strcat3

Usage

Library

Standard C Library (libc, -lc)

Admonishment

YOU SHOULD ALMOST CERTAINLY USE strlcat() INSTEAD.

See "Security Considerations" below.

Overview

The strcat() and strncat() functions append a copy of the null-terminated string s2 to the end of the null-terminated string s1, then add a terminating ‘\0’. The string s1 must have sufficient space to hold the result.

The strncat() function appends not more than n characters from s2, and then adds a terminating ‘\0’.

The source and destination strings should not overlap, as the behavior is undefined.

Return values

The strcat() and strncat() functions return the pointer s1.

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
}