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strlcpy(3) [freebsd man page]

STRLCPY(3)						   BSD Library Functions Manual 						STRLCPY(3)

NAME
strlcpy, strlcat -- size-bounded string copying and concatenation LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <string.h> size_t strlcpy(char * restrict dst, const char * restrict src, size_t size); size_t strlcat(char * restrict dst, const char * restrict src, size_t size); DESCRIPTION
The strlcpy() and strlcat() functions copy and concatenate strings with the same input parameters and output result as snprintf(3). They are designed to be safer, more consistent, and less error prone replacements for the easily misused functions strncpy(3) and strncat(3). strlcpy() and strlcat() take the full size of the destination buffer and guarantee NUL-termination if there is room. Note that room for the NUL should be included in dstsize. strlcpy() copies up to dstsize - 1 characters from the string src to dst, NUL-terminating the result if dstsize is not 0. strlcat() appends string src to the end of dst. It will append at most dstsize - strlen(dst) - 1 characters. It will then NUL-terminate, unless dstsize is 0 or the original dst string was longer than dstsize (in practice this should not happen as it means that either dstsize is incorrect or that dst is not a proper string). If the src and dst strings overlap, the behavior is undefined. RETURN VALUES
Besides quibbles over the return type (size_t versus int) and signal handler safety (snprintf(3) is not entirely safe on some systems), the following two are equivalent: n = strlcpy(dst, src, len); n = snprintf(dst, len, "%s", src); Like snprintf(3), 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. If the return value is >= dstsize, the output string has been truncated. It is the caller's responsibility to handle this. 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(). As a matter of fact, the first version of this manual page got it wrong. SEE ALSO
snprintf(3), strncat(3), strncpy(3), wcslcpy(3) HISTORY
The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, and FreeBSD 3.3. BSD
November 4, 2013 BSD

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STRLCPY(3)						   BSD Library Functions Manual 						STRLCPY(3)

NAME
strlcpy, strlcat -- size-bounded string copying and concatenation LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <string.h> size_t strlcpy(char * restrict dst, const char * restrict src, size_t size); size_t strlcat(char * restrict dst, const char * restrict src, size_t size); DESCRIPTION
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(3) and strncat(3). 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. 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(). As a matter of fact, the first version of this manual page got it wrong. SEE ALSO
snprintf(3), strncat(3), strncpy(3), wcslcpy(3) HISTORY
The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, and made their appearance in FreeBSD 3.3. BSD
June 22, 1998 BSD

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