strlcpy(3) - NetBSD Manual Pages

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STRLCPY(3)              NetBSD Library Functions Manual             STRLCPY(3)

strlcpy, strlcat -- size-bounded string copying and concatenation
Standard C Library (libc, -lc)
#include <string.h> size_t strlcpy(char *dst, const char *src, size_t size); size_t strlcat(char *dst, const char *src, size_t size);
The strlcpy() and strlcat() functions copy and concatenate NUL-terminated strings respectively. The strlcpy() function copies up to size - 1 bytes 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. Relation to strncpy(3) and strncat(3) Unlike strncpy(3) and strncat(3), strlcpy() and strlcat() are guaranteed 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 you should include a byte for the NUL in size. WARNING: Also unlike strncpy(3) and strncat(3), strlcpy() and strlcat() are not guaranteed to initialize all size bytes of dst -- bytes past dst[strlen(src) + 1] are left uninitialized. This can lead to security vulnerabilities such as leaking secrets from uninitialized stack or heap buffers. WARNING: 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. Applications handling fixed-width fields with (possibly empty) NUL padding, instead of NUL-terminated C strings, MUST use strncpy(3) and strncat(3) instead. Attempting to use strlcpy() or strlcat() for these cases can lead to crashes or security vulnerabilities from buffer overruns.
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 trunca- tion detection simple. Note however, that if strlcat() traverses size bytes without finding a NUL, the length of the string is considered to be size and the destina- tion 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 incor- rect or that dst is not a proper ``C'' string). The check exists to pre- vent potential security problems in incorrect code.
The following code fragment illustrates the simple case: char *s, *p, buf[BUFSIZ]; ... strlcpy(buf, s, sizeof(buf)); 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 we know how many bytes we copied the first time, we can speed things 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().
snprintf(3), strncat(3), strncpy(3) Todd C. Miller and Theo de Raadt, "strlcpy and strlcat -- Consistent, Safe, String Copy and Concatenation", Proceedings of the FREENIX Track: 1999 USENIX Annual Technical Conference, USENIX Association,, June 6-11, 1999.
The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, then in NetBSD 1.4.3 and FreeBSD 3.3. NetBSD 10.99 August 11, 2023 NetBSD 10.99
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