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digitalmars.D.bugs - [Issue 5251] New: Const C file

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5251

           Summary: Const C file
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



I think this program is supposed to work, because fprintf() and fclose() don't
modify 'fout':


import std.c.stdio: fopen, fclose, fprintf;
void main() {
    const fout = fopen("test.txt", "w");
    fprintf(fout, "%d", 10); // ERR
    fclose(fout); // ERR
}



But DMD 2.050 shows the errors:

test.d(4): Error: function core.stdc.stdio.fprintf (shared(_iobuf)* stream, in
const(char*) format,...) is not callable using argument types
(const(shared(const(_iobuf))*),string,int)
test.d(4): Error: cannot implicitly convert expression (fout) of type
const(shared(const(_iobuf))*) to shared(_iobuf)*
test.d(5): Error: function core.stdc.stdio.fclose (shared(_iobuf)* stream) is
not callable using argument types (const(shared(const(_iobuf))*))
test.d(5): Error: cannot implicitly convert expression (fout) of type
const(shared(const(_iobuf))*) to shared(_iobuf)*

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5251


Steven Schveighoffer <schveiguy yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |schveiguy yahoo.com
         Resolution|                            |INVALID



08:58:52 PST ---


 I think this program is supposed to work, because fprintf() and fclose() don't
 modify 'fout':


Yes they do.  fprintf and fclose deal with a memory-allocated buffer that will
be used to optimize I/O throughput.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 Yes they do.  fprintf and fclose deal with a memory-allocated buffer that will
 be used to optimize I/O throughput.


If this bug report is invalid then thank you for closing it. I am not expert
enough on this.

But I don't understand what you have said. Even if fprintf and fclose deal with
a memory-allocated buffer, do they modify the value of 'fout'?

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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10:11:55 PST ---
const is transitive, so if you imagine a FILE having this structure:

struct FILE
{
   int fd;
   ubyte[] buffer;
}

Now, if FILE is const, then buffer is const(ubyte[]), so how does fprintf write
to that buffer?

What you are looking for is head-const (the variable cannot change, but
everything it points to can), which has some good uses, but does not exist in
D.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 const is transitive, so if you imagine a FILE having this structure:
 
 struct FILE
 {
    int fd;
    ubyte[] buffer;
 }
 
 Now, if FILE is const, then buffer is const(ubyte[]), so how does fprintf write
 to that buffer?
 
 What you are looking for is head-const (the variable cannot change, but
 everything it points to can), which has some good uses, but does not exist in
 D.


I understand, you are right, thank you.

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prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5251


nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nfxjfg gmail.com





 What you are looking for is head-const (the variable cannot change, but
 everything it points to can), which has some good uses, but does not exist in
 D.


It exists to some degree in D1:

final int[] a = [1,2];
a[0] = 1;  //works
a = [5,6]; //Error: cannot modify final variable 'a'

Apparently this was disabled in D2.

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