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STPCPY(3) Library Functions Manual STPCPY(3)

stpcpy, stpncpy
copy strings

#include <string.h>

char *
stpcpy(char *dst, const char *src);

char *
stpncpy(char *dst, const char *src, size_t len);

The stpcpy() and stpncpy() functions copy the string src to dst (including the terminating ‘\0’ character).

The stpncpy() function copies not more than len characters into dst, appending ‘\0’ characters if src is less than len characters long, and not terminating dst if the length of src is greater than or equal to len.

If the src and dst strings overlap, the behavior is undefined.

The stpcpy() function returns a pointer to the terminating ‘\0’ character written into dst.

The stpncpy() function returns a pointer to the first ‘\0’ character written into dst, or to &dst[len] if the length of src is greater than or equal to len.

The most common use of stpcpy() is to build up a string from multiple elements. The following example builds up a pathname from directory and file components using stpcpy():
char *dir, *file, pname[PATH_MAX];

...

if (strlen(dir) + strlen("/") + strlen(file) >= sizeof(pname))
	goto toolong;
stpcpy(stpcpy(stpcpy(pname, dir), "/"), file);

However, the size check required to avoid a buffer overflow is error prone since the check can become out of sync with the code that performs the copy.

One might expect that stpncpy() could be safely used instead, but it suffers from the same defects as strncpy(). The example below using stpncpy() is even more prone to error and will not detect when truncation occurs:

char *dir, *file, pname[PATH_MAX];
char *p1, *p2;

...

p1 = stpncpy(pname, dir, sizeof(pname) - 1);
p2 = stpncpy(p1, "/", sizeof(pname) - 1 - (p1 - pname));
stpncpy(p2, file, sizeof(pname) - 1 - (p2 - pname));
pname[sizeof(pname) - 1] = '\0';

A safer (and simpler) approach is to use snprintf():

char *dir, *file, pname[PATH_MAX];
int len;

...

len = snprintf(pname, sizeof(pname), "%s/%s", dir, file);
if (len >= sizeof(pname))
	goto toolong;

In most cases, it is better to use snprintf(), strlcpy(), or strlcat().

snprintf(3), strcpy(3), strlcpy(3), strncpy(3)

The stpcpy() and stpncpy() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The function stpcpy() first appeared in the Lattice C AmigaDOS compiler (1986 or earlier). The function stpncpy() first appeared in the GNU C library version 1.07 (1993). Both functions have been available since OpenBSD 5.1.
February 23, 2014 OpenBSD-current