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strncat(3)
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, over‐
writing 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-termi‐
nated.
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;
}
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).
allbox; lbw19 lb lb l l l. Interface Attribute Value T{ strcat(),
strncat() T} 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 termi‐
nating 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. strl‐
cat() 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 tar‐
get, then manually copying the bytes from each source string while
maintaining a pointer to the end of the target string will provide bet‐
ter 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 + 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");
}
}
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 5.02 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 2019-08-02 STRCAT(3)