strncat (3)
Leading comments
Copyright 1993 David Metcalfe (david@prism.demon.co.uk) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Sin...
NAME
strcat, strncat - concatenate two stringsSYNOPSIS
#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 (aq\0aq) 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] != aq\0aq ; i++)
dest[dest_len + i] = src[i];
dest[dest_len + i] = aq\0aq;
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 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.