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strncat (3)

>> strncat (3) ( FreeBSD man: Библиотечные вызовы )

strncat (3) ( Русские man: Библиотечные вызовы )

strncat (3) ( Linux man: Библиотечные вызовы )

strncat (3) ( POSIX man: Библиотечные вызовы )

BSD mandoc

NAME



strcat

 - concatenate strings

LIBRARY

Lb 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

The strcat ();
and 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 `\0' The string Fa s must have sufficient space to hold the result.

The strncat ();
function appends not more than Fa count characters from Fa append , and then adds a terminating `\0'

RETURN VALUES

The strcat ();
and strncat ();
functions return the pointer Fa s .

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. (See the FSA.)

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
}