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STRNCPY(3) Library Functions Manual STRNCPY(3)

strncpycopy part of a string to another

#include <string.h>

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

The () function copies not more than len characters from the string src to the buffer dst. If src is less than len characters long, it fills the remaining buffer with ‘\0’ characters. If the length of src is greater than or equal to len, dst will be NUL-terminated.

() NUL terminates the destination string when the length of the source string is less than the length parameter.

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

The strncpy() function returns dst.

The following sets chararray to “abc\0\0\0”:

(void)strncpy(chararray, "abc", 6);

The following sets chararray to “abcdef”, without a NUL-terminator:

(void)strncpy(chararray, "abcdefgh", 6);

The following sequence copies as many characters from input to buf as will fit, and then NUL terminates the result by hand:

char buf[BUFSIZ];

(void)strncpy(buf, input, sizeof(buf) - 1);
buf[sizeof(buf) - 1] = '\0';

By now it is clear that strncpy is dangerously easy to misuse. The strlcpy(3) function is safer for this kind of operation:

if (strlcpy(buf, input, sizeof(buf)) >= sizeof(buf))
        goto toolong;

strlcpy(3), wcscpy(3), wcslcpy(3)

The strncpy() function conforms to ANSI X3.159-1989 (“ANSI C89”).

The strncpy() function first appeared in Version 7 AT&T UNIX.

April 19, 2014 OpenBSD-7.3