Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
| From | BartC <bc@freeuk.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: How to copy a string safely and efficiently in C? |
| Date | 2016-09-13 15:59 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <nr948u$rbv$1@dont-email.me> (permalink) |
| References | <0b30b0cc-f7c5-4928-8acb-56bf2364797b@googlegroups.com> <nr7lnl$71m$1@reader2.panix.com> |
On 13/09/2016 02:45, Dan Cross wrote:
> semantics: it was originally written for copying data into
> fixed-width fields of data structures that were written to disk
> (actually as part of a filesystem). Thus, it zeros out the part of
> the destination after the end of the source if the source is shorter
> than the buffer size
That's handy to do anyway, whether you're going to write to disk or not.
For example, for making full use of small char-arrays within structs.
Then, if you only have a 4-char field, you want to be able to use all 4
chars, instead of needing an odd 5-char field or being limited to an odd
3-char string.
Nothing to do with the subject (and I just fancied writing some C), but
the following code uses store- and get- functions for copying normal
terminated strings into packed strings and extracting them again.
But it stores them as counted strings (storing a length within the
packed string without sacrificing a byte, or a top bit, to do so.).
Truncation is reported (only because it was mentioned in the
sub-thread!). However unused characters within the field are not zeroed.
#include<stdio.h>
#include<string.h>
void setfslength(char* s, int m, int n){
// encode length n of packed string within m-char field, within
// last two bytes of field. Strings can be up to m characters so
// using the entire field.
// a,b The last two chars of the fixed string
// ---
// 0,N Length is N
// 0,0 Length is 0 (special case of 0,N)
// X,0 Length is M-1
// X,Y Length is M
// NOTE: this only works for m in 2..256, and the string can't
// contain zero bytes.
if (m == n); // x,y
else if (n == m-1)
*(s+m-1) = 0; // x,0
else { // 0,n
*(s+m-2) = 0;
*(s+m-1) = n;
}
}
int getfslength(char* s, int m) {
// s points to a packed string encoded with length at its end.
// m is the max length (m>=2)
// return the encoded length
s += m-1;
if (*(s-1) == 0)
return *s;
else if (*s == 0)
return m-1;
else
return m;
}
int storepackedstring(char* dest, int width, char* source) {
// store C string into packed fixed-length non-nul-terminated
// field of size 'width' chars.
// return 1 (OK), 0 (truncated)
// width must be 2 to 256
int slen = strlen(source);
int status;
if (slen>width) {
status = 0;
slen = width;
}
else
status = 1;
if (slen)
memcpy(dest,source,slen);
setfslength(dest,width,slen);
return status;
}
int getpackedstring(char* dest, int width, char* source) {
// copy packed string in fixed width field to normal C terminated
// string. dest must point to at least width+1 characters (but
// maximum space needed is 257 characters).
// returns length of packed string
int slen = getfslength(source, width);
if (slen)
memcpy(dest,source,slen);
*(dest+slen) = 0;
return slen;
}
int main(void) {
typedef struct {
char A[4];
char B[2];
char C[26];
} R;
R S;
char str[257];
int len;
#define STR1 "bartc"
#define STR2 "XY"
#define STR3 "ABC"
if (!storepackedstring(S.A, sizeof S.A, STR1))
printf("%s truncated\n",STR1);
if (!storepackedstring(S.B, sizeof S.B, STR2))
printf("%s truncated\n",STR2);
if (!storepackedstring(S.C, sizeof S.C, STR3))
printf("%s truncated\n",STR3);
len = getpackedstring(str, sizeof S.A, S.A);
printf("S.A = %s Length: %d\n", str, len);
len = getpackedstring(str, sizeof S.B, S.B);
printf("S.B = %s Length: %d\n", str, len);
len = getpackedstring(str, sizeof S.C, S.C);
printf("S.C = %s Length: %d\n", str, len);
}
--
Bartc
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
How to copy a string safely and efficiently in C? will hunt <glovepm@gmail.com> - 2016-09-01 18:50 -0700
Re: How to copy a string safely and efficiently in C? John Gordon <gordon@panix.com> - 2016-09-02 01:58 +0000
Re: How to copy a string safely and efficiently in C? will hunt <glovepm@gmail.com> - 2016-09-01 19:40 -0700
Re: How to copy a string safely and efficiently in C? Geoff <geoff@invalid.invalid> - 2016-09-01 20:29 -0700
Re: How to copy a string safely and efficiently in C? James Kuyper <jameskuyper@verizon.net> - 2016-09-02 12:29 -0400
Re: How to copy a string safely and efficiently in C? asetofsymbols@gmail.com - 2016-09-12 13:09 -0700
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-12 15:30 -0700
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-01 23:08 -0700
Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-04 23:59 -0700
Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-01 21:12 -0600
Re: How to copy a string safely and efficiently in C? <william@wilbur.25thandClement.com> - 2016-09-01 22:12 -0700
Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-01 23:58 -0700
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 10:42 +0100
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 22:23 -0700
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-04 23:20 -0700
Re: How to copy a string safely and efficiently in C? mark.bluemel@gmail.com - 2016-09-05 01:03 -0700
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-05 01:46 -0700
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-05 10:30 +0100
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-10 11:25 -0700
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-11 06:23 -0700
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-11 15:41 -0700
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-11 23:40 -0700
Re: How to copy a string safely and efficiently in C? Ian Collins <ian-news@hotmail.com> - 2016-09-12 21:04 +1200
Re: How to copy a string safely and efficiently in C? Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-12 09:21 +0000
Re: How to copy a string safely and efficiently in C? Ian Collins <ian-news@hotmail.com> - 2016-09-12 21:33 +1200
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-12 10:41 +0100
Re: How to copy a string safely and efficiently in C? Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-12 10:48 +0000
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-12 07:04 -0700
Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 12:36 +0100
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 14:24 +0100
Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 14:51 +0100
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 23:42 -0700
Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 18:05 +0100
Re: How to copy a string safely and efficiently in C? Ike Naar <ike@iceland.freeshell.org> - 2016-09-02 19:48 +0000
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 01:45 +0000
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 11:06 +0100
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:09 +0000
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 15:30 +0100
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:52 +0000
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:54 +0000
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 17:18 +0100
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 18:15 +0000
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 20:30 -0700
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-15 14:03 +0000
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-15 12:32 -0700
Re: How to copy a string safely and efficiently in C? luser droog <luser.droog@gmail.com> - 2016-09-15 12:45 -0700
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 13:00 -0700
Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-15 14:52 -0600
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 16:04 -0700
Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-15 15:52 -0600
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 17:07 -0700
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 15:53 -0700
Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-15 22:17 +0100
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 17:01 -0700
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 16:36 -0700
Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 19:19 -0700
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 21:39 -0700
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-16 06:17 -0700
Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-16 07:04 -0700
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-16 09:11 -0700
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-22 22:24 -0700
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-19 15:15 +0000
Re: How to copy a string safely and efficiently in C? Richard Damon <Richard@Damon-Family.org> - 2016-09-13 08:19 -0400
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:07 +0000
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 07:57 -0700
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 17:33 +0100
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 18:13 +0000
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 11:30 -0700
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 20:03 +0000
Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 13:38 -0700
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 21:39 +0000
Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 23:26 +0100
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 16:14 -0700
Re: How to copy a string safely and efficiently in C? BartC <bc@freeuk.com> - 2016-09-13 15:59 +0100
Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 16:07 -0700
Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-14 00:24 +0000
csiph-web