Given the following:

=====a======
void main(string[] args)
{
    FILE* fp = fopen(args[1].ptr, "r");
    if (!fp) throw new Exception("fopen");
}

=====b======
void main(string[] args)
{
    FILE* fp = fopen(args[1].dup.ptr, "r");
    if (!fp) throw new Exception("fopen");
}

Why does a fail but b succeed?

On 9/3/20 1:47 PM, Curious wrote:
> Given the following:
> 
> =====a======
> void main(string[] args)
> {
>      FILE* fp = fopen(args[1].ptr, "r");
>      if (!fp) throw new Exception("fopen");
> }
> 
> =====b======
> void main(string[] args)
> {
>      FILE* fp = fopen(args[1].dup.ptr, "r");
>      if (!fp) throw new Exception("fopen");
> }
> 
> Why does a fail but b succeed?

try `toStringz`:
```D
import std.string : toStringz;

void main(string[] args)
{
     FILE* fp = fopen(args[1].toStringz, "r");
     if (!fp) throw new Exception("fopen");
}
```
The reason is that args are D strings (that contains no terminating 0) but `fopen` gets C string (null terminated) so your `a` variant fails because the filename becomes wrong as there is no terminating 0. Your `b` variant works in fact accidentally because after duplication in new memory after filename 0 appears due to random reason (for example all that memory area zeroed by allocator).

On Thursday, 3 September 2020 at 10:47:04 UTC, Curious wrote:
> Given the following:
>
> =====a======
> void main(string[] args)
> {
>     FILE* fp = fopen(args[1].ptr, "r");
>     if (!fp) throw new Exception("fopen");
> }
>
> =====b======
> void main(string[] args)
> {
>     FILE* fp = fopen(args[1].dup.ptr, "r");
>     if (!fp) throw new Exception("fopen");
> }
>
> Why does a fail but b succeed?

version b works by accident/UB. You need to null terminate your filename if you use the C library functions:

---
void main(string[] args)
{
    FILE* fp = fopen((args[1] ~ '\0').ptr, "r");
    if (!fp) throw new Exception("fopen");
}
---

otherwise what you get as args are D dynamic arrays (a payload made of .ptr and .length) so you can use std.file or std.stdio to open a file using the "D main" arguments (it's not the like "C main").

---
void main(string[] args)
{
    import std.stdio;
    File f = File(args[1], "r");
}
---

On Thursday, 3 September 2020 at 11:12:49 UTC, user1234 wrote:
> On Thursday, 3 September 2020 at 10:47:04 UTC, Curious wrote:
>
> otherwise what you get as args are D dynamic arrays (a payload made of .ptr and .length) so you can use std.file or std.stdio to open a file using the "D main" arguments (it's not the like "C main").
>
> ---
> void main(string[] args)
> {
>     import std.stdio;
>     File f = File(args[1], "r");
> }
> ---

Okay. Got it!

Thanks