C string handling
C standard library (libc) |
---|
General topics |
Miscellaneous headers |
The
The only support for strings in the programming language proper is that the compiler translates quoted string constants into null-terminated strings.
Definitions
A string is defined as a contiguous sequence of
Generally, the term string means a string where the code unit is of type char
, which is exactly 8 bits on all modern machines.
wchar_t
, which is 16 or 32 bits on modern machines. This was intended for Unicode but it is increasingly common to use UTF-8Strings 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 during compilation.[2] 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.[3]
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 (char*
) can contain non-
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++.)[4] C++11 and C11 add two types with explicit widths char16_t
and char32_t
.[5]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 char foo[512] = "φωωβαρ";
(UTF-8) or wchar_t foo[512] = L"φωωβαρ";
(UTF-16 or UTF-32, depends on wchar_t
) is implementation defined,[6] and may require that the source code be in the same encoding, especially for char
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 u8
is available that guarantees UTF-8 for a bytestring literal, as in char foo[512] = u8"φωωβαρ";
.[7] 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[who?] 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
const
string pointer and returning a non-const
pointer within the string. To correct this, some have been separated into two overloaded functionsConstants and types
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 UCS-2) character, but is now only enough to represent a UTF-16 code unit, which can be half a code point. On other platforms it is defined as 32-bit and a Unicode code point always fits. The C standard only requires that wchar_t be wide enough to hold the widest character set among the supported system locales[9] and be greater or equal in size to char,[10]
|
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. |
char8_t[11] | Part of the C standard since C23, in <uchar.h>, a type that is suitable for storing UTF-8 characters.[12] |
char16_t[13] | Part of the C standard since C11,[14] in <uchar.h>, a type capable of holding 16 bits even if wchar_t is another size. If the macro __STDC_UTF_16__ is defined as 1, the type is used for UTF-16 on that system. This is always the case in C23.[15] C++ does not define such a macro, but the type is always used for UTF-16 in that language.[16]
|
char32_t[13] | Part of the C standard since C11,[17] in <uchar.h>, a type capable of holding 32 bits even if wchar_t is another size. If the macro __STDC_UTF_32__ is defined as 1, the type is used for UTF-32 on that system. This is always the case in C23.[15] C++ does not define such a macro, but the type is always used for UTF-32 in that language.[16]
|
mbstate_t | Contains all the information about the conversion state required from one call to a function to the other. |
Functions
Byte string |
Wide string |
Description[note 1] | |
---|---|---|---|
String manipulation |
strcpy[18] | wcscpy[19] | Copies one string to another |
strncpy[20] | wcsncpy[21] | Writes exactly n bytes, copying from source or adding nulls | |
strcat[22] | wcscat[23] | Appends one string to another | |
strncat[24] | wcsncat[25] | Appends no more than n bytes from one string to another | |
strxfrm[26] | wcsxfrm[27] | Transforms a string according to the current locale | |
String examination |
strlen[28] | wcslen[29] | Returns the length of the string |
strcmp[30] | wcscmp[31] | Compares two strings (three-way comparison) | |
strncmp[32] | wcsncmp[33] | Compares a specific number of bytes in two strings | |
strcoll[34] | wcscoll[35] | Compares two strings according to the current locale | |
strchr[36] | wcschr[37] | Finds the first occurrence of a byte in a string | |
strrchr[38] | wcsrchr[39] | Finds the last occurrence of a byte in a string | |
strspn[40] | wcsspn[41] | Returns the number of initial bytes in a string that are in a second string | |
strcspn[42] | wcscspn[43] | Returns the number of initial bytes in a string that are not in a second string | |
strpbrk[44] | wcspbrk[45] | Finds in a string the first occurrence of a byte in a set | |
strstr[46] | wcsstr[47] | Finds the first occurrence of a substring in a string | |
strtok[48] | wcstok[49] | Splits a string into tokens | |
Miscellaneous | strerror[50] | — | Returns a string containing a message derived from an error code |
Memory manipulation |
memset[51] | wmemset[52] | Fills a buffer with a repeated byte. Since C23, memset_explicit() was added to erase sensitive data. |
memcpy[53] | wmemcpy[54] | Copies one buffer to another. Since C23, memccpy() was added to efficiently concatenate strings. | |
memmove[55] | wmemmove[56] | Copies one buffer to another, possibly overlapping, buffer | |
memcmp[57] | wmemcmp[58] | Compares two buffers (three-way comparison) | |
memchr[59] | wmemchr[60] | Finds the first occurrence of a byte in a buffer | |
|
Multibyte functions
Name | Description |
---|---|
mblen[61] | Returns the number of bytes in the next multibyte character |
mbtowc[62] | Converts the next multibyte character to a wide character |
wctomb[63] | Converts a wide character to its multibyte representation |
mbstowcs[64] | Converts a multibyte string to a wide string |
wcstombs[65] | Converts a wide string to a multibyte string |
btowc[66] | Converts a single-byte character to wide character, if possible |
wctob[67] | Converts a wide character to a single-byte character, if possible |
mbsinit[68] | Checks if a state object represents initial state |
mbrlen[69] | Returns the number of bytes in the next multibyte character, given state |
mbrtowc[70] | Converts the next multibyte character to a wide character, given state |
wcrtomb[71] | Converts a wide character to its multibyte representation, given state |
mbsrtowcs[72] | Converts a multibyte string to a wide string, given state |
wcsrtombs[73] | Converts a wide string to a multibyte string, given state |
mbrtoc8[74] | Converts the next multibyte character to a UTF-8 character, given state |
c8rtomb[75] | Converts a single code point from UTF-8 to a narrow multibyte character representation, given state |
mbrtoc16[76] | Converts the next multibyte character to a UTF-16 character, given state |
c16rtomb[77] | Converts a single code point from UTF-16 to a narrow multibyte character representation, given state |
mbrtoc32[78] | Converts the next multibyte character to a UTF-32 character, given state |
c32rtomb[79] | Converts a single code point from UTF-32 to a narrow multibyte character representation, given state |
These functions all need a mbstate_t 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 mb encodings, but modern ones such as UTF-8 do not need this. However these functions were designed on the assumption that the wc encoding is not a variable-width encoding and thus are designed to deal with exactly one wchar_t at a time, passing it by value rather than using a string pointer. As UTF-16 is a variable-width encoding, the mbstate_t has been reused to keep track of surrogate pairs in the wide encoding, though the caller must still detect and call mbtowc twice for a single character.[80][81][82] 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
Byte string |
Wide string |
Description[note 1] |
---|---|---|
atof[83] | — | converts a string to a floating-point value ('atof' means 'ASCII to float') |
atoi atol atoll[84] |
— | converts a string to an integer (C99) ('atoi' means 'ASCII to integer') |
strtof (C99)[85] strtod[86] strtold (C99)[87] |
wcstof (C99)[88] wcstod[89] wcstold (C99)[90] |
converts a string to a floating-point value |
strtol strtoll[91] |
wcstol wcstoll[92] |
converts a string to a signed integer |
strtoul strtoull[93] |
wcstoul wcstoull[94] |
converts a string to an unsigned integer |
|
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++).
The functions strchr
, bsearch
, strpbrk
, strrchr
, strstr
, memchr
and their wide counterparts are not
const
string pointer and return a non-const
pointer within the string. This has been fixed in C23.[95]
Also, since the Normative Amendment 1 (C95), atoxx
functions are considered subsumed by strtoxxx
functions, for which reason neither C95 nor any later standard provides wide-character versions of these functions. The argument against atoxx
is that they do not differentiate between an error and a 0
.[96]
Popular extensions
Name | Platform | Description |
---|---|---|
bzero[97][98] | BSD
|
Fills a buffer with zero bytes, deprecated by memset |
memccpy[99] | SVID, POSIX | Part of the C standard since C23, copies between two non-overlapping memory areas, stopping when a given byte is found. |
mempcpy[100] | GNU | a variant of memcpy returning a pointer to the byte following the last written byte |
strcasecmp[101] | POSIX, BSD | case-insensitive versions of strcmp |
strcat_s[102] | Windows | a variant of strcat that checks the destination buffer size before copying |
strcpy_s[103] | Windows | a variant of strcpy that checks the destination buffer size before copying |
strdup & strndup[104] | POSIX | Part of the C standard since C23, allocates and duplicates a string |
strerror_r[105] | POSIX 1, GNU | a variant of strerror that is thread-safe. The GNU version is incompatible with the POSIX one. |
stricmp[106] | Windows | case-insensitive versions of strcmp |
strlcpy[107] | BSD, Solaris | a variant of strcpy that truncates the result to fit in the destination buffer[108] |
strlcat[107] | BSD, Solaris | a variant of strcat that truncates the result to fit in the destination buffer[108] |
strsignal[109] | POSIX:2008 | returns string representation of a signal code . Not thread safe.
|
strtok_r[110] | POSIX | a variant of strtok that is thread-safe |
Replacements
Despite the well-established need to replace strcat
[22] and strcpy
[18] with functions that do not allow buffer overflows, no accepted standard has arisen. This is partly due to the mistaken belief by many C programmers that strncat
and strncpy
have the desired behavior; however, neither function was designed for this (they were intended to manipulate null-padded fixed-size string buffers, a data format less commonly used in modern software), and the behavior and arguments are non-intuitive and often written incorrectly even by expert programmers.[108]
The most popular
Sometimes memcpy
[53] or memmove
[55] are used, as they may be more efficient than strcpy
as they do not repeatedly check for NUL (this is less true on modern processors). Since they need a buffer length as a parameter, correct setting of this parameter can avoid buffer overflows.
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).[123] 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,[124] which usually aborts the program.[125][126] Some functions perform destructive operations before calling the runtime-constraint handler; for example, strcat_s
sets the destination to the empty string,[127] which can make it difficult to recover from error conditions or debug them. 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.[128] Although open-source implementations of these functions are available, these functions are not present in common Unix C libraries.[129] Experience with these functions has shown significant problems with their adoption and errors in usage, so the removal of Annex K is proposed for the next revision of the C standard.[130] Usage of memset_s
has also been suggested as a way to avoid unwanted compiler optimizations.[131][132]
See also
- C syntax § Strings – source code syntax, including backslash escape sequences
- String functions
- Perl Compatible Regular Expressions (PCRE)
Notes
- ^ On GitHub, there are 7,813,206 uses of
strlcpy
, versus 38,644 uses ofstrcpy_s
(and 15,286,150 uses ofstrcpy
).[citation needed]
References
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{{cite web}}
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This [strlcpy and strlcat] API has been adopted by most modern operating systems and many standalone software packages [...]. The notable exception is the GNU standard C library, glibc, whose maintainer steadfastly refuses to include these improved APIs, labelling them "horribly inefficient BSD crap", despite prior evidence that they are faster is most cases than the APIs they replace.
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- ^ Todd C. Miller. "strlcat.c". BSD Cross Reference.
- ^ Lovell, Martyn. "Repel Attacks on Your Code with the Visual Studio 2005 Safe C and C++ Libraries". Retrieved 13 February 2015.
- ^ "The C11 standard draft" (PDF). §K.3.1.4p2. Retrieved 13 February 2013.
{{cite web}}
: CS1 maint: location (link) - ^ "The C11 standard draft" (PDF). §K.3.6.1.1p4. Retrieved 13 February 2013.
{{cite web}}
: CS1 maint: location (link) - ^ "Parameter Validation".
- ^ "The C11 standard draft" (PDF). §K.3.7.2.1p4. Retrieved 13 February 2013.
{{cite web}}
: CS1 maint: location (link) - ^ Danny Kalev. "They're at it again". InformIT. Archived from the original on 15 January 2012. Retrieved 10 November 2011.
- ^ Safe C Library. "The Safe C Library provides bound checking memory and string functions per ISO/IEC TR24731". Sourceforge. Retrieved 6 March 2013.
- ^ "Field Experience With Annex K — Bounds Checking Interfaces". Retrieved 5 November 2015.
- ^ "MSC06-C. Beware of compiler optimizations". SEI CERT C Coding Standard.
- ^ FreeBSD Library Functions Manual –
External links
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/df/Wikibooks-logo-en-noslogan.svg/40px-Wikibooks-logo-en-noslogan.svg.png)
- Fast memcpy in C, multiple C coding examples to target different types of CPU instruction architectures