C string handling

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C string handling refers to a group of functions implementing operations on strings in the C standard library. Various operations, such as copying, concatenation, tokenization and searching are supported.

The only support for strings in the C programming language itself is that the compiler will translate a quoted string constant into a null-terminated string, which is stored in static memory. However, the C standard library provides a large number of functions designed to manipulate these null-terminated strings. These functions are so popular and used so often that they are usually considered part of the definition of C.

Definitions[edit]

A string is a contiguous sequence of code units terminated by the first zero code (written '\0' and corresponding to the ASCII null character). In C, there are two types of strings: string, which is sometimes called byte string which uses the type chars as code units (one char is at least 8 bits), and wide string[1] which uses the type wchar_t as code units.

A common misconception is that all char arrays are strings, because string literals are converted to arrays during the compilation (or translation) phase.[2] It is important to remember that a string ends at the first zero code unit. An array or string literal that contains a zero before the last byte therefore contains a string, or possibly several strings, but is not itself a string.[3] Conversely, it is possible to create a char array that is not null-terminated and is thus not a string: char is often used as a small integer when needing to save memory.

The term pointer to a string is used in C to describe a pointer to the initial (lowest-addressed) byte of a string.[1] In C, pointers are used to pass strings to functions. Documentation (including this page) will often use the term string to mean pointer to a string.[citation needed]

The term length of a string is used in C to describe the number of bytes preceding the zero byte.[1] strlen is a standardised function commonly used to determine the length of a string. A common mistake is to not realize that a string uses one more unit of memory than this length, in order to store the zero that ends the string.

Character encodings[edit]

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.

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 length 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 char foo[512] = "φωωβαρ";(UTF-8) or wchar_t foo[512] = L"φωωβαρ"; (UTF-16 or UTF-32) is implementation defined,[4] and may require that the source code be in the same encoding. 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.

Overview of functions[edit]

Most of the functions that operate on C strings are defined in the string.h (cstring header in C++). Functions that operate on C wide strings are defined in the wchar.h (cwchar header 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 buffer overflows, leading programmers to prefer safer, possibly less portable variants, of which some popular ones are listed here. 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.

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 any other byte encoding. 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.

Constants and types[edit]

Name Notes
NULL macro expanding to the null pointer constant; that is, a constant representing a pointer value which is guaranteed not to be a valid address of an object in memory.
wchar_t type used for a code unit in a wide strings, usually either 16 or 32 bits.
wint_t integer type that can hold any value of a wchar_t as well as the value of the macro WEOF. This type is unchanged by integral promotions. Usually a 32 bit signed value.
mbstate_t contains all the information about the conversion state required from one call to a function to the other.

Functions[edit]

Byte
string
Wide
string
Description[note 1]
String
manipulation
strcpy[5] wcscpy[6] copies one string to another
strncpy[7] wcsncpy[8] writes exactly n bytes/wchar_t, copying from source or adding nulls
strcat[9] wcscat[10] appends one string to another
strncat[11] wcsncat[12] appends no more than n bytes/wchar_t from one string to another
strxfrm[13] wcsxfrm[14] transforms a string according to the current locale
String
examination
strlen[15] wcslen[16] returns the length of the string
strcmp[17] wcscmp[18] compares two strings
strncmp[19] wcsncmp[20] compares a specific number of bytes/wchar_t in two strings
strcoll[21] wcscoll[22] compares two strings according to the current locale
strchr[23] wcschr[24] finds the first occurrence of a byte/wchar_t in a string
strrchr[25] wcsrchr[26] finds the last occurrence of a byte/wchar_t in a string
strspn[27] wcsspn[28] finds in a string the first occurrence of a byte/wchar_t not in a set
strcspn[29] wcscspn[30] finds in a string the last occurrence of a byte/wchar_t not in a set
strpbrk[31] wcspbrk[32] finds in a string the first occurrence of a byte/wchar_t in a set
strstr[33] wcsstr[34] finds the first occurrence of a substring in a string
strtok[35] wcstok[36] splits string into tokens
Miscellaneous strerror[37] N/A returns a string containing a message derived from an error code
Memory
manipulation
memset[38] wmemset[39] fills a buffer with a repeated byte/wchar_t
memcpy[40] wmemcpy[41] copies one buffer to another
memmove[42] wmemmove[43] copies one buffer to another, possibly overlapping, buffer
memcmp[44] wmemcmp[45] compares two buffers
memchr[46] wmemchr[47] finds the first occurrence of a byte/wchar_t in a buffer
  1. ^ Here string refers either to byte string or wide string

Multibyte functions[edit]

Name Description
mblen[48] returns the number of bytes in the next multibyte character
mbtowc[49] converts the next multibyte character to a wide character
wctomb[50] converts a wide character to its multibyte representation
mbstowcs[51] converts a multibyte string to a wide string
wcstombs[52] converts a wide string to a multibyte string
btowc[53] convert a single-byte character to wide character, if possible
wctob[54] convert a wide character to a single-byte character, if possible
mbsinit[55] checks if a state object represents initial state
mbrlen[56] returns the number of bytes in the next multibyte character, given state
mbrtowc[57] converts the next multibyte character to a wide character, given state
wcrtomb[58] converts a wide character to its multibyte representation, given state
mbsrtowcs[59] converts a multibyte string to a wide string, given state
wcsrtombs[60] converts a wide string to a multibyte string, given state

"state" is used by encodings that rely on history such as shift states. This is not needed by UTF-8 or UTF-32. UTF-16 uses them to keep track of surrogate pairs and to hide the fact that it actually is a multi-word encoding.

Numeric conversions[edit]

Byte
string
Wide
string
Description[note 1]
atof[61] N/A converts a string to a floating-point value
atoi
atol
atoll[62]
N/A converts a string to an integer (C99)
strtof (C99)[63]
strtod[64]
strtold (C99)[65]
wcstof (C99)[66]
wcstod[67]
wcstold (C99)[68]
converts a string to a floating-point value
strtol
strtoll[69]
wcstol
wcstoll[70]
converts a string to a signed integer
strtoul
strtoull[71]
wcstoul
wcstoull[72]
converts a string to an unsigned integer
  1. ^ Here string refers either to byte string or wide string

The C standard library contains several functions for numeric conversions. The functions that deal with byte strings are defined in the stdlib.h header (cstdlib header in C++). The functions that deal with wide strings are defined in the wchar.h header (cwchar header in C++). Note that the strtoxxx functions are not const-correct, since they accept a const string pointer and return a non-const pointer within the string.

Popular extensions[edit]

Name Platform Description
memccpy[73] SVID, POSIX copies up to specified number of bytes between two memory areas, which must not overlap, stopping when a given byte is found.
mempcpy[74] GNU a variant of memcpy returning a pointer to the byte following the last written byte
strcasecmp[75] POSIX, BSD case-insensitive versions of strcmp
strcat_s[76] C (2011) and ISO/IEC WDTR 24731 a variant of strcat that checks the destination buffer size before copying
strcpy_s[77] C (2011) and ISO/IEC WDTR 24731 a variant of strcpy that checks the destination buffer size before copying
strdup[78] POSIX allocates and duplicates a string
strerror_r[79] POSIX 1, GNU a variant of strerror that is thread-safe. GNU version is incompatible with POSIX one.
stricmp[80] Various case-insensitive versions of strcmp
strlcpy[81] BSD a variant of strcpy that truncates the result to fit in the destination buffer[82]
strlcat[83] BSD a variant of strcat that truncates the result to fit in the destination buffer[82]
strsignal[84] POSIX:2008 returns string representation of a signal code. Not thread safe.
strtok_r[85] POSIX a variant of strtok that is thread-safe

Strcat/strcpy replacements[edit]

Despite the well-established need to replace strcat and strcpy with functions that do not overflow buffers, no accepted standard has arisen. Partly this is due to the mistaken belief by many C programmers that strncat and strncpy have the desired behavior (neither function was designed for this and the behavior and arguments are non-intuitive and often written incorrectly even by expert programmers).[82]

strcat_s and strcpy_s functions are defined in the C11 (Annex K), and in ISO/IEC WDTR 24731. With these two functions, an error indicator is returned upon buffer overflow and the output buffer is set to a zero-length string (which destroys data in the case of strcat_s). These functions attracted considerable criticism because initially they were implemented only on Windows. At the same time, warning messages produced by Microsoft Visual C++—​which suggested programmers to use these functions instead of standard ones—​have been speculated by some to be a Microsoft's attempt to lock developers into its platform.[86][87][88] Open source implementations for these functions are available.[89] The absence of these functions from the standard libraries used in Unix-based and Unix-like operating systems is due to the consensus that the design of these functions is incorrect.

The more popular strlcat and strlcpy functions have been criticized on the basis that they encourage use of C strings and thus create more problems than they solve.[90][91] Consequently they have not been included in the GNU C library (used by software on Linux), although they are implemented in OpenBSD, FreeBSD, NetBSD, Solaris, Mac OS X, and QNX. The lack of GNU C library support has not stopped various library authors from using it and bundling a replacement, among other SDL, glib2, ffmpeg, rsync, and even internally in the Linux kernel. Open source implementations for these functions are available.[92][93]

See also[edit]

References[edit]

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