate kind (char or wchar_t). Consequently, a byte string () can contain non-NUL characters in ASCII or any ASCII extension, but not characters in encodings such as UTF-16 (even though a 16-bit code unit might be nonzero, its high or low byte might be zero). The encodings that can be stored in wide strings are defined by the width of wchar_t. In most implementations, wchar_t is at least 16 bits, and so all 16-bit encodings, such as UCS-2, can be stored. If wchar_t is 32-bits, then 32-bit encodings, such as UTF-32, can be stored. (The standard requires a "type that holds any wide character", which on Windows no longer holds true since the UCS-2 to UTF-16 shift. This was recognized as a defect in the standard and fixed in C++.) C++11 and C11 add two types with explicit widths and . Variable-width encodings can be used in both byte strings and wide strings. String length and offsets are measured in bytes or wchar_t, not in "characters", which can be confusing to beginning programmers. UTF-8 and Shift JIS are often used in C byte strings, while UTF-16 is often used in C wide strings when wchar_t is 16 bits. Truncating strings with variable-width characters using functions like strncpy can produce invalid sequences at the end of the string. This can be unsafe if the truncated parts are interpreted by code that assumes the input is valid. Support for Unicode literals such as (UTF-8) or (UTF-16 or UTF-32, depends on ) is implementation defined, and may require that the source code be in the same encoding, especially for where compilers might just copy whatever is between the quotes. Some compilers or editors will require entering all non-ASCII characters as \xNN sequences for each byte of UTF-8, and/or \uNNNN for each word of UTF-16. Since C11 (and C++11), a new literal prefix is available that guarantees UTF-8 for a bytestring literal, as in . Since C++20 and C23, a char8_t type was added that is meant to store UTF-8 characters and the types of u8 prefixed character and string literals were changed to char8_t and char8_t[] respectively. ==Features==

Terminology In historical documentation the term "character" was often used instead of "byte" for C strings, which leads many to believe that these functions somehow do not work for UTF-8. In fact all lengths are defined as being in bytes and this is true in all implementations, and these functions work as well with UTF-8 as with single-byte encodings. The BSD documentation has been fixed to make this clear, but POSIX, Linux, and Windows documentation still uses "character" in many places where "byte" or "wchar_t" is the correct term. Functions for handling memory buffers can process sequences of bytes that include null-byte as part of the data. Names of these functions typically start with mem, as opposite to the str prefix. Headers Most of the functions that operate on C strings are declared in the string.h header (cstring in C++), while functions that operate on C wide strings are declared in the wchar.h header (cwchar in C++). These headers also contain declarations of functions used for handling memory buffers; the name is thus something of a misnomer. Functions declared in string.h are extremely popular since, as a part of the C standard library, they are guaranteed to work on any platform which supports C. However, some security issues exist with these functions, such as potential buffer overflows when not used carefully and properly, causing the programmers to prefer safer and possibly less portable variants, out of which some popular ones are listed below. Some of these functions also violate const-correctness by accepting a const string pointer and returning a non-const pointer within the string. To correct this, some have been separated into two overloaded functions in the C++ version of the standard library. Constants and types Functions Multibyte functions These functions all need a object, originally in static memory (making the functions not be thread-safe) and in later additions the caller must maintain. This was originally intended to track shift states in the encodings, but modern ones such as UTF-8 do not need this. However these functions were designed on the assumption that the encoding is not a variable-width encoding and thus are designed to deal with exactly one at a time, passing it by value rather than using a string pointer. As UTF-16 is a variable-width encoding, the has been reused to keep track of surrogate pairs in the wide encoding, though the caller must still detect and call twice for a single character. Later additions to the standard admit that the only conversion programmers are interested in is between UTF-8 and UTF-16 and directly provide this. Numeric conversions The C standard library contains several functions for numeric conversions. The functions that deal with byte strings are defined in the header ( header in C++). The functions that deal with wide strings are defined in the header ( header in C++). The functions , , , , , and their wide counterparts are not const-correct, since they accept a string pointer and return a non- pointer within the string. This has been fixed in C23. Also, since the Normative Amendment 1 (C95), functions are considered subsumed by functions, for which reason neither C95 nor any later standard provides wide-character versions of these functions. The argument against is that they do not differentiate between an error and a . ==Popular extensions==

Several related standards have extended the string handling functionality provided by standard C, including BSD, SVID, and POSIX. Some such functions have been added in more recent C standards (e.g. C11 and C23). ==strlcpy, strlcat==

Despite the well-established need to replace strcat and strlcpy functions, which appeared in OpenBSD 2.4 in December, 1998. encouraging the use of C strings (instead of some superior alternative form of string), and hiding other potential errors. Even while glibc hadn't added support, strlcat and strlcpy have been implemented in a number of other C libraries including ones for OpenBSD, FreeBSD, NetBSD, Solaris, OS X, and QNX, as well as in alternative C libraries for Linux, such as libbsd, introduced in 2008, and musl, introduced in 2011, and the source code added directly to other projects such as SDL, GLib, ffmpeg, rsync, and even internally in the Linux kernel. This did change in 2024, the glibc FAQ notes that as of glibc 2.38, the code has been committed and thereby added. These functions were standardized as part of POSIX.1-2024, the Austin Group Defect Tracker ID 986 tracked some discussion about such plans for POSIX. As part of its 2004 Security Development Lifecycle, Microsoft introduced a family of "secure" functions including strcpy_s and strcat_s (along with many others). These functions were standardized with some minor changes as part of the optional C11 (Annex K) proposed by ISO/IEC WDTR 24731. These functions perform various checks including whether the string is too long to fit in the buffer. If the checks fail, a user-specified "runtime-constraint handler" function is called, which usually aborts the program. These functions attracted considerable criticism because initially they were implemented only on Windows and at the same time warning messages started to be produced by Microsoft Visual C++ suggesting use of these functions instead of standard ones. This has been speculated by some to be an attempt by Microsoft to lock developers into its platform. Experience with these functions has shown significant problems with their adoption and errors in usage, so the removal of Annex K was proposed for the next revision of the C standard. Usage of has been suggested as a way to avoid unwanted compiler optimizations. ==See also==