freebsd-dev/lib/libc/string/strcpy.3

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.\"     @(#)strcpy.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd August 9, 2001
.Dt STRCPY 3
.Os
.Sh NAME
.Nm strcpy , strncpy
.Nd copy strings
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In string.h
.Ft char *
.Fn strcpy "char *dst" "const char *src"
.Ft char *
.Fn strncpy "char *dst" "const char *src" "size_t len"
.Sh DESCRIPTION
The
.Fn strcpy
function
copies the string
.Fa src
to
.Fa dst
(including the terminating
.Ql \e0
character).
.Pp
The
.Fn strncpy
function copies not more than
.Fa len
characters from
.Fa src
into
.Fa dst ,
appending
.Ql \e0
characters if
.Fa src
is less than
.Fa len
characters long, and
.Em not
terminating
.Fa dst
otherwise.
.Sh RETURN VALUES
The
.Fn strcpy
and
.Fn strncpy
functions
return
.Fa dst .
.Sh EXAMPLES
The following sets
.Va chararray
to
.Dq Li abc\e0\e0\e0 :
.Bd -literal -offset indent
char chararray[6];

(void)strncpy(chararray, "abc", sizeof(chararray));
.Ed
.Pp
The following sets
.Va chararray
to
.Dq Li abcdef :
.Bd -literal -offset indent
char chararray[6];

(void)strncpy(chararray, "abcdefgh", sizeof(chararray));
.Ed
.Pp
Note that it does
.Em not
.Tn NUL
terminate
.Va chararray
because the length of the source string is greater than or equal
to the length parameter.
.Pp
The following copies as many characters from
.Va input
to
.Va buf
as will fit and
.Tn NUL
terminates the result.
Because
.Fn strncpy
does
.Em not
guarantee to
.Tn NUL
terminate the string itself, this must be done explicitly.
.Bd -literal -offset indent
char buf[1024];

(void)strncpy(buf, input, sizeof(buf) - 1);
buf[sizeof(buf) - 1] = '\e0';
.Ed
.Pp
This could be better achieved using
.Xr strlcpy 3 ,
as shown in the following example:
.Pp
.Dl "(void)strlcpy(buf, input, sizeof(buf));"
.Pp
Note that because
.Xr strlcpy 3
is not defined in any standards, it should
only be used when portability is not a concern.
.Sh SECURITY CONSIDERATIONS
The
.Fn strcpy
function is easily misused in a manner which enables malicious users
to arbitrarily change a running program's functionality through a
buffer overflow attack.
(See
the FSA.)
.Pp
Avoid using
.Fn strcpy .
Instead, use
.Fn strncpy 
or
.Fn strlcpy
and ensure that no more characters are copied to the destination buffer
than it can hold.
Don't forget to NUL-terminate the destination buffer,
as
.Fn strncpy
will not terminate the destination string if it is truncated.
.Pp
Note that
.Fn strncpy
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:
.Pp
.Bd -literal
void
foo(const char *arbitrary_string)
{
        char onstack[8];

#if defined(BAD)
        /*
         * This first strcpy is bad behavior.  Don't use strcpy()!
         */
        (void)strcpy(onstack, arbitrary_string);     /* BAD! */
#elif defined(BETTER)
        /*
         * The following two lines demonstrate better use of
         * strncpy().
         */
        (void)strncpy(onstack, arbitrary_string, sizeof(onstack) - 1);
        onstack[sizeof(onstack - 1)] = '\\0';
#elif defined(BEST)
        /*
         * These lines are even more robust due to testing for
         * truncation.
         */
        if (strlen(arbitrary_string) + 1 > sizeof(onstack))
                err(1, "onstack would be truncated");
        (void)strncpy(onstack, arbitrary_string, sizeof(onstack));
#endif
}
.Ed
.Sh SEE ALSO
.Xr bcopy 3 ,
.Xr memccpy 3 ,
.Xr memcpy 3 ,
.Xr memmove 3 ,
.Xr strlcpy 3
.Rs
.%T "The FreeBSD Security Architecture"
.%J "/usr/share/doc/{to be decided}"
.Re
.Sh STANDARDS
The
.Fn strcpy
and
.Fn strncpy
functions
conform to
.St -isoC .