www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - fread return value?

reply Andy Valencia <dont spam.me> writes:
Scratching my head here.  fread() really appears to be the 
standard C idea of an fread(), so it returns a count.  Instead, 
it returns 0.  The buf[] does indeed have the expected content, 
but I'd like to understand what's up before just changing my code 
to assume 0 is a success value?

(Linux, ldc2 seen on versions 1.40.0 and 1.41.0)

Thanks,
Andy

```d
import core.stdc.stdio : fopen, fread, fclose;
import std.string : toStringz;

void main(string[] argv) {
     import std.stdio : writeln;

     foreach(a; argv[1 .. $]) {
         auto f = fopen(a.toStringz(), "r".ptr);
         ubyte[1024] buf;
         auto res = fread(&buf[0], buf.length, 1, f);
         writeln(res, ": ", buf);
         f.fclose();
     }
}
```
Jul 30
next sibling parent monkyyy <crazymonkyyy gmail.com> writes:
On Wednesday, 30 July 2025 at 19:45:20 UTC, Andy Valencia wrote:

 Scratching my head here.  fread() really appears to be the 
 standard C idea of an fread(), so it returns a count.  Instead, 
 it returns 0.  The buf[] does indeed have the expected content, 
 but I'd like to understand what's up before just changing my 
 code to assume 0 is a success value?

 (Linux, ldc2 seen on versions 1.40.0 and 1.41.0)

 Thanks,
 Andy

 ```d
 import core.stdc.stdio : fopen, fread, fclose;
 import std.string : toStringz;

 void main(string[] argv) {
     import std.stdio : writeln;

     foreach(a; argv[1 .. $]) {
         auto f = fopen(a.toStringz(), "r".ptr);
         ubyte[1024] buf;
         auto res = fread(&buf[0], buf.length, 1, f);
         writeln(res, ": ", buf);
         f.fclose();
     }
 }
 ```


in code I have running its used as a count and has to be greater 
then 0


 fread(&buf[0], buf.length, 1, f);


from mine: `auto 
ret=fread(&data[length],1,data.length-length,ptr);`
youve swizzled the arguments
Jul 30
prev sibling next sibling parent reply 0xEAB <desisma heidel.beer> writes:
On Wednesday, 30 July 2025 at 19:45:20 UTC, Andy Valencia wrote:

 but I'd like to understand what's up before just changing my 
 code to assume 0 is a success value?


AFAICT your code is wrong.
The parameters of `fread()` are `ptr`, `size`, `n`, `stream`.
Your code is attempting to read `1` unit with a size of 
`buf.length`. If the stream has less then a single unit of size 
`buf.length` left, it has obviously read 0 units. Which it 
reports.
Jul 30
parent Andy Valencia <dont spam.me> writes:
On Wednesday, 30 July 2025 at 20:03:37 UTC, 0xEAB wrote:

 On Wednesday, 30 July 2025 at 19:45:20 UTC, Andy Valencia wrote:

 but I'd like to understand what's up before just changing my 
 code to assume 0 is a success value?


 AFAICT your code is wrong.
 The parameters of `fread()` are `ptr`, `size`, `n`, `stream`.
 Your code is attempting to read `1` unit with a size of 
 `buf.length`. If the stream has less then a single unit of size 
 `buf.length` left, it has obviously read 0 units. Which it 
 reports.


(Looks at manual.  Looks at code.  SMH.)

Thanks.  I've been writing C code since K&R, but I guess it _has_ 
been a while.  Sorry to bug you for such a dumb one!

Andy
Jul 30
prev sibling parent =?UTF-8?Q?Ali_=C3=87ehreli?= <acehreli yahoo.com> writes:
On 7/30/25 12:45 PM, Andy Valencia wrote:

 Scratching my head here.  fread()


I haven't used anything other than rawRead() for that purpose. rawRead() 
returns a slice of what it's just read:

import std.algorithm;
import std.exception;
import std.file;
import std.format;
import std.stdio;

void main(string[] args) {
     enforce(args.length == 2, "Please provide the name of the file to 
read.");

     auto file = File(args[1], "r");
     auto size = getSize(file.name);

     auto buffer = new ubyte[1024 * 1024];
     while (size) {
         const readSize = min(size, buffer.length);
         auto read = file.rawRead(buffer[0..readSize]);
         enforce(read.length == readSize, format!"Read %,s bytes instead 
of %,s."(read.length, readSize));

         writefln!"Read %,s bytes"(read.length);

         size -= read.length;
     }
}

Ali
Jul 30