strcat, strncat - concatenate two strings
The strcat() function appends the
src string to the
dest string, overwriting the terminating null byte ('\0') at the end of
dest, and then adds a terminating null byte. The strings may not overlap, and the
dest string must have enough space for the result. If
dest is not large enough, program behavior is unpredictable;
buffer overruns are a favorite avenue for attacking secure programs.
The strncat() function is similar, except that
it will use at most
n bytes from
src does not need to be null-terminated if it contains
n or more bytes.
As with strcat(), the resulting string in
dest is always null-terminated.
n or more bytes, strncat() writes
n+1 bytes to
src plus the terminating null byte). Therefore, the size of
dest must be at least
A simple implementation of strncat() might be:
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
Some systems (the BSDs, Solaris, and others) provide the following function:
This function appends the null-terminated string
src to the string
dest, copying at most
src, and adds a terminating null byte to the result,
size is less than
strlen(dest). This function fixes the buffer overrun problem of strcat(), but the caller must still handle the possibility of data loss if
size is too small. The function returns the length of the string strlcat() tried to create; if the return value is greater than or equal to
size, data loss occurred. If data loss matters, the caller
must either check the arguments before the call, or test the function return value. strlcat() is not present in glibc and is not standardized by POSIX, but is available on Linux via the
libbsd library. Because strcat() and strncat() must find the null byte that terminates the string
dest using a search that starts at the beginning of the string, the execution time of these functions scales according to the length of the string
dest. This can be demonstrated by running the program below. (If the goal is to concatenate many strings to one target, then manually copying the bytes from each source string while maintaining a pointer to the end of the target string will provide better performance.)
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