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digitalmars.D.learn - Call C function - Access violation

reply TheDGuy <loder.feuer googlemail.com> writes:
I get an access violation with this code:

extern(C) char* write(char* text);

void main(string[] args){
	char[] text = "Hello World".dup; //.dup converts string to char[]
	text ~= '\0'; //append

	char* result = write(text.ptr); //you need .ptr
	const(char)[] s = cstr2dstr(result);
	writeln(s);
	readln(); //keep the window open
}

auto cstr2dstr(inout(char)* cstr)
{
	return cstr ? cstr[0 .. strlen(cstr)] : "";
}

--

#include std.stdio;

char* write(char* text){
	return text;
}
Jan 03 2016
next sibling parent reply anonymous <anonymous example.com> writes:
On 03.01.2016 13:30, TheDGuy wrote:

 I get an access violation with this code:

 extern(C) char* write(char* text);

 void main(string[] args){
      char[] text = "Hello World".dup; //.dup converts string to char[]
      text ~= '\0'; //append

      char* result = write(text.ptr); //you need .ptr
      const(char)[] s = cstr2dstr(result);
      writeln(s);
      readln(); //keep the window open
 }

 auto cstr2dstr(inout(char)* cstr)
 {
      return cstr ? cstr[0 .. strlen(cstr)] : "";
 }

 --

 #include std.stdio;

 char* write(char* text){
      return text;
 }


Works for me after adding the needed imports and removing the wrong 
include from the C file. Is this really the actual code you're running? 
Doesn't your C compiler reject that include? gcc does.
Jan 03 2016
parent reply TheDGuy <loder.feuer googlemail.com> writes:
 Works for me after adding the needed imports and removing the 
 wrong include from the C file. Is this really the actual code 
 you're running? Doesn't your C compiler reject that include? 
 gcc does.


Okay, i think this C code should work (checked with cpp.sh):

#import <stdio.h>
char* write(char* text){
	return text;
}
int main(void){
     return 0;
}


but i still get the access violation.
Jan 03 2016
parent reply anonymous <anonymous example.com> writes:
On 03.01.2016 14:01, TheDGuy wrote:

 Okay, i think this C code should work (checked with cpp.sh):

 #import <stdio.h>
 char* write(char* text){
      return text;
 }
 int main(void){
      return 0;
 }


Uh, no. 1) In C it's include, not import. 2) Now you have two main 
functions, that can't work.

You shouldn't get a segfault, though. You should get some compile/link 
error. Are you compiling the right files? Can the segfault be from 
something other than your program?
Jan 03 2016
parent anonymous <anonymous example.com> writes:
On 03.01.2016 14:12, anonymous wrote:

 You shouldn't get a segfault, though. You should get some compile/link
 error. Are you compiling the right files? Can the segfault be from
 something other than your program?


Oh, I see what's probably happening:

You compile the D program, but you don't compile and/or don't link the C 
object file. It segfaults then when trying to call the C function.

You need to compile the C file and pass it to dmd or the linker. For 
example:

gcc -c -otest.c.o test.c
dmd test.d test.c.o
./test
Jan 03 2016
prev sibling parent reply Gary Willoughby <dev nomad.so> writes:
On Sunday, 3 January 2016 at 12:30:34 UTC, TheDGuy wrote:

 I get an access violation with this code:

 ...


There are a few things you can do to improve your code to make it 
easier to debug.

1. When converting a D string to a char pointer for use with C, 
use `std.string.toStringz`:



2. When converting a char pointer to a D string use 
`std.conv.to!(string)`:



3. Define C-style char pointers in D as `const(char)*`.
4. Don't use the variable name `text` as it conflicts with 
`std.conv.text`.


I think I've noticed one problem with the code above. You are 
using `text.ptr`. You shouldn't do that because you are passing a 
pointer not an array. Just use `text`.
Jan 03 2016
parent reply Gary Willoughby <dev nomad.so> writes:
On Sunday, 3 January 2016 at 13:23:25 UTC, Gary Willoughby wrote:

 I think I've noticed one problem with the code above. You are 
 using `text.ptr`. You shouldn't do that because you are passing 
 a pointer not an array. Just use `text`.


Forget this line, my mistake. Use `toStringz` and pass a pointer 
instead of an array.
Jan 03 2016
parent reply TheDGuy <loder.feuer googlemail.com> writes:
On Sunday, 3 January 2016 at 13:25:04 UTC, Gary Willoughby wrote:

 On Sunday, 3 January 2016 at 13:23:25 UTC, Gary Willoughby 
 wrote:

 I think I've noticed one problem with the code above. You are 
 using `text.ptr`. You shouldn't do that because you are 
 passing a pointer not an array. Just use `text`.


 Forget this line, my mistake. Use `toStringz` and pass a 
 pointer instead of an array.


Hi and thanks for your answer. My code looks now like this:

void main(string[] args){
	const(char)* val = "Hello World".std.string.toStringz;
	char* result = write(val);
	const(char)[] s = cstr2dstr(result);
	writeln(s);
	readln(); //keep the window open
}

But now i get the error: "function expected before(), not package 
std of type void" (refering to line 2).

And if i define the variable 'value' as 'const(char)*' i also 
have to rewrite my C-function to accept const(char)*...
Jan 03 2016
parent reply Gary Willoughby <dev nomad.so> writes:
On Sunday, 3 January 2016 at 19:24:46 UTC, TheDGuy wrote:

 On Sunday, 3 January 2016 at 13:25:04 UTC, Gary Willoughby 
 wrote:

 On Sunday, 3 January 2016 at 13:23:25 UTC, Gary Willoughby 
 wrote:

 I think I've noticed one problem with the code above. You are 
 using `text.ptr`. You shouldn't do that because you are 
 passing a pointer not an array. Just use `text`.


 Forget this line, my mistake. Use `toStringz` and pass a 
 pointer instead of an array.


 Hi and thanks for your answer. My code looks now like this:

 void main(string[] args){
 	const(char)* val = "Hello World".std.string.toStringz;
 	char* result = write(val);
 	const(char)[] s = cstr2dstr(result);
 	writeln(s);
 	readln(); //keep the window open
 }

 But now i get the error: "function expected before(), not 
 package std of type void" (refering to line 2).

 And if i define the variable 'value' as 'const(char)*' i also 
 have to rewrite my C-function to accept const(char)*...


Use an import.

import std.string;
import std.conv;

void main(string[] args) {
     auto value  = toStringz("Hello World");
     auto result = write(value);
     auto s      = to!(string)(result);
     writeln(s);
}

Also all string literals in D are zero terminated so you could 
write the call like this:

auto result = write("Hello World");
Jan 03 2016
parent reply TheDGuy <loder.feuer googlemail.com> writes:
 Use an import.

 import std.string;
 import std.conv;

 void main(string[] args) {
     auto value  = toStringz("Hello World");
     auto result = write(value);
     auto s      = to!(string)(result);
     writeln(s);
 }

 Also all string literals in D are zero terminated so you could 
 write the call like this:

 auto result = write("Hello World");


Okay, thank you very much!

But now i get the access violation error again.

If i type:
gcc -c -otest.c.o

the 'test.c.o' file is generated but if i type:

dmd main.d test.c.o i get: 'Error: unrecognized file extension o'?
Jan 03 2016
parent reply anonymous <anonymous example.com> writes:
On 03.01.2016 21:32, TheDGuy wrote:

 If i type:
 gcc -c -otest.c.o

 the 'test.c.o' file is generated but if i type:

 dmd main.d test.c.o i get: 'Error: unrecognized file extension o'?


You're probably on Windows then? dmd doesn't recognize the .o extension 
on Windows, use .obj instead.
Jan 03 2016
parent reply TheDGuy <loder.feuer googlemail.com> writes:
On Sunday, 3 January 2016 at 21:20:35 UTC, anonymous wrote:

 On 03.01.2016 21:32, TheDGuy wrote:

 If i type:
 gcc -c -otest.c.o

 the 'test.c.o' file is generated but if i type:

 dmd main.d test.c.o i get: 'Error: unrecognized file extension 
 o'?


 You're probably on Windows then? dmd doesn't recognize the .o 
 extension on Windows, use .obj instead.


If i rename "test.o" to "test.obj" i get:

'Error: Module or Dictionary corrupt'
Jan 03 2016
parent anonymous <anonymous example.com> writes:
On 03.01.2016 22:37, TheDGuy wrote:

 If i rename "test.o" to "test.obj" i get:

 'Error: Module or Dictionary corrupt'


My guess is, that means that dmd can't handle the object file format 
that gcc produces.

Disclaimer: I don't know much about object formats, gcc, and Windows. I 
may be mistaken in what I write here.

I think there are three object file formats that are relevant for us:

* OMF: the format dmd uses by default on 32 bit Windows,
* COFF: the format dmd uses on 64 bit Windows and in 32 bit mode with 
the -m32mscoff switch,
* ELF: the format gcc uses by default (on Linux at least, maybe on 
Windows too).

You should probably try to find out what gcc produces (by reading gcc 
documentation, I supppose).

Then you need to get your D compiler and your C compiler to produce the 
same format.

As mentioned, dmd has some switches that affect the used/expected object 
format: -m32 (OMF) -m64 (COFF) -m32mscoff (COFF).

You can also try to get gcc to produce another format. I don't know how 
to do that.

Or you can use another C compiler that produces a different format than gcc.

I'd expect Digital Mars's (i.e. Walter's) dmc to be compatible with dmd 
and produce OMF files. Looks like there's a free download:
http://www.digitalmars.com/download/freecompiler.html

dmd's COFF mode is supposed to be compatible with Microsoft's Visual 
Studio. So that would be another option.
Jan 03 2016