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[thirdparty/man-pages.git] / man3 / strcat.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 18:11:47 1993 by Rik Faith (faith@cs.unc.edu)
.\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
.\"     Improve discussion of strncat().
.TH STRCAT 3  2012-07-19 "GNU" "Linux Programmer's Manual"
.SH NAME
strcat, strncat \- concatenate two strings
.SH SYNOPSIS
.nf
.B #include <string.h>
.sp
.BI "char *strcat(char *" dest ", const char *" src );
.sp
.BI "char *strncat(char *" dest ", const char *" src ", size_t " n );
.fi
.SH DESCRIPTION
The
.BR strcat ()
function appends the \fIsrc\fP string to the
\fIdest\fP string,
overwriting the terminating null byte (\(aq\\0\(aq) at the end of
\fIdest\fP, and then adds a terminating null byte.
The strings may not overlap, and the \fIdest\fP string must have
enough space for the result.
If
.I dest
is not large enough, program behavior is unpredictable;
.IR "buffer overruns are a favorite avenue for attacking secure programs" .
.PP
The
.BR strncat ()
function is similar, except that
.IP * 3
it will use at most \fIn\fP bytes from \fIsrc\fP; and
.IP *
\fIsrc\fP does not need to be null-terminated if it contains
\fIn\fP or more bytes.
.PP
As with
.BR strcat (),
the resulting string in \fIdest\fP is always null-terminated.
.PP
If \fIsrc\fP contains \fIn\fP or more bytes,
.BR strncat ()
writes \fIn+1\fP bytes to \fIdest\fP (\fIn\fP
from \fIsrc\fP plus the terminating null byte).
Therefore, the size of \fIdest\fP must be at least
\fIstrlen(dest)+n+1\fP.

A simple implementation of
.BR strncat ()
might be:
.in +4n
.nf

char*
strncat(char *dest, const char *src, size_t n)
{
    size_t dest_len = strlen(dest);
    size_t i;

    for (i = 0 ; i < n && src[i] != \(aq\\0\(aq ; i++)
        dest[dest_len + i] = src[i];
    dest[dest_len + i] = \(aq\\0\(aq;

    return dest;
}
.fi
.in
.SH RETURN VALUE
The
.BR strcat ()
and
.BR strncat ()
functions return a pointer to the resulting string \fIdest\fP.
.SH CONFORMING TO
SVr4, 4.3BSD, C89, C99.
.SH NOTES
Some systems (the BSDs, Solaris, and others) provide the following function:

    size_t strlcat(char *dest, const char *src, size_t size);

This function appends the null-terminated string
.I src
to the string
.IR dest ,
copying at most
.IR "size\-strlen(dest)\-1"
from
.IR src ,
and adds a null terminator to the result,
.I unless
.IR size
is less than
.IR strlen(dest) .
This function fixes the buffer overrun problem of
.BR strcat (),
but the caller must still handle the possibility of data loss if
.I size
is too small.
The function returns the length of the string
.BR strlcat ()
tried to create; if the return value is greater than or equal to
.IR size ,
data loss occurred.
If data loss matters, the caller
.I must
either check the arguments before the call, or test the function return value.
.BR strlcat ()
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 SEE ALSO
.BR bcopy (3),
.BR memccpy (3),
.BR memcpy (3),
.BR strcpy (3),
.BR string (3),
.BR strncpy (3),
.BR wcscat (3),
.BR wcsncat (3)