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Re: Splitting strings into arrays with sscanf

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scott@slp53.sl.home (Scott Lurndal) writes:
> DFS <nospam@dfs.com> writes:
[...]
>>Done, and then it compiles, but when run it crashes on line 14:
>
> In C, a string is an array of characters terminated by
> an ASCII (or UTF-8) NUL byte.

C doesn't mention ASCII (other than a couple of non-normative
footnotes).  A string is terminated by a null character, i.e., a
character with the value zero ('\0').

[...]

>>	char line[50]    ;
>>	char lbls[4]     ;
>
> This declaration reserves four bytes of memory (four 1-byte characters).
>
>>	char data[10][4] ;
>>	
>>	memset(lbls, 0, sizeof(lbls));
>
> This sets all four bytes to zero.

Which means that lbls contains a string of length 0.  The original code:
    char lbls[4] = "";
did exactly the same thing.

>>	memset(data, 0, sizeof(data));
>>	
>>	strcpy(line,"Name,Quarter,Product,Sales");
>>	sscanf(line, "%s,%s,%s,%s", lbls[0],lbls[1],lbls[2],lbls[3]);
>
> This line (the scanf) is the root of your problem.    The arguments to
> scanf when the format character is '%s' must be the first address of a
> character vector sized sufficiently to hold the value being scanned.
>
> Passing lbls[0] passes the value zero as the argument.   When sscanf
> processes the first '%s', it will take that zero and use it as the
> address of the first byte in which to store the matching data.  Since
> the value zero maps to the NULL pointer on pretty much all current
> systems, you'll get a segmentation violation when attempting to store
> the result.

More simply, "%s" requires an argument of type char*, but it's being
given an argument of type int (lbls[0] is of type char, which promotes
to int).  The behavior is undefined.

> If you had, instead, written:
>
>    sscanf(line, "%s,%s,%s,%s", &lbls[0], &lbls[1], &lbls[2], &lbls[3]);
>
> Then the correct 'char *' arguments would have been passed to the sscanf
> function.   However, there is only one byte of storage available at each
> of those pointers, so you'll end up corrupting your stack.

&lbls[0] points to the beginning of a 4-byte object, but the other
arguments point to overlapping locations within that object.  If you wrote:

    sscanf(line, "%s", &lbls[0]);

it would be fine as long as the scanned data is no more than 3 bytes
long (an additional byte is needed for the trailing '\0').

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Splitting strings into arrays with sscanf DFS <nospam@dfs.com> - 2017-06-16 13:33 -0400
  Re: Splitting strings into arrays with sscanf scott@slp53.sl.home (Scott Lurndal) - 2017-06-16 17:46 +0000
    Re: Splitting strings into arrays with sscanf DFS <nospam@dfs.com> - 2017-06-16 14:02 -0400
      Re: Splitting strings into arrays with sscanf scott@slp53.sl.home (Scott Lurndal) - 2017-06-16 18:15 +0000
        Re: Splitting strings into arrays with sscanf Keith Thompson <kst-u@mib.org> - 2017-06-16 12:28 -0700
    Re: Splitting strings into arrays with sscanf Keith Thompson <kst-u@mib.org> - 2017-06-16 12:20 -0700
  Re: Splitting strings into arrays with sscanf Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-06-16 13:47 -0400
    Re: Splitting strings into arrays with sscanf scott@slp53.sl.home (Scott Lurndal) - 2017-06-16 18:01 +0000
      Re: Splitting strings into arrays with sscanf Barry Schwarz <schwarzb@dqel.com> - 2017-06-16 12:03 -0700
        Re: Splitting strings into arrays with sscanf "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-16 15:33 -0400
          Re: Splitting strings into arrays with sscanf Keith Thompson <kst-u@mib.org> - 2017-06-16 13:16 -0700
      Re: Splitting strings into arrays with sscanf Keith Thompson <kst-u@mib.org> - 2017-06-16 12:31 -0700
      Re: Splitting strings into arrays with sscanf "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-16 15:45 -0400
  Re: Splitting strings into arrays with sscanf "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-16 14:07 -0400
  Re: Splitting strings into arrays with sscanf "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-16 14:22 -0400
    Re: Splitting strings into arrays with sscanf DFS <nospam@dfs.com> - 2017-06-16 15:36 -0400
      Re: Splitting strings into arrays with sscanf "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-16 17:17 -0400
  Re: Splitting strings into arrays with sscanf Barry Schwarz <schwarzb@dqel.com> - 2017-06-16 12:06 -0700
  Re: Splitting strings into arrays with sscanf Real Troll <Real.Troll@Trolls.com> - 2017-06-16 17:30 -0400
  Re: Splitting strings into arrays with sscanf ike@iceland.freeshell.org - 2017-06-17 16:49 +0000
  Re: Splitting strings into arrays with sscanf Richard Damon <Richard@Damon-Family.org> - 2017-06-17 18:25 -0400

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