strcat, strncat - concatenate two strings
#include <string.h>
char *strcat(char *dest, const char *src);
char *strncat(char *dest, const char *src, size_t n);
The strcat() function appends the src
string to the dest
string, overwriting the terminating null
byte ('\0') at the end of dest
, and then adds a terminating
null byte. The strings may not overlap, and the dest
string
must have enough space for the result. If dest
is not large
enough, program behavior is unpredictable; buffer overruns are a
favorite avenue for attacking secure programs.
The strncat() function is similar, except that
it will use at most n
bytes from src
;
and
src
does not need to be null-terminated if it contains
n
or more bytes.
As with strcat(), the resulting string in
dest
is always null-terminated.
If src
contains n
or more bytes,
strncat() writes n+1
bytes to dest
(n
from src
plus the terminating null byte).
Therefore, the size of dest
must be at least
strlen(dest)+n+1
.
A simple implementation of strncat() might be:
char *
strncat(char *dest, const char *src, size_t n)
{
size_t dest_len = strlen(dest);
size_t i;
for (i = 0 ; i < n && src[i] != '\0' ; i++)
dest[dest_len + i] = src[i];
dest[dest_len + i] = '\0';
return dest;
}
Because strcat() and strncat() must
find the null byte that terminates the string dest
using a
search that starts at the beginning of the string, the execution time of
these functions scales according to the length of the string
dest
. This can be demonstrated by running the program below.
(If the goal is to concatenate many strings to one target, then manually
copying the bytes from each source string while maintaining a pointer to
the end of the target string will provide better performance.)
#include <stdint.h>
#include <string.h>
#include <time.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
#define LIM 4000000
char p[LIM + 1]; /* +1 for terminating null byte */
time_t base;
base = time(NULL);
p[0] = '\0';
for (int j = 0; j < LIM; j++) {
if ((j % 10000) == 0)
printf("%d %jd\n", j, (intmax_t) (time(NULL) - base));
strcat(p, "a");
}
}
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
strcat(), strncat() | Thread safety | MT-Safe |
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
Some systems (the BSDs, Solaris, and others) provide the following function:
size_t strlcat(char *dest, const char *src, size_t size);
This function appends the null-terminated string src
to the
string dest
, copying at most size-strlen(dest)-1
from
src
, and adds a terminating null byte to the result,
unless
size
is less than strlen(dest)
. This
function fixes the buffer overrun problem of strcat(),
but the caller must still handle the possibility of data loss if
size
is too small. The function returns the length of the
string strlcat() tried to create; if the return value
is greater than or equal to size
, data loss occurred. If data
loss matters, the caller must
either check the arguments before
the call, or test the function return value. strlcat()
is not present in glibc and is not standardized by POSIX, but is
available on Linux via the libbsd
library.
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