STRCAT(3) Linux Programmer's Manual STRCAT(3)
NAME
strcat, strncat - concatenate two strings
SYNOPSIS
#include <string.h>
char *strcat(char *dest, const char *src);
char *strncat(char *dest, const char *src, size_t n);
DESCRIPTION
The strcat() function appends the src string to the dest string, overwriting the terminating null byte (' ') 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] != ' ' ; i++)
dest[dest_len + i] = src[i];
dest[dest_len + i] = ' ';
return dest;
}
RETURN VALUE
The strcat() and strncat() functions return a pointer to the resulting string dest.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
+--------------------+---------------+---------+
|Interface | Attribute | Value |
+--------------------+---------------+---------+
|strcat(), strncat() | Thread safety | MT-Safe |
+--------------------+---------------+---------+
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
NOTES
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 terminat-
ing 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.
EXAMPLE
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.)
Program source
#include <string.h>
#include <time.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
#define LIM 4000000
int j;
char p[LIM];
time_t base;
base = time(NULL);
p[0] = ' ';
for (j = 0; j < LIM; j++) {
if ((j % 10000) == 0)
printf("%d %ld
", j, (long) (time(NULL) - base));
strcat(p, "a");
}
}
SEE ALSO
bcopy(3), memccpy(3), memcpy(3), strcpy(3), string(3), strncpy(3), wcscat(3), wcsncat(3)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the
latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
GNU
2017-09-15 STRCAT(3)