strlcat(), strlcpy -- size-bounded string copying and concatenation

SYNOPSIS

#include <string.h>

size_t strlcat(char *dest, const char *src, size_t size);

size_t strlcpy(char *dest, const char *src, size_t size);

The strlcpy() and strlcat() functions copy and concatenate strings respectively. They are designed to be safer, more consistent, and less error prone replacements for strncpy() and strncat(). Unlike those functions, strlcpy() and strlcat() take the full size of the buffer (not just the length) and guarantee to NUL-terminate the result (as long as size is larger than 0 or, in the case of strlcat(), as long as there is at least one byte free in dst). Note that a byte for the NUL should be included in size. Also note that strlcpy() and strlcat() only operate on true "C" strings. This means that for strlcpy() src must be NUL-terminated and for strlcat() both src and dst must be NUL-terminated.

The strlcpy() function copies up to size - 1 characters from the NUL-terminated string src to dst, NUL-terminating the result.

The strlcat() function appends the NUL-terminated string src to the end of dst. It will append at most size - strlen(dst) - 1 bytes, NUL-terminating the result.

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

PARAMETERS

dst: Is the null-terminated string to be modified or appended.
src: Is the null-terminated string to be copied.
size: Nothing will be written beyond dst+size-1.

RETURN VALUES

The strlcpy() and strlcat() functions return the total length of the string they tried to create. For strlcpy() that means the length of src. For strlcat() that means the initial length of dst plus the length of src. While this may seem somewhat confusing, it was done to make truncation detection simple.

Note however, that if strlcat() traverses size characters without finding a NUL, the length of the string is considered to be size and the destination string will not be NUL-terminated (since there was no space for the NUL). This keeps strlcat() from running off the end of a string. In practice this should not happen (as it means that either size is incorrect or that dst is not a proper "C" string). The check exists to prevent potential security problems in incorrect code.

EXAMPLES

The following code fragment illustrates the simple case:

           char *s, *p, buf[BUFSIZ];

           ...

           (void)strlcpy(buf, s, sizeof(buf));
           (void)strlcat(buf, p, sizeof(buf));

To detect truncation, perhaps while building a pathname, something like the following might be used:

           char *dir, *file, pname[MAXPATHLEN];

           ...

           if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
                   goto toolong;
           if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
                   goto toolong;

Since it is known how many characters were copied the first time, things can be sped up a bit by using a copy instead of an append

           char *dir, *file, pname[MAXPATHLEN];
           size_t n;

           ...

           n = strlcpy(pname, dir, sizeof(pname));
           if (n >= sizeof(pname))
                   goto toolong;
           if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
                   goto toolong;

However, one may question the validity of such optimizations, as they defeat the whole purpose of strlcpy() and strlcat().

strlcat(), strlcpy
size-bounded string copying and concatenation

Function

SYNOPSIS

DESCRIPTION

PARAMETERS

RETURN VALUES

EXAMPLES

MULTITHREAD SAFETY LEVEL

PORTING ISSUES

AVAILABILITY

SEE ALSO

strlcat(), strlcpy size-bounded string copying and concatenation

Function

strlcat(), strlcpy
size-bounded string copying and concatenation