STRCAT(3) | Library Functions Manual | STRCAT(3) |
strcat
, strncat
— concatenate strings
Standard C Library (libc, -lc)
#include
<string.h>
char *
strcat
(char *restrict s1,
const char *restrict s2);
char *
strncat
(char *restrict s1,
const char *restrict s2, size_t
n);
YOU SHOULD ALMOST CERTAINLY USE
strlcat
()
INSTEAD.
See "Security Considerations" below.
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.
The strcat
() and
strncat
() functions return the pointer
s1.
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 }
bcopy(3), memccpy(3), memcpy(3), memmove(3), strcpy(3), strlcat(3), strlcpy(3), wcscat(3)
The strcat
() and
strncat
() functions conform to
ISO/IEC 9899:1990
(“ISO C90”).
December 1, 2009 | macOS 14.6 |