Node:Copying and Concatenation, Next:String/Array Comparison, Previous:String Length, Up:String and Array Utilities
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. The str
and mem
functions are declared in the
header file string.h
while the wstr
and wmem
functions are declared in the file wchar.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 Formatted Output Functions) and scanf
(see Formatted Input Functions).
void * memcpy (void *restrict to, const void *restrict from, size_t size) | Function |
The 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 Here is an example of how you might use struct foo *oldarray, *newarray; int arraysize; ... memcpy (new, old, arraysize * sizeof (struct foo)); |
wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restruct wfrom, size_t size) | Function |
The wmemcpy function copies size wide characters from the object
beginning at wfrom into the object beginning at wto. The
behavior of this function is undefined if the two arrays wto and
wfrom overlap; use wmemmove instead if overlapping is possible.
The following is a possible implementation of wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t)); } The value returned by This function was introduced in Amendment 1 to ISO C90. |
void * mempcpy (void *restrict to, const void *restrict from, size_t size) | Function |
The mempcpy function is nearly identical to the memcpy
function. It copies size bytes from the object beginning at
from into the object pointed to by to. But instead of
returning the value of to it returns a pointer to the byte
following the last written byte in the object beginning at to.
I.e., the value is ((void *) ((char *) to + size)) .
This function is useful in situations where a number of objects shall be
copied to consecutive memory positions.
void * combine (void *o1, size_t s1, void *o2, size_t s2) { void *result = malloc (s1 + s2); if (result != NULL) mempcpy (mempcpy (result, o1, s1), o2, s2); return result; } This function is a GNU extension. |
wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) | Function |
The wmempcpy function is nearly identical to the wmemcpy
function. It copies size wide characters from the object
beginning at wfrom into the object pointed to by wto. But
instead of returning the value of wto it returns a pointer to the
wide character following the last written wide character in the object
beginning at wto. I.e., the value is wto + size .
This function is useful in situations where a number of objects shall be copied to consecutive memory positions. The following is a possible implementation of wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t)); } This function is a GNU extension. |
void * memmove (void *to, const void *from, size_t size) | Function |
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.
The value returned by |
wchar_t * wmemmove (wchar *wto, const wchar_t *wfrom, size_t size) | Function |
wmemmove copies the size wide characters at wfrom
into the size wide characters at wto, even if those two
blocks of space overlap. In the case of overlap, memmove is
careful to copy the original values of the wide characters in the block
at wfrom, including those wide characters which also belong to the
block at wto.
The following is a possible implementation of wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t)); } The value returned by This function is a GNU extension. |
void * memccpy (void *restrict to, const void *restrict from, int c, size_t size) | Function |
This function copies no more than size bytes from from to to, stopping if a byte matching c is found. The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from. |
void * memset (void *block, int c, size_t size) | Function |
This function copies the value of c (converted to an
unsigned char ) into each of the first size bytes of the
object beginning at block. It returns the value of block.
|
wchar_t * wmemset (wchar_t *block, wchar_t wc, size_t size) | Function |
This function copies the value of wc into each of the first size wide characters of the object beginning at block. It returns the value of block. |
char * strcpy (char *restrict to, const char *restrict from) | Function |
This copies characters from the string from (up to and including
the terminating null character) into the string to. Like
memcpy , this function has undefined results if the strings
overlap. The return value is the value of to.
|
wchar_t * wcscpy (wchar_t *restrict wto, const wchar_t *restrict wfrom) | Function |
This copies wide characters from the string wfrom (up to and
including the terminating null wide character) into the string
wto. Like wmemcpy , this function has undefined results if
the strings overlap. The return value is the value of wto.
|
char * strncpy (char *restrict to, const char *restrict from, size_t size) | Function |
This function is similar to strcpy but always copies exactly
size characters into to.
If the length of from is more than size, then If the length of from is less than size, then The behavior of Using |
wchar_t * wcsncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) | Function |
This function is similar to wcscpy but always copies exactly
size wide characters into wto.
If the length of wfrom is more than size, then
If the length of wfrom is less than size, then
The behavior of Using |
char * strdup (const char *s) | Function |
This function copies the null-terminated string s into a newly
allocated string. The string is allocated using malloc ; see
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.
|
wchar_t * wcsdup (const wchar_t *ws) | Function |
This function copies the null-terminated wide character string ws
into a newly allocated string. The string is allocated using
malloc ; see Unconstrained Allocation. If malloc
cannot allocate space for the new string, wcsdup returns a null
pointer. Otherwise it returns a pointer to the new wide character
string.
This function is a GNU extension. |
char * strndup (const char *s, size_t size) | Function |
This function is similar to strdup but always copies at most
size characters into the newly allocated string.
If the length of s is more than size, then This function is different to
|
char * stpcpy (char *restrict to, const char *restrict from) | Function |
This function is like strcpy , except that it returns a pointer to
the end of the string to (that is, the address of the terminating
null character to + strlen (from) ) rather than the beginning.
For example, this program uses #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. The function is
declared in |
wchar_t * wcpcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom) | Function |
This function is like wcscpy , except that it returns a pointer to
the end of the string wto (that is, the address of the terminating
null character wto + strlen (wfrom) ) rather than the beginning.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. The behavior of
|
char * stpncpy (char *restrict to, const char *restrict from, size_t size) | Function |
This function is similar to stpcpy but copies always exactly
size characters into to.
If the length of from is more then size, then If the length of from is less than size, then This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behavior is undefined if the strings overlap. The function is
declared in |
wchar_t * wcpncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) | Function |
This function is similar to wcpcpy but copies always exactly
wsize characters into wto.
If the length of wfrom is more then size, then
If the length of wfrom is less than size, then This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behavior is undefined if the strings overlap.
|
char * strdupa (const char *s) | Macro |
This macro is similar to strdup but allocates the new string
using alloca instead of malloc (see Variable Size Automatic). This means of course the returned string has the same
limitations as any block of memory allocated using alloca .
For obvious reasons #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 This function is only available if GNU CC is used. |
char * strndupa (const char *s, size_t size) | Macro |
This function is similar to strndup but like strdupa it
allocates the new string using alloca
see Variable Size Automatic. The same advantages and limitations
of strdupa are valid for strndupa , too.
This function is implemented only as a macro, just like
|
char * strcat (char *restrict to, const char *restrict from) | Function |
The 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 char * strcat (char *restrict to, const char *restrict from) { strcpy (to + strlen (to), from); return to; } This function has undefined results if the strings overlap. |
wchar_t * wcscat (wchar_t *restrict wto, const wchar_t *restrict wfrom) | Function |
The wcscat function is similar to wcscpy , except that the
characters from wfrom are concatenated or appended to the end of
wto, instead of overwriting it. That is, the first character from
wfrom overwrites the null character marking the end of wto.
An equivalent definition for wchar_t * wcscat (wchar_t *wto, const wchar_t *wfrom) { wcscpy (wto + wcslen (wto), wfrom); return wto; } This function has undefined results if the strings overlap. |
Programmers using the strcat
or wcscat
function (or the
following strncat
or wcsncar
functions for that matter)
can easily be recognized as lazy and reckless. In almost all situations
the lengths of the participating strings are known (it better should be
since how can one otherwise ensure the allocated size of the buffer is
sufficient?) Or at least, one could know them if one keeps track of the
results of the various function calls. But then it is very inefficient
to use strcat
/wcscat
. A lot of time is wasted finding the
end of the destination string so that the actual copying can start.
This is a common example:
/* This function concatenates arbitrarily many strings. The last parameter must beNULL
. */ char * concat (const char *str, ...) { va_list ap, ap2; size_t total = 1; const char *s; char *result; va_start (ap, str); /* Actuallyva_copy
, but this is the name more gcc versions understand. */ __va_copy (ap2, ap); /* Determine how much space we need. */ for (s = str; s != NULL; s = va_arg (ap, const char *)) total += strlen (s); va_end (ap); result = (char *) malloc (total); if (result != NULL) { result[0] = '\0'; /* Copy the strings. */ for (s = str; s != NULL; s = va_arg (ap2, const char *)) strcat (result, s); } va_end (ap2); return result; }
This looks quite simple, especially the second loop where the strings
are actually copied. But these innocent lines hide a major performance
penalty. Just imagine that ten strings of 100 bytes each have to be
concatenated. For the second string we search the already stored 100
bytes for the end of the string so that we can append the next string.
For all strings in total the comparisons necessary to find the end of
the intermediate results sums up to 5500! If we combine the copying
with the search for the allocation we can write this function more
efficient:
char * concat (const char *str, ...) { va_list ap; size_t allocated = 100; char *result = (char *) malloc (allocated); char *wp; if (allocated != NULL) { char *newp; va_start (ap, atr); wp = result; for (s = str; s != NULL; s = va_arg (ap, const char *)) { size_t len = strlen (s); /* Resize the allocated memory if necessary. */ if (wp + len + 1 > result + allocated) { allocated = (allocated + len) * 2; newp = (char *) realloc (result, allocated); if (newp == NULL) { free (result); return NULL; } wp = newp + (wp - result); result = newp; } wp = mempcpy (wp, s, len); } /* Terminate the result string. */ *wp++ = '\0'; /* Resize memory to the optimal size. */ newp = realloc (result, wp - result); if (newp != NULL) result = newp; va_end (ap); } return result; }
With a bit more knowledge about the input strings one could fine-tune
the memory allocation. The difference we are pointing to here is that
we don't use strcat
anymore. We always keep track of the length
of the current intermediate result so we can safe us the search for the
end of the string and use mempcpy
. Please note that we also
don't use stpcpy
which might seem more natural since we handle
with strings. But this is not necessary since we already know the
length of the string and therefore can use the faster memory copying
function. The example would work for wide characters the same way.
Whenever a programmer feels the need to use strcat
she or he
should think twice and look through the program whether the code cannot
be rewritten to take advantage of already calculated results. Again: it
is almost always unnecessary to use strcat
.
char * strncat (char *restrict to, const char *restrict from, size_t size) | Function |
This function is like 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 char * strncat (char *to, const char *from, size_t size) { to[strlen (to) + size] = '\0'; strncpy (to + strlen (to), from, size); return to; } The behavior of |
wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) | Function |
This function is like wcscat 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 wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { wto[wcslen (to) + size] = L'\0'; wcsncpy (wto + wcslen (wto), wfrom, size); return wto; } The behavior of |
Here is an example showing the use of strncpy
and strncat
(the wide character version is equivalent). 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
void bcopy (const void *from, void *to, size_t size) | Function |
This is a partially obsolete alternative for memmove , derived from
BSD. Note that it is not quite equivalent to memmove , because the
arguments are not in the same order and there is no return value.
|
void bzero (void *block, size_t size) | Function |
This is a partially obsolete alternative for memset , derived from
BSD. Note that it is not as general as memset , because the only
value it can store is zero.
|