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Operations on strings (or arrays of characters) are an important part of
many programs. The GNU C library provides an extensive set of string
utility functions, including functions for copying, concatenating,
comparing, and searching strings. Many of these functions can also
operate on arbitrary regions of storage; for example, the memcpy
function can be used to copy the contents of any kind of array.
It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.
For instance, you could easily compare one string to another in two
lines of C code, but if you use the built-in strcmp function,
you're less likely to make a mistake. And, since these library
functions are typically highly optimized, your program may run faster
too.
This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.
A string is an array of char objects. But string-valued
variables are usually declared to be pointers of type char *.
Such variables do not include space for the text of a string; that has
to be stored somewhere else--in an array variable, a string constant,
or dynamically allocated memory (see section Memory Allocation). It's up to
you to store the address of the chosen memory space into the pointer
variable. Alternatively you can store a null pointer in the
pointer variable. The null pointer does not point anywhere, so
attempting to reference the string it points to gets an error.
By convention, a null character, '\0', marks the end of a
string. For example, in testing to see whether the char *
variable p points to a null character marking the end of a string,
you can write !*p or *p == '\0'.
A null character is quite different conceptually from a null pointer,
although both are represented by the integer 0.
String literals appear in C program source as strings of
characters between double-quote characters (`"'). In ISO C,
string literals can also be formed by string concatenation:
"a" "b" is the same as "ab". Modification of string
literals is not allowed by the GNU C compiler, because literals
are placed in read-only storage.
Character arrays that are declared const cannot be modified
either. It's generally good style to declare non-modifiable string
pointers to be of type const char *, since this often allows the
C compiler to detect accidental modifications as well as providing some
amount of documentation about what your program intends to do with the
string.
The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocation size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.
A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.
This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters.
Functions that operate on arbitrary blocks of memory have names
beginning with `mem' (such as memcpy) and invariably take an
argument which specifies the size (in bytes) of the block of memory to
operate on. The array arguments and return values for these functions
have type void *, and as a matter of style, the elements of these
arrays are referred to as "bytes". You can pass any kind of pointer
to these functions, and the sizeof operator is useful in
computing the value for the size argument.
In contrast, functions that operate specifically on strings have names
beginning with `str' (such as strcpy) and look for a null
character to terminate the string instead of requiring an explicit size
argument to be passed. (Some of these functions accept a specified
maximum length, but they also check for premature termination with a
null character.) The array arguments and return values for these
functions have type char *, and the array elements are referred
to as "characters".
In many cases, there are both `mem' and `str' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str' functions, unless you already know the length of the string in advance.
You can get the length of a string using the strlen function.
This function is declared in the header file `string.h'.
strlen function returns the length of the null-terminated
string s. (In other words, it returns the offset of the terminating
null character within the array.)
For example,
strlen ("hello, world")
=> 12
When applied to a character array, the strlen function returns
the length of the string stored there, not its allocation size. You can
get the allocation size of the character array that holds a string using
the sizeof operator:
char string[32] = "hello, world";
sizeof (string)
=> 32
strlen (string)
=> 12
You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. These functions are declared in the header file `string.h'.
A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.
Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.
All functions that have problems copying between overlapping arrays are
explicitly identified in this manual. In addition to functions in this
section, there are a few others like sprintf (see section Formatted Output Functions) and scanf (see section Formatted Input Functions).
memcpy function copies size bytes from the object
beginning at from into the object beginning at to. The
behavior of this function is undefined if the two arrays to and
from overlap; use memmove instead if overlapping is possible.
The value returned by memcpy is the value of to.
Here is an example of how you might use memcpy to copy the
contents of an array:
struct foo *oldarray, *newarray; int arraysize; ... memcpy (new, old, arraysize * sizeof (struct foo));
memmove copies the size bytes at from into the
size bytes at to, even if those two blocks of space
overlap. In the case of overlap, memmove is careful to copy the
original values of the bytes in the block at from, including those
bytes which also belong to the block at to.
unsigned char) into each of the first size bytes of the
object beginning at block. It returns the value of block.
memcpy, this function has undefined results if the strings
overlap. The return value is the value of to.
strcpy but always copies exactly
size characters into to.
If the length of from is more than size, then strncpy
copies just the first size characters. Note that in this case
there is no null terminator written into to.
If the length of from is less than size, then strncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behavior is rarely useful, but it
is specified by the ISO C standard.
The behavior of strncpy is undefined if the strings overlap.
Using strncpy as opposed to strcpy is a way to avoid bugs
relating to writing past the end of the allocated space for to.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, strncpy will
waste a considerable amount of time copying null characters.
malloc; see
section Unconstrained Allocation. If malloc cannot allocate space
for the new string, strdup returns a null pointer. Otherwise it
returns a pointer to the new string.
strdup but always copies at most
size characters into the newly allocated string.
If the length of s is more than size, then strndup
copies just the first size characters and adds a closing null
terminator. Otherwise all characters are copied and the string is
terminated.
This function is different to strncpy in that it always
terminates the destination string.
strcpy, except that it returns a pointer to
the end of the string to (that is, the address of the terminating
null character) rather than the beginning.
For example, this program uses stpcpy to concatenate `foo'
and `bar' to produce `foobar', which it then prints.
#include <string.h>
#include <stdio.h>
int
main (void)
{
char buffer[10];
char *to = buffer;
to = stpcpy (to, "foo");
to = stpcpy (to, "bar");
puts (buffer);
return 0;
}
This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.
Its behavior is undefined if the strings overlap.
stpcpy but copies always exactly
size characters into to.
If the length of from is more then size, then stpncpy
copies just the first size characters and returns a pointer to the
character directly following the one which was copied last. Note that in
this case there is no null terminator written into to.
If the length of from is less than size, then stpncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behaviour is rarely useful, but it
is implemented to be useful in contexts where this behaviour of the
strncpy is used. stpncpy returns a pointer to the
first written null character.
This function is not part of ISO or POSIX but was found useful while developing GNU C Library itself.
Its behaviour is undefined if the strings overlap.
strdup but allocates the new string
using alloca instead of malloc
see section Automatic Storage with Variable Size. This means of course the returned
string has the same limitations as any block of memory allocated using
alloca.
For obvious reasons strdupa is implemented only as a macro. I.e.,
you cannot get the address of this function. Despite this limitations
it is a useful function. The following code shows a situation where
using malloc would be a lot more expensive.
#include <paths.h>
#include <string.h>
#include <stdio.h>
const char path[] = _PATH_STDPATH;
int
main (void)
{
char *wr_path = strdupa (path);
char *cp = strtok (wr_path, ":");
while (cp != NULL)
{
puts (cp);
cp = strtok (NULL, ":");
}
return 0;
}
Please note that calling strtok using path directly is
illegal.
This function is only available if GNU CC is used.
strndup but like strdupa it
allocates the new string using alloca
see section Automatic Storage with Variable Size. The same advantages and limitations
of strdupa are valid for strndupa, too.
This function is implemented only as a macro which means one cannot get the address of it.
strndupa is only available if GNU CC is used.
strcat function is similar to strcpy, except that the
characters from from are concatenated or appended to the end of
to, instead of overwriting it. That is, the first character from
from overwrites the null character marking the end of to.
An equivalent definition for strcat would be:
char *
strcat (char *to, const char *from)
{
strcpy (to + strlen (to), from);
return to;
}
This function has undefined results if the strings overlap.
strcat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
The strncat function could be implemented like this:
char *
strncat (char *to, const char *from, size_t size)
{
strncpy (to + strlen (to), from, size);
return to;
}
The behavior of strncat is undefined if the strings overlap.
Here is an example showing the use of strncpy and strncat.
Notice how, in the call to strncat, the size parameter
is computed to avoid overflowing the character array buffer.
#include <string.h>
#include <stdio.h>
#define SIZE 10
static char buffer[SIZE];
main ()
{
strncpy (buffer, "hello", SIZE);
puts (buffer);
strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
puts (buffer);
}
The output produced by this program looks like:
hello hello, wo
memmove, derived from
BSD. Note that it is not quite equivalent to memmove, because the
arguments are not in the same order.
memset, derived from
BSD. Note that it is not as general as memset, because the only
value it can store is zero.
You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See section Searching and Sorting, for an example of this.
Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater".
The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'.
All of these functions are declared in the header file `string.h'.
memcmp compares the size bytes of memory
beginning at a1 against the size bytes of memory beginning
at a2. The value returned has the same sign as the difference
between the first differing pair of bytes (interpreted as unsigned
char objects, then promoted to int).
If the contents of the two blocks are equal, memcmp returns
0.
On arbitrary arrays, the memcmp function is mostly useful for
testing equality. It usually isn't meaningful to do byte-wise ordering
comparisons on arrays of things other than bytes. For example, a
byte-wise comparison on the bytes that make up floating-point numbers
isn't likely to tell you anything about the relationship between the
values of the floating-point numbers.
You should also be careful about using memcmp to compare objects
that can contain "holes", such as the padding inserted into structure
objects to enforce alignment requirements, extra space at the end of
unions, and extra characters at the ends of strings whose length is less
than their allocated size. The contents of these "holes" are
indeterminate and may cause strange behavior when performing byte-wise
comparisons. For more predictable results, perform an explicit
component-wise comparison.
For example, given a structure type definition like:
struct foo
{
unsigned char tag;
union
{
double f;
long i;
char *p;
} value;
};
you are better off writing a specialized comparison function to compare
struct foo objects instead of comparing them with memcmp.
strcmp function compares the string s1 against
s2, returning a value that has the same sign as the difference
between the first differing pair of characters (interpreted as
unsigned char objects, then promoted to int).
If the two strings are equal, strcmp returns 0.
A consequence of the ordering used by strcmp is that if s1
is an initial substring of s2, then s1 is considered to be
"less than" s2.
strcmp, except that differences in case
are ignored.
strcasecmp is derived from BSD.
strncmp, except that differences in case
are ignored.
strncasecmp is a GNU extension.
strcmp, except that no more than
size characters are compared. In other words, if the two strings are
the same in their first size characters, the return value is zero.
Here are some examples showing the use of strcmp and strncmp.
These examples assume the use of the ASCII character set. (If some
other character set--say, EBCDIC--is used instead, then the glyphs
are associated with different numeric codes, and the return values
and ordering may differ.)
strcmp ("hello", "hello")
=> 0 /* These two strings are the same. */
strcmp ("hello", "Hello")
=> 32 /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
=> -15 /* The character 'h' comes before 'w'. */
strcmp ("hello", "hello, world")
=> -44 /* Comparing a null character against a comma. */
strncmp ("hello", "hello, world", 5)
=> 0 /* The initial 5 characters are the same. */
strncmp ("hello, world", "hello, stupid world!!!", 5)
=> 0 /* The initial 5 characters are the same. */
memcmp, derived from BSD.
In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'.
You can use the functions strcoll and strxfrm (declared in
the header file `string.h') to compare strings using a collation
ordering appropriate for the current locale. The locale used by these
functions in particular can be specified by setting the locale for the
LC_COLLATE category; see section Locales and Internationalization.
In the standard C locale, the collation sequence for strcoll is
the same as that for strcmp.
Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.
The function strcoll performs this translation implicitly, in
order to do one comparison. By contrast, strxfrm performs the
mapping explicitly. If you are making multiple comparisons using the
same string or set of strings, it is likely to be more efficient to use
strxfrm to transform all the strings just once, and subsequently
compare the transformed strings with strcmp.
strcoll function is similar to strcmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
Here is an example of sorting an array of strings, using strcoll
to compare them. The actual sort algorithm is not written here; it
comes from qsort (see section Array Sort Function). The job of the
code shown here is to say how to compare the strings while sorting them.
(Later on in this section, we will show a way to do this more
efficiently using strxfrm.)
/* This is the comparison function used withqsort. */ int compare_elements (char **p1, char **p2) { return strcoll (*p1, *p2); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings (char **array, int nstrings) { /* Sorttemp_arrayby comparing the strings. */ qsort (array, sizeof (char *), nstrings, compare_elements); }
strxfrm transforms string using the collation
transformation determined by the locale currently selected for
collation, and stores the transformed string in the array to. Up
to size characters (including a terminating null character) are
stored.
The behavior is undefined if the strings to and from overlap; see section Copying and Concatenation.
The return value is the length of the entire transformed string. This
value is not affected by the value of size, but if it is greater
or equal than size, it means that the transformed string did not
entirely fit in the array to. In this case, only as much of the
string as actually fits was stored. To get the whole transformed
string, call strxfrm again with a bigger output array.
The transformed string may be longer than the original string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, strxfrm simply returns the number of characters that would
be the length of the transformed string. This is useful for determining
what size string to allocate. It does not matter what to is if
size is zero; to may even be a null pointer.
Here is an example of how you can use strxfrm when
you plan to do many comparisons. It does the same thing as the previous
example, but much faster, because it has to transform each string only
once, no matter how many times it is compared with other strings. Even
the time needed to allocate and free storage is much less than the time
we save, when there are many strings.
struct sorter { char *input; char *transformed; };
/* This is the comparison function used with qsort
to sort an array of struct sorter. */
int
compare_elements (struct sorter *p1, struct sorter *p2)
{
return strcmp (p1->transformed, p2->transformed);
}
/* This is the entry point--the function to sort
strings using the locale's collating sequence. */
void
sort_strings_fast (char **array, int nstrings)
{
struct sorter temp_array[nstrings];
int i;
/* Set up temp_array. Each element contains
one input string and its transformed string. */
for (i = 0; i < nstrings; i++)
{
size_t length = strlen (array[i]) * 2;
char *transformed;
size_t transformed_lenght;
temp_array[i].input = array[i];
/* First try a buffer perhaps big enough. */
transformed = (char *) xmalloc (length);
/* Transform array[i]. */
transformed_length = strxfrm (transformed, array[i], length);
/* If the buffer was not large enough, resize it
and try again. */
if (transformed_length >= length)
{
/* Allocate the needed space. +1 for terminating
NUL character. */
transformed = (char *) xrealloc (transformed,
transformed_length + 1);
/* The return value is not interesting because we know
how long the transformed string is. */
(void) strxfrm (transformed, array[i], transformed_length + 1);
}
temp_array[i].transformed = transformed;
}
/* Sort temp_array by comparing transformed strings. */
qsort (temp_array, sizeof (struct sorter),
nstrings, compare_elements);
/* Put the elements back in the permanent array
in their sorted order. */
for (i = 0; i < nstrings; i++)
array[i] = temp_array[i].input;
/* Free the strings we allocated. */
for (i = 0; i < nstrings; i++)
free (temp_array[i].transformed);
}
Compatibility Note: The string collation functions are a new feature of ISO C. Older C dialects have no equivalent feature.
This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file `string.h'.
unsigned char) in the initial size bytes of the
object beginning at block. The return value is a pointer to the
located byte, or a null pointer if no match was found.
strchr function finds the first occurrence of the character
c (converted to a char) in the null-terminated string
beginning at string. The return value is a pointer to the located
character, or a null pointer if no match was found.
For example,
strchr ("hello, world", 'l')
=> "llo, world"
strchr ("hello, world", '?')
=> NULL
The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the c argument.
index is another name for strchr; they are exactly the same.
strrchr is like strchr, except that it searches
backwards from the end of the string string (instead of forwards
from the front).
For example,
strrchr ("hello, world", 'l')
=> "ld"
rindex is another name for strrchr; they are exactly the same.
strchr, except that it searches haystack for a
substring needle rather than just a single character. It
returns a pointer into the string haystack that is the first
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
For example,
strstr ("hello, world", "l")
=> "llo, world"
strstr ("hello, world", "wo")
=> "world"
strstr, but needle and haystack are byte
arrays rather than null-terminated strings. needle-len is the
length of needle and haystack-len is the length of
haystack.
This function is a GNU extension.
strspn ("string span") function returns the length of the
initial substring of string that consists entirely of characters that
are members of the set specified by the string skipset. The order
of the characters in skipset is not important.
For example,
strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
=> 5
strcspn ("string complement span") function returns the length
of the initial substring of string that consists entirely of characters
that are not members of the set specified by the string stopset.
(In other words, it returns the offset of the first character in string
that is a member of the set stopset.)
For example,
strcspn ("hello, world", " \t\n,.;!?")
=> 5
strpbrk ("string pointer break") function is related to
strcspn, except that it returns a pointer to the first character
in string that is a member of the set stopset instead of the
length of the initial substring. It returns a null pointer if no such
character from stopset is found.
For example,
strpbrk ("hello, world", " \t\n,.;!?")
=> ", world"
It's fairly common for programs to have a need to do some simple kinds
of lexical analysis and parsing, such as splitting a command string up
into tokens. You can do this with the strtok function, declared
in the header file `string.h'.
strtok.
The string to be split up is passed as the newstring argument on
the first call only. The strtok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same string are indicated by passing a null pointer as
the newstring argument. Calling strtok with another
non-null newstring argument reinitializes the state information.
It is guaranteed that no other library function ever calls strtok
behind your back (which would mess up this internal state information).
The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.
On the next call to strtok, the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to strtok.
If the end of the string newstring is reached, or if the remainder of
string consists only of delimiter characters, strtok returns
a null pointer.
Warning: Since strtok alters the string it is parsing,
you always copy the string to a temporary buffer before parsing it with
strtok. If you allow strtok to modify a string that came
from another part of your program, you are asking for trouble; that
string may be part of a data structure that could be used for other
purposes during the parsing, when alteration by strtok makes the
data structure temporarily inaccurate.
The string that you are operating on might even be a constant. Then
when strtok tries to modify it, your program will get a fatal
signal for writing in read-only memory. See section Program Error Signals.
This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.
The function strtok is not reentrant. See section Signal Handling and Nonreentrant Functions, for
a discussion of where and why reentrancy is important.
Here is a simple example showing the use of strtok.
#include <string.h> #include <stddef.h> ... char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token; ... token = strtok (string, delimiters); /* token => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */
The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy.
strtok this function splits the string into several
tokens which can be accessed be successive calls to strtok_r.
The difference is that the information about the next token is not set
up in some internal state information. Instead the caller has to
provide another argument save_ptr which is a pointer to a string
pointer. Calling strtok_r with a null pointer for
newstring and leaving save_ptr between the calls unchanged
does the job without limiting reentrancy.
This function was proposed for POSIX.1b and can be found on many systems which support multi-threading.
strsep move the pointer along the tokens
separated by delimiter, returning the address of the next token
and updating string_ptr to point to the beginning of the next
token.
This function was introduced in 4.3BSD and therefore is widely available.
Here is how the above example looks like when strsep is used.
#include <string.h> #include <stddef.h> ... char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; ... running = string; token = strsep (&running, delimiters); /* token => "words" */ token = strsep (&running, delimiters); /* token => "separated" */ token = strsep (&running, delimiters); /* token => "by" */ token = strsep (&running, delimiters); /* token => "spaces" */ token = strsep (&running, delimiters); /* token => "and" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => NULL */
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