mirror of https://github.com/mkerrisk/man-pages
236 lines
5.4 KiB
Groff
236 lines
5.4 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" References consulted:
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.\" Linux libc source code
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.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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.\" 386BSD man pages
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.\" Modified Sat Jul 24 18:11:47 1993 by Rik Faith (faith@cs.unc.edu)
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.\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
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.\" Improve discussion of strncat().
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.TH STRCAT 3 2016-07-17 "GNU" "Linux Programmer's Manual"
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.SH NAME
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strcat, strncat \- concatenate two strings
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.SH SYNOPSIS
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.nf
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.B #include <string.h>
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.sp
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.BI "char *strcat(char *" dest ", const char *" src );
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.sp
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.BI "char *strncat(char *" dest ", const char *" src ", size_t " n );
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.fi
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.SH DESCRIPTION
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The
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.BR strcat ()
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function appends the
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.I src
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string to the
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.I dest
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string,
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overwriting the terminating null byte (\(aq\\0\(aq) at the end of
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.IR dest ,
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and then adds a terminating null byte.
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The strings may not overlap, and the
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.I dest
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string must have
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enough space for the result.
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If
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.I dest
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is not large enough, program behavior is unpredictable;
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.IR "buffer overruns are a favorite avenue for attacking secure programs" .
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.PP
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The
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.BR strncat ()
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function is similar, except that
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.IP * 3
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it will use at most
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.I n
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bytes from
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.IR src ;
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and
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.IP *
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.I src
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does not need to be null-terminated if it contains
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.I n
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or more bytes.
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.PP
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As with
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.BR strcat (),
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the resulting string in
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.I dest
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is always null-terminated.
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.PP
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If
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.IR src
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contains
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.I n
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or more bytes,
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.BR strncat ()
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writes
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.I n+1
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bytes to
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.I dest
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.RI ( n
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from
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.I src
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plus the terminating null byte).
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Therefore, the size of
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.I dest
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must be at least
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.IR "strlen(dest)+n+1" .
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A simple implementation of
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.BR strncat ()
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might be:
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.in +4n
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.nf
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char *
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strncat(char *dest, const char *src, size_t n)
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{
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size_t dest_len = strlen(dest);
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size_t i;
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for (i = 0 ; i < n && src[i] != \(aq\\0\(aq ; i++)
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dest[dest_len + i] = src[i];
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dest[dest_len + i] = \(aq\\0\(aq;
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return dest;
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}
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.fi
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.in
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.SH RETURN VALUE
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The
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.BR strcat ()
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and
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.BR strncat ()
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functions return a pointer to the resulting string
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.IR dest .
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lbw19 lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR strcat (),
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.BR strncat ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
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.SH NOTES
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Some systems (the BSDs, Solaris, and others) provide the following function:
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size_t strlcat(char *dest, const char *src, size_t size);
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This function appends the null-terminated string
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.I src
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to the string
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.IR dest ,
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copying at most
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.IR "size\-strlen(dest)\-1"
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from
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.IR src ,
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and adds a terminating null byte to the result,
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.I unless
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.IR size
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is less than
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.IR strlen(dest) .
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This function fixes the buffer overrun problem of
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.BR strcat (),
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but the caller must still handle the possibility of data loss if
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.I size
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is too small.
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The function returns the length of the string
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.BR strlcat ()
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tried to create; if the return value is greater than or equal to
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.IR size ,
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data loss occurred.
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If data loss matters, the caller
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.I must
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either check the arguments before the call, or test the function return value.
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.BR strlcat ()
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is not present in glibc and is not standardized by POSIX,
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.\" https://lwn.net/Articles/506530/
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but is available on Linux via the
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.IR libbsd
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library.
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.\"
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.SH EXAMPLE
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Because
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.BR strcat ()
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and
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.BR strncat ()
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must find the null byte that terminates the string
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.I dest
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using a search that starts at the beginning of the string,
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the execution time of these functions
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scales according to the length of the string
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.IR dest .
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This can be demonstrated by running the program below.
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(If the goal is to concatenate many strings to one target,
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then manually copying the bytes from each source string
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while maintaining a pointer to the end of the target string
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will provide better performance.)
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.\"
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.SS Program source
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\&
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.nf
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#include <string.h>
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#include <time.h>
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#include <stdio.h>
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int
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main(int argc, char *argv[])
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{
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#define LIM 4000000
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int j;
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char p[LIM];
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time_t base;
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base = time(NULL);
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p[0] = \(aq\\0\(aq;
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for (j = 0; j < LIM; j++) {
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if ((j % 10000) == 0)
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printf("%d %ld\\n", j, (long) (time(NULL) \- base));
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strcat(p, "a");
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}
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}
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.fi
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.BR
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.\"
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.SH SEE ALSO
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.BR bcopy (3),
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.BR memccpy (3),
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.BR memcpy (3),
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.BR strcpy (3),
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.BR string (3),
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.BR strncpy (3),
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.BR wcscat (3),
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.BR wcsncat (3)
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