xref: /freebsd/lib/libc/string/strcat.3 (revision dc36d6f9bb1753f3808552f3afd30eda9a7b206a)

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.Dd April 3, 2022
.Dt STRCAT 3
.Os
.Sh NAME
.Nm strcat ,
.Nm strncat
.Nd concatenate strings
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In string.h
.Ft char *
.Fn strcat "char * restrict s" "const char * restrict append"
.Ft char *
.Fn strncat "char * restrict s" "const char * restrict append" "size_t count"
.Sh DESCRIPTION
The
.Fn strcat
and
.Fn strncat
functions
append a copy of the null-terminated string
.Fa append
to the end of the null-terminated string
.Fa s ,
then add a terminating
.Ql \e0 .
The string
.Fa s
must have sufficient space to hold the result.
If
.Fa s
and
.Fa append
overlap, the results are undefined.
.Pp
The
.Fn strncat
function
appends not more than
.Fa count
characters from
.Fa append ,
and then adds a terminating
.Ql \e0 .
If
.Fa s
and
.Fa append
overlap, the results are undefined.
.Sh RETURN VALUES
The
.Fn strcat
and
.Fn strncat
functions
return the pointer
.Fa s .
.Sh SEE ALSO
.Xr bcopy 3 ,
.Xr memccpy 3 ,
.Xr memcpy 3 ,
.Xr memmove 3 ,
.Xr strcpy 3 ,
.Xr strlcat 3 ,
.Xr strlcpy 3 ,
.Xr wcscat 3
.Sh STANDARDS
The
.Fn strcat
and
.Fn strncat
functions
conform to
.St -isoC .
.Sh HISTORY
The
.Fn strcat
function first appeared in the Programmer's Workbench (PWB/UNIX)
and was ported to
.At v7 ;
.Fn strncat
first appeared in
.At v7 .
.Sh SECURITY CONSIDERATIONS
The
.Fn 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.
.Pp
Avoid using
.Fn strcat .
Instead, use
.Fn strncat
or
.Fn strlcat
and ensure that no more characters are copied to the destination buffer
than it can hold.
.Pp
Note that
.Fn 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:
.Bd -literal
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
}
.Ed