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Groups > comp.lang.c > #147449
| From | James Kuyper <jameskuyper@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: memcpy not memcpying? |
| Date | 2019-11-01 09:24 -0400 |
| Organization | A noiseless patient Spider |
| Message-ID | <qphblr$leg$1@dont-email.me> (permalink) |
| References | <2LVuF.7$BF1.0@fx26.iad> <qSVuF.94$kE3.74@fx36.iad> |
On 11/1/19 8:49 AM, DFS wrote:
> On 11/1/2019 8:42 AM, DFS wrote:
>> Just trying to combine 2 arrays:
>> ====================================================================
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <string.h>
>>
>> int i = 0;
>>
>> void combinearrays(int a, int b, char *x[], char *y[], char *z[]) {
>> memcpy(z, x, a);
It would appear from context that you intend to copy a elements from the
array x to the array z. The third element of memcpy() indicates how many
bytes to copy, not the number of elements to copy. What is the element
type of those arrays? Do you have any particular reason to think that
the size of that type is 1?
>> memcpy(z+a, y, b);
>> }
>>
>> int main(int argc, char *argv[]) {
>> char *x[] = {"adf1","fg7","Dxx","eff","Mnn","opp","zaa"};
>> char *y[] = {"A","c","p","S","x"};
>> int j = sizeof(x)/sizeof(x[0]);
>> int k = sizeof(y)/sizeof(y[0]);
>>
>> char **z = malloc(j+k);
>
> combinearrays(j,k,x,y,z); //forgot this line
On a typical modern implementation of C, sizeof(char*) is often 4, so
I'll use that value in the following explanation.
At this point, j should have a value of 7, while k should have a value of 5.
Therefore, your first memcpy() call copies the first 7 bytes of x into
z. That would copy x[0] into z[0], and the first three bytes of x[1]
into z[1]. The last byte of z[1], and all of the bytes of z[2], z[3],
z[4], z[5] and z[6] would remain uninitialized.
The second memcpy() call copies the first 5 bytes of y into z, starting
at z+7. That would copy y[0] into z[7], and the first byte of y[1] into
z[8]. The remaining 3 bytes of z[8], and all of the bytes of z[9],
z[10], and z[11] are uninitialized.
>> i=0;
>> while (i<j) printf("%s ",x[i++]);
>> printf("\n");
>>
>> i=0;
>> while (i<k) printf("%s ",y[i++]);
>> printf("\n");
>>
>> i=0;
As a result of those uninitialized bytes, z[1] isn't even required to
contain a valid pointer value, and the behavior of z[i++] is therefore
undefined as soon as i==1.
>> while (i<j+k) printf("%s ",z[i++]);
>>
>> free(z);
>> exit(0);
>> }
...
> adf1 fg7 Dxx eff Mnn opp zaa
> A c p S x
> adf1 Çn
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memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 08:42 -0400
Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 08:49 -0400
Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:24 -0400
Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 09:46 -0400
Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:49 -0400
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-01 10:19 -0700
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-01 21:14 +0100
Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 17:16 -0400
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-01 22:42 +0000
Re: memcpy not memcpying? Richard Damon <Richard@Damon-Family.org> - 2019-11-01 21:10 -0400
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 11:36 +0000
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 02:15 -0700
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-02 13:04 +0100
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 13:43 +0000
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-02 17:24 +0100
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 17:00 +0000
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 13:06 -0700
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 21:05 +0000
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 22:40 +0000
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 15:59 -0700
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-02 23:47 +0000
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-02 18:22 -0700
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 13:27 +0000
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-03 17:46 +0100
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 17:20 +0000
Re: memcpy not memcpying? Manfred <noname@add.invalid> - 2019-11-03 19:44 +0100
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 19:36 +0000
Re: memcpy not memcpying? Manfred <noname@add.invalid> - 2019-11-03 22:57 +0100
Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-03 22:39 +0000
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-03 23:06 +0100
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 23:28 +0000
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-04 11:00 +0100
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 11:14 +0000
Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 11:22 +0000
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-04 13:10 +0100
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-04 10:03 -0800
Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 19:36 +0000
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-05 10:04 -0800
Re: memcpy not memcpying? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-04 20:58 -0800
Re: memcpy not memcpying? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-04 22:01 -0800
Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-04 12:56 +0100
Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-04 09:50 -0500
Re: memcpy not memcpying? scott@slp53.sl.home (Scott Lurndal) - 2019-11-04 20:26 +0000
Re: memcpy not memcpying? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-04 04:34 -0800
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 13:05 +0000
Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 14:09 +0000
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 15:11 +0000
Re: memcpy not memcpying? Melzzzzz <Melzzzzz@zzzzz.com> - 2019-11-04 15:15 +0000
Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 15:43 +0000
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 20:20 +0000
Re: memcpy not memcpying? Ian Collins <ian-news@hotmail.com> - 2019-11-03 14:37 +1300
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 11:01 +0000
Re: memcpy not memcpying? Öö Tiib <ootiib@hot.ee> - 2019-11-03 04:03 -0800
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 12:52 +0000
Re: memcpy not memcpying? Öö Tiib <ootiib@hot.ee> - 2019-11-03 23:34 -0800
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 09:36 +0000
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-04 16:20 +0000
Re: memcpy not memcpying? Ian Collins <ian-news@hotmail.com> - 2019-11-04 07:46 +1300
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-03 12:26 -0800
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-03 17:31 +0100
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-03 17:05 +0000
Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 09:14 -0800
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-03 12:32 -0800
Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 14:59 -0800
Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 15:19 -0800
Re: memcpy not memcpying? already5chosen@yahoo.com - 2019-11-03 17:00 -0800
Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-02 11:16 -0400
Re: memcpy not memcpying? David Brown <david.brown@hesbynett.no> - 2019-11-02 17:26 +0100
Re: memcpy not memcpying? Barry Schwarz <schwarzb@delq.com> - 2019-11-01 06:39 -0700
Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 09:56 -0400
Re: memcpy not memcpying? Barry Schwarz <schwarzb@delq.com> - 2019-11-01 12:39 -0700
Re: memcpy not memcpying? guinness.tony@gmail.com - 2019-11-01 06:49 -0700
Re: memcpy not memcpying? jak <nospam@please.ty> - 2019-11-01 20:40 +0100
Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 14:26 +0100
Re: memcpy not memcpying? Keith Thompson <kst-u@mib.org> - 2019-11-01 10:22 -0700
Re: memcpy not memcpying? Bart <bc@freeuk.com> - 2019-11-01 19:44 +0000
Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 21:31 +0100
Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:39 -0400
Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 14:42 +0100
Re: memcpy not memcpying? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-01 09:48 -0400
Re: memcpy not memcpying? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-01 06:47 -0700
Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 14:58 +0100
Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 10:22 -0400
Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 18:38 +0100
Re: memcpy not memcpying? scott@slp53.sl.home (Scott Lurndal) - 2019-11-01 17:54 +0000
Re: memcpy not memcpying? Autist <autist69@gmail.com> - 2019-11-01 20:24 +0100
Re: memcpy not memcpying? DFS <nospam@dfs.com> - 2019-11-01 15:24 -0400
Re: memcpy not memcpying? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-01 23:19 +0000
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