344 lines
7.2 KiB
HTML
344 lines
7.2 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of STRCPY</TITLE>
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</HEAD><BODY>
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<H1>STRCPY</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2019-03-06<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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strcpy, strncpy - copy a string
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/string.h">string.h</A>></B>
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<B>char *strcpy(char *</B><I>dest</I><B>, const char *</B><I>src</I><B>);</B>
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<B>char *strncpy(char *</B><I>dest</I><B>, const char *</B><I>src</I><B>, size_t </B><I>n</I><B>);</B>
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</PRE>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>strcpy</B>()
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function copies the string pointed to by
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<I>src</I>,
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including the terminating null byte ('\0'),
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to the buffer pointed to by
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<I>dest</I>.
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The strings may not overlap, and the destination string
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<I>dest</I>
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must be large enough to receive the copy.
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<I>Beware of buffer overruns!</I>
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(See BUGS.)
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<P>
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The
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<B>strncpy</B>()
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function is similar, except that at most
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<I>n</I>
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bytes of
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<I>src</I>
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are copied.
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<B>Warning</B>:
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If there is no null byte
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among the first
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<I>n</I>
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bytes of
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<I>src</I>,
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the string placed in
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<I>dest</I>
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will not be null-terminated.
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<P>
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If the length of
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<I>src</I>
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is less than
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<I>n</I>,
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<B>strncpy</B>()
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writes additional null bytes to
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<I>dest</I>
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to ensure that a total of
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<I>n</I>
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bytes are written.
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<P>
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A simple implementation of
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<B>strncpy</B>()
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might be:
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<P>
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char *
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strncpy(char *dest, const char *src, size_t n)
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{
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<BR> size_t i;
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<P>
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<BR> for (i = 0; i < n && src[i] != '\0'; i++)
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<BR> dest[i] = src[i];
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<BR> for ( ; i < n; i++)
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<BR> dest[i] = '\0';
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<P>
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<BR> return dest;
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}
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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The
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<B>strcpy</B>()
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and
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<B>strncpy</B>()
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functions return a pointer to
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the destination string
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<I>dest</I>.
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<A NAME="lbAF"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>strcpy</B>(),
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<B>strncpy</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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Some programmers consider
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<B>strncpy</B>()
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to be inefficient and error prone.
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If the programmer knows (i.e., includes code to test!)
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that the size of
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<I>dest</I>
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is greater than
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the length of
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<I>src</I>,
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then
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<B>strcpy</B>()
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can be used.
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<P>
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One valid (and intended) use of
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<B>strncpy</B>()
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is to copy a C string to a fixed-length buffer
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while ensuring both that the buffer is not overflowed
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and that unused bytes in the destination buffer are zeroed out
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(perhaps to prevent information leaks if the buffer is to be
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written to media or transmitted to another process via an
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interprocess communication technique).
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<P>
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If there is no terminating null byte in the first
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<I>n</I>
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bytes of
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<I>src</I>,
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<B>strncpy</B>()
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produces an unterminated string in
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<I>dest</I>.
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If
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<I>buf</I>
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has length
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<I>buflen</I>,
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you can force termination using something like the following:
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<P>
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if (buflen > 0) {
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<BR> strncpy(buf, str, buflen - 1);
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<BR> buf[buflen - 1]= '\0';
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}
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<P>
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(Of course, the above technique ignores the fact that, if
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<I>src</I>
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contains more than
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<I>buflen - 1</I>
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bytes, information is lost in the copying to
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<I>dest</I>.)
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<A NAME="lbAI"> </A>
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<H3>strlcpy()</H3>
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Some systems (the BSDs, Solaris, and others) provide the following function:
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<P>
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<BR> size_t strlcpy(char *dest, const char *src, size_t size);
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<P>
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This function is similar to
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<B>strncpy</B>(),
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but it copies at most
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<I>size-1</I>
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bytes to
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<I>dest</I>,
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always adds a terminating null byte,
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and does not pad the destination with (further) null bytes.
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This function fixes some of the problems of
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<B>strcpy</B>()
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and
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<B>strncpy</B>(),
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but the caller must still handle the possibility of data loss if
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<I>size</I>
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is too small.
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The return value of the function is the length of
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<I>src</I>,
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which allows truncation to be easily detected:
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if the return value is greater than or equal to
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<I>size</I>,
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truncation occurred.
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If loss of data matters, the caller
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<I>must</I>
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either check the arguments before the call,
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or test the function return value.
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<B>strlcpy</B>()
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is not present in glibc and is not standardized by POSIX,
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but is available on Linux via the
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<I>libbsd</I>
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library.
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<A NAME="lbAJ"> </A>
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<H2>BUGS</H2>
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If the destination string of a
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<B>strcpy</B>()
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is not large enough, then anything might happen.
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Overflowing fixed-length string buffers is a favorite cracker technique
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for taking complete control of the machine.
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Any time a program reads or copies data into a buffer,
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the program first needs to check that there's enough space.
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This may be unnecessary if you can show that overflow is impossible,
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but be careful: programs can get changed over time,
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in ways that may make the impossible possible.
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?3+bcopy">bcopy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+memccpy">memccpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+memcpy">memcpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+memmove">memmove</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+stpcpy">stpcpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+stpncpy">stpncpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+strdup">strdup</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+string">string</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+wcscpy">wcscpy</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+wcsncpy">wcsncpy</A></B>(3)
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<A NAME="lbAL"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="5"><A HREF="#lbAF">ATTRIBUTES</A><DD>
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<DT id="6"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="7"><A HREF="#lbAH">NOTES</A><DD>
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<DL>
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<DT id="8"><A HREF="#lbAI">strlcpy()</A><DD>
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</DL>
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<DT id="9"><A HREF="#lbAJ">BUGS</A><DD>
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<DT id="10"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="11"><A HREF="#lbAL">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:58 GMT, March 31, 2021
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</BODY>
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</HTML>
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