strncpy(3) manual page

Manual Pages — STRCPY

NAME

stpcpy, stpncpy, strcpy, strncpy – copy strings

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <string.h>

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

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

char *
strcpy(char * restrict dst, const char * restrict src);

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

DESCRIPTION

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

The strncpy() and stpncpy() functions copy at most len characters from src into dst. If src is less than len characters long, the remainder of dst is filled with ‘\0’ characters. Otherwise, dst is not terminated.

For all of strcpy(), strncpy(), stpcpy(), and stpncpy(), the result is undefined if src and dst overlap.

RETURN VALUES

The strcpy() and strncpy() functions return dst. The stpcpy() and stpncpy() functions return a pointer to the terminating ‘\0’ character of dst. If stpncpy() does not terminate dst with a NUL character, it instead returns a pointer to dst[n] (which does not necessarily refer to a valid memory location.)

EXAMPLES

The following sets chararray to "abc\0\0\0":

char chararray[6];

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

The following sets chararray to "abcdef":

char chararray[6];

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

Note that it does not NUL terminate chararray because the length of the source string is greater than or equal to the length argument.

The following copies as many characters from input to buf as will fit and NUL terminates the result. Because strncpy() does not guarantee to NUL terminate the string itself, this must be done explicitly.

char buf[1024];

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

This could be better achieved using strlcpy(3), as shown in the following example:

(void)strlcpy(buf, input, sizeof(buf));

STANDARDS

The strcpy() and strncpy() functions conform to ISO/IEC 9899:1990 ("ISO C90"). The stpcpy() and stpncpy() functions conform to IEEE Std 1003.1-2008 ("POSIX.1").

HISTORY

The stpcpy() function first appeared in FreeBSD 4.4, and stpncpy() was added in FreeBSD 8.0 .

SECURITY CONSIDERATIONS

All of the functions documented in this manual page are easily misused in a manner which enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack.

It is strongly suggested that the strlcpy() function be used in almost all cases.

For some, but not all, fixed-length records, non-terminated strings may be both valid and desirable. In that specific case, the strncpy() function may be most sensible.

STRCPY (3)

June 6, 2018

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