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;
}
The
strcat() and
strncat() functions return a pointer to the
resulting string
dest.
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.
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 <string.h>
#include <time.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
#define LIM 4000000
int j;
char p[LIM + 1]; /* +1 for terminating null byte */
time_t base;
base = time(NULL);
p[0] = '\0';
for (j = 0; j < LIM; j++) {
if ((j % 10000) == 0)
printf("%d %ld\n", j, (long) (time(NULL) - base));
strcat(p, "a");
}
}
bcopy(3),
memccpy(3),
memcpy(3),
strcpy(3),
string(3),
strncpy(3),
wcscat(3),
wcsncat(3)