How To Read an C Void pointer in Ada?

Question

There is Following C-Function with void pointer as Argument:

int read(void *buf, size_t num);

int => returns if the read operation was successfully or not.
void *buf => c void unsigned byte buffer pointer as function argument.
size_t num => size of the byte buffer which should be filled up.

Currently I have the following Ada Implementation:

with Ada.Text_IO; with System; with Interfaces.C; use Ada.Text_IO; use Interfaces.C; procedure Main is -- Imported C function function Read(Buf:System.Address; Num:int) return int; pragma Import(C, Read, "read"); -- Byte Array Type type Byte_Type is mod 2**8; for Byte_Type'Size use 8; type Byte_Array_Type is array(Positive range <>) of Byte_Type; -- Initialize Buffer_Check:int; Buffer_Size:Positive:=10; Buffer_Array:Byte_Array_Type(1 .. Buffer_Size):=(others => 0); --initialise array with zero begin Buffer_Check:=Read(Buffer_Array'Address, int(Buffer_Size)); if Buffer_Check /= 0 then Put_Line("Read success"); for K in Buffer_Array'First .. Buffer_Array'Last loop Put_Line(Integer'Image(Integer(Byte_Type(Buffer_Array(K))))); end loop; else Put_Line("Read Failed"); end if; end Main;

The Buffer_Array doesn't get filled up as expected. It would be Great if some Ada Enthusiast have some hint or Idea.

"doesn’t get filled up as expected" - what actually happens? What OS are you using? Which library is read() imported from? — Simon Wright
– Simon Wright, Commented Jan 1, 2021 at 17:49
Output : Read success 0 0 0 0 0 0 0 0 0 0 OS:Slackware14.04 64 bit, read is an example, but i want to import SSL_read from openssl.org/docs/man1.1.1/man3/SSL_read.html — mcxbain
– mcxbain, Commented Jan 1, 2021 at 18:54
SSL_read Works now. Nothing was wrong with the Ada Code. It was an false c implementation of my own c example function. Thank you Simon to have a look at the code. — mcxbain
– mcxbain, Commented Jan 1, 2021 at 19:50

Jeffrey R. Carter · Accepted Answer · 2021-01-03 10:11:42Z

Assuming that an Ada private type System.Address and a C pointer are compatible is extremely non-portable and unnecessary. Declare your types with convention C and your out parameter as an out parameter:

pragma Convention (C, Byte_Type); pragma Convention (C, Byte_Array_Type); function Read (Buf : out Byte_Array_Type; Num : in int) return int; pragma Import (C, Read, "read");

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