NAME
strcat, strncat
— concatenate strings
LIBRARY
library “libc”
SYNOPSIS
#include
<string.h>
char *
strcat(char
* restrict s, const char
* restrict append);
char *
strncat(char
* restrict s, const char
* restrict append, size_t
count);
DESCRIPTION
Thestrcat()
and strncat() functions append a copy of the
null-terminated string append to the end of the
null-terminated string s, then add a terminating
‘\0’. The string s
must have sufficient space to hold the result. If s and
append overlap, the results are undefined.
The
strncat()
function appends not more than count characters from
append, and then adds a terminating
‘\0’. If s and
append overlap, the results are undefined.
RETURN VALUES
The strcat() and
strncat() functions return the pointer
s.
SEE ALSO
bcopy(3), memccpy(3), memcpy(3), memmove(3), strcpy(3), strlcat(3), strlcpy(3), wcscat(3)
STANDARDS
The strcat() and
strncat() functions conform to
ISO/IEC 9899:1990
(“ISO C90”).
SECURITY CONSIDERATIONS
The strcat() function is easily misused in
a manner which enables malicious users to arbitrarily change a running
program's functionality through a buffer overflow attack.
Avoid using strcat(). Instead, use
strncat() or strlcat() and
ensure that no more characters are copied to the destination buffer than it
can hold.
Note that strncat() can also be
problematic. It may be a security concern for a string to be truncated at
all. Since the truncated string will not be as long as the original, it may
refer to a completely different resource and usage of the truncated resource
could result in very incorrect behavior. Example:
void
foo(const char *arbitrary_string)
{
char onstack[8];
#if defined(BAD)
/*
* This first strcat is bad behavior. Do not use strcat!
*/
(void)strcat(onstack, arbitrary_string); /* BAD! */
#elif defined(BETTER)
/*
* The following two lines demonstrate better use of
* strncat().
*/
(void)strncat(onstack, arbitrary_string,
sizeof(onstack) - strlen(onstack) - 1);
#elif defined(BEST)
/*
* These lines are even more robust due to testing for
* truncation.
*/
if (strlen(arbitrary_string) + 1 >
sizeof(onstack) - strlen(onstack))
err(1, "onstack would be truncated");
(void)strncat(onstack, arbitrary_string,
sizeof(onstack) - strlen(onstack) - 1);
#endif
}