.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" References consulted:
.\" Linux libc source code
.\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
.\" Improve discussion of strncpy().
.\"
-.TH STRCPY 3 2007-06-15 "GNU" "Linux Programmer's Manual"
+.TH STRCPY 3 2019-03-06 "GNU" "Linux Programmer's Manual"
.SH NAME
strcpy, strncpy \- copy a string
.SH SYNOPSIS
.nf
.B #include <string.h>
-.sp
+.PP
.BI "char *strcpy(char *" dest ", const char *" src );
-.sp
+.PP
.BI "char *strncpy(char *" dest ", const char *" src ", size_t " n );
.fi
.SH DESCRIPTION
The
.BR strcpy ()
-function copies the string pointed to by \fIsrc\fP,
-including the terminating null byte ('\\0'),
-to the buffer pointed to by \fIdest\fP.
+function copies the string pointed to by
+.IR src ,
+including the terminating null byte (\(aq\e0\(aq),
+to the buffer pointed to by
+.IR dest .
The strings may not overlap, and the destination string
-\fIdest\fP must be large enough to receive the copy.
+.I dest
+must be large enough to receive the copy.
+.IR "Beware of buffer overruns!"
+(See BUGS.)
.PP
The
.BR strncpy ()
function is similar, except that at most
-\fIn\fP bytes of \fIsrc\fP are copied.
+.I n
+bytes of
+.I src
+are copied.
.BR Warning :
If there is no null byte
-among the first \fIn\fP bytes of \fIsrc\fP,
-the string placed in \fIdest\fP will not be null terminated.
+among the first
+.I n
+bytes of
+.IR src ,
+the string placed in
+.I dest
+will not be null-terminated.
.PP
If the length of
.I src
is less than
.IR n ,
.BR strncpy ()
-pads the remainder of
+writes additional null bytes to
.I dest
-with null bytes.
+to ensure that a total of
+.I n
+bytes are written.
.PP
A simple implementation of
.BR strncpy ()
might be:
-.in +0.25i
-.nf
-
-char*
-strncpy(char *dest, const char *src, size_t n){
+.PP
+.in +4n
+.EX
+char *
+strncpy(char *dest, const char *src, size_t n)
+{
size_t i;
- for (i = 0 ; i < n && src[i] != '\\0' ; i++)
+ for (i = 0; i < n && src[i] != \(aq\e0\(aq; i++)
dest[i] = src[i];
- for ( ; i < n ; i++)
- dest[i] = '\\0';
+ for ( ; i < n; i++)
+ dest[i] = \(aq\e0\(aq;
return dest;
}
-.fi
+.EE
.in
-.SH "RETURN VALUE"
+.SH RETURN VALUE
The
.BR strcpy ()
and
.BR strncpy ()
functions return a pointer to
-the destination string \fIdest\fP.
-.SH "CONFORMING TO"
-SVr4, 4.3BSD, C89, C99.
+the destination string
+.IR dest .
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbw19 lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR strcpy (),
+.BR strncpy ()
+T} Thread safety MT-Safe
+.TE
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
.SH NOTES
Some programmers consider
.BR strncpy ()
to be inefficient and error prone.
If the programmer knows (i.e., includes code to test!)
-that the size of \fIdest\fP is greater than
-the length of \fIsrc\fP, then
+that the size of
+.I dest
+is greater than
+the length of
+.IR src ,
+then
.BR strcpy ()
can be used.
-
-If there is no terminating null byte in the first \fIn\fP
-characters of \fIsrc\fP,
+.PP
+One valid (and intended) use of
.BR strncpy ()
-produces an unterminated string in \fIdest\fP.
-Programmers often prevent this mistake by forcing termination
-as follows:
-.in +0.25i
-.nf
-
-strncpy(buf, str, n);
-if (n > 0)
- buf[n - 1]= '\\0';
-.fi
+is to copy a C string to a fixed-length buffer
+while ensuring both that the buffer is not overflowed
+and that unused bytes in the destination buffer are zeroed out
+(perhaps to prevent information leaks if the buffer is to be
+written to media or transmitted to another process via an
+interprocess communication technique).
+.PP
+If there is no terminating null byte in the first
+.I n
+bytes of
+.IR src ,
+.BR strncpy ()
+produces an unterminated string in
+.IR dest .
+If
+.I buf
+has length
+.IR buflen ,
+you can force termination using something like the following:
+.PP
+.in +4n
+.EX
+if (buflen > 0) {
+ strncpy(buf, str, buflen \- 1);
+ buf[buflen \- 1]= \(aq\e0\(aq;
+}
+.EE
.in
+.PP
+(Of course, the above technique ignores the fact that, if
+.I src
+contains more than
+.I "buflen\ \-\ 1"
+bytes, information is lost in the copying to
+.IR dest .)
+.\"
+.SS strlcpy()
+Some systems (the BSDs, Solaris, and others) provide the following function:
+.PP
+ size_t strlcpy(char *dest, const char *src, size_t size);
+.PP
+.\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
+.\" "strlcpy and strlcat - consistent, safe, string copy and concatenation"
+.\" 1999 USENIX Annual Technical Conference
+This function is similar to
+.BR strncpy (),
+but it copies at most
+.I size\-1
+bytes to
+.IR dest ,
+always adds a terminating null byte,
+and does not pad the destination with (further) null bytes.
+This function fixes some of the problems of
+.BR strcpy ()
+and
+.BR strncpy (),
+but the caller must still handle the possibility of data loss if
+.I size
+is too small.
+The return value of the function is the length of
+.IR src ,
+which allows truncation to be easily detected:
+if the return value is greater than or equal to
+.IR size ,
+truncation occurred.
+If loss of data matters, the caller
+.I must
+either check the arguments before the call,
+or test the function return value.
+.BR strlcpy ()
+is not present in glibc and is not standardized by POSIX,
+.\" https://lwn.net/Articles/506530/
+but is available on Linux via the
+.IR libbsd
+library.
.SH BUGS
If the destination string of a
.BR strcpy ()
-is not large enough
-(that is, if the programmer was stupid or lazy, and failed to check
-the size before copying) then anything might happen.
-Overflowing fixed length strings is a favorite cracker technique.
-.SH "SEE ALSO"
+is not large enough, then anything might happen.
+Overflowing fixed-length string buffers is a favorite cracker technique
+for taking complete control of the machine.
+Any time a program reads or copies data into a buffer,
+the program first needs to check that there's enough space.
+This may be unnecessary if you can show that overflow is impossible,
+but be careful: programs can get changed over time,
+in ways that may make the impossible possible.
+.SH SEE ALSO
.BR bcopy (3),
.BR memccpy (3),
.BR memcpy (3),
.BR memmove (3),
+.BR stpcpy (3),
+.BR stpncpy (3),
+.BR strdup (3),
+.BR string (3),
.BR wcscpy (3),
.BR wcsncpy (3)
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