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digitalmars.D.learn - use of C memmove

reply spir <denis.spir gmail.com> writes:
Hello,

I'm trying to use C's memmove as a tool to delete or insert a slice from/into 
an array. But I cannot manage to do it: systematic segmentation fault.
What is wrong below?

import std.c.string : memmove;
// void *memmove(void *dest, const void *src, size_t n);

void moveEnd (E) (E[] elements, size_t start, int offset) {
     // Length must be known before possible extension.
     auto length = elements.length;

     // If move up, extend array to make place.
     if (offset > 0)
         elements.length += offset;

     // Move slice.
     auto dest = cast(void*)(&(elements[start + offset]));
     auto source = cast(void*)(&(elements[start]));
     size_t size = length - start;
     memmove(dest, source, size);    // segfault ***

     // If move down, compress array.
     if (offset < 0)
         elements.length += offset;
}

unittest {
     string s = "012--3456789";
     // Remove slice.
     s.moveEnd(5, -2);
     writeln(s);
}

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Apr 07 2011
next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Thu, 07 Apr 2011 13:09:05 -0400, spir <denis.spir gmail.com> wrote:


 Hello,

 I'm trying to use C's memmove as a tool to delete or insert a slice  
 from/into an array. But I cannot manage to do it: systematic  
 segmentation fault.
 What is wrong below?

 import std.c.string : memmove;
 // void *memmove(void *dest, const void *src, size_t n);

 void moveEnd (E) (E[] elements, size_t start, int offset) {
      // Length must be known before possible extension.
      auto length = elements.length;

      // If move up, extend array to make place.
      if (offset > 0)
          elements.length += offset;

      // Move slice.
      auto dest = cast(void*)(&(elements[start + offset]));
      auto source = cast(void*)(&(elements[start]));
      size_t size = length - start;
      memmove(dest, source, size);    // segfault ***

      // If move down, compress array.
      if (offset < 0)
          elements.length += offset;
 }

 unittest {
      string s = "012--3456789";
      // Remove slice.
      s.moveEnd(5, -2);
      writeln(s);
 }


Two problems.  One is, the memmove size_t n is number of *bytes*, not  
number of elements as you have expected.  You probably would have noticed  
this quickly if the other problem wasn't there.

The other problem is, strings literals are immutable.  On Windows, this  
code may have worked, but Linux protects the pages of static data, so  
writing to a string literal creates a seg fault.

Try this:

auto s = "012--3456789".dup; // convert to char[]

To fix first problem use memmove(dest, source, size * (E).sizeof);

-Steve
Apr 07 2011
parent spir <denis.spir gmail.com> writes:
On 04/07/2011 08:12 PM, Steven Schveighoffer wrote:

 On Thu, 07 Apr 2011 13:09:05 -0400, spir <denis.spir gmail.com> wrote:


 Hello,

 I'm trying to use C's memmove as a tool to delete or insert a slice from/into
 an array. But I cannot manage to do it: systematic segmentation fault.
 What is wrong below?

 import std.c.string : memmove;
 // void *memmove(void *dest, const void *src, size_t n);

 void moveEnd (E) (E[] elements, size_t start, int offset) {
 // Length must be known before possible extension.
 auto length = elements.length;

 // If move up, extend array to make place.
 if (offset > 0)
 elements.length += offset;

 // Move slice.
 auto dest = cast(void*)(&(elements[start + offset]));
 auto source = cast(void*)(&(elements[start]));
 size_t size = length - start;
 memmove(dest, source, size); // segfault ***

 // If move down, compress array.
 if (offset < 0)
 elements.length += offset;
 }

 unittest {
 string s = "012--3456789";
 // Remove slice.
 s.moveEnd(5, -2);
 writeln(s);
 }


 Two problems. One is, the memmove size_t n is number of *bytes*, not number of
 elements as you have expected. You probably would have noticed this quickly if
 the other problem wasn't there.

 The other problem is, strings literals are immutable. On Windows, this code may
 have worked, but Linux protects the pages of static data, so writing to a
 string literal creates a seg fault.

 Try this:

 auto s = "012--3456789".dup; // convert to char[]

 To fix first problem use memmove(dest, source, size * (E).sizeof);


Thank you very much, Steven!

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Apr 07 2011
prev sibling parent reply Kagamin <spam here.lot> writes:
spir Wrote:


      // If move down, compress array.
      if (offset < 0)
          elements.length += offset;


ow, addAssign works on length?
Apr 07 2011
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Thu, 07 Apr 2011 14:44:28 -0400, Kagamin <spam here.lot> wrote:


 spir Wrote:


      // If move down, compress array.
      if (offset < 0)
          elements.length += offset;


 ow, addAssign works on length?


Since 12/09 :)

http://www.digitalmars.com/d/2.0/changelog.html#new2_037

-Steve
Apr 07 2011