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digitalmars.D.learn - Why doesn't this piece of code work?

reply SvGaming <sven.kovac1 outlook.com> writes:
I want to ask the user to select their USB drive by typing the 
mount location. For some reason that does not work and just skips 
over the user input part.
```d
void writeusb() {
	writeln("Here is a list of your mounted drives: ");
	auto mounts = executeShell("cat /proc/mounts | grep media");
	writeln(mounts.output);
	writef("Type the path to your USB drive: ");
	string cont = readln;
	writeln(a);
	writeln("A file selection menu will now appear so you can select 
an ISO to write.");
	auto seliso = executeShell("zenity --file-selection");
	writeln(seliso.output);
}
```
I am clueless as to what I need to do here? Can anyone help?
May 15 2022
next sibling parent reply kdevel <kdevel vogtner.de> writes:
On Sunday, 15 May 2022 at 11:10:41 UTC, SvGaming wrote:

 I want to ask the user to select their USB drive by typing the 
 mount location. For some reason that does not work and just 
 skips over the user input part.
 ```d
 void writeusb() {
 	writeln("Here is a list of your mounted drives: ");
 	auto mounts = executeShell("cat /proc/mounts | grep media");
 	writeln(mounts.output);
 	writef("Type the path to your USB drive: ");
 	string cont = readln;
 	writeln(a);
 	writeln("A file selection menu will now appear so you can 
 select an ISO to write.");
 	auto seliso = executeShell("zenity --file-selection");
 	writeln(seliso.output);
 }
 ```
 I am clueless as to what I need to do here? Can anyone help?


```
import std.stdio;
import std.process;

int main ()
{
    writeln("Here is a list of your mounted drives: ");
    auto mounts = executeShell("cat /proc/mounts | grep media");
    writeln(mounts.output);
    writef("Type the path to your USB drive: ");
    string cont = readln;
    writeln(cont); // there was no variable named "a"
    writeln("A file selection menu will now appear so you can 
select an ISO to write.");
//   auto seliso = executeShell("zenity --file-selection");
//   writeln(seliso.output);
    return 0;
}
```

This code runs as expected.
May 15 2022
parent reply SvGaming <sven.kovac1 outlook.com> writes:
On Sunday, 15 May 2022 at 12:13:14 UTC, kdevel wrote:

 On Sunday, 15 May 2022 at 11:10:41 UTC, SvGaming wrote:

 [...]


 ```
 import std.stdio;
 import std.process;

 int main ()
 {
    writeln("Here is a list of your mounted drives: ");
    auto mounts = executeShell("cat /proc/mounts | grep media");
    writeln(mounts.output);
    writef("Type the path to your USB drive: ");
    string cont = readln;
    writeln(cont); // there was no variable named "a"
    writeln("A file selection menu will now appear so you can 
 select an ISO to write.");
 //   auto seliso = executeShell("zenity --file-selection");
 //   writeln(seliso.output);
    return 0;
 }
 ```

 This code runs as expected.


Strange. It does not for me. I even tried different compilers.
It simply does not ask for user input here where it is supposed 
to:
```d
writef("Type the path to your USB drive: ");
string cont = readln;
writeln(cont); // there was no variable named "a"
```
May 15 2022
parent reply kdevel <kdevel vogtner.de> writes:
On Sunday, 15 May 2022 at 12:19:22 UTC, SvGaming wrote:
[...]

 This code runs as expected.


 Strange. It does not for me. I even tried different compilers.
 It simply does not ask for user input here where it is supposed 
 to:
 ```d
 writef("Type the path to your USB drive: ");
 string cont = readln;
 writeln(cont); // there was no variable named "a"
 ```


Install the `strace` program (I assume you are running Linux) and 
start your program under strace:

```
    $ strace -f <name of the executable>
```

Then examine the system calls. On my machine I get this:

```
[...]
mmap(NULL, 4194304, PROT_READ|PROT_WRITE, 
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff6beb000
read(4, "", 4096)                       = 0
--- SIGCHLD (Child exited)   0 (0) ---
read(4, "", 4096)                       = 0
wait4(26713, [{WIFEXITED(s) && WEXITSTATUS(s) == 1}], 0, NULL) = 
26713
close(4)                                = 0
munmap(0x7ffff7ff5000, 4096)            = 0
write(1, "\n", 1
)                       = 1
fstat(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 10), ...}) = 
0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 
-1, 0) = 0x7ffff7ff5000
write(1, "Type the path to your USB drive:"..., 33Type the path 
to your USB drive: ) = 33
read(0,
```

Here the program waits for input.
May 15 2022
parent reply SvGaming <sven.kovac1 outlook.com> writes:
On Sunday, 15 May 2022 at 12:27:45 UTC, kdevel wrote:

 On Sunday, 15 May 2022 at 12:19:22 UTC, SvGaming wrote:
 [...]

 [...]


 Install the `strace` program (I assume you are running Linux) 
 and start your program under strace:

 [...]


I am so confused right now. It works if that code is the main 
function.
But in the full program that is only one of the functions and is 
not the main function.
May 15 2022
next sibling parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 5/15/22 08:27, SvGaming wrote:


 I am so confused right now. It works if that code is the main function.
 But in the full program that is only one of the functions and is not the
 main function.


Could it be that the main() function exits before calling that other 
function?

You can sprinkle simple writeln calls to figure out exactly where it 
goes wrong:

void main() {
   writeln("entered main");
   writeln("before calling foo");
   foo();
   writeln("exiting main");
}

Ali
May 15 2022
prev sibling parent reply kdevel <kdevel vogtner.de> writes:
On Sunday, 15 May 2022 at 15:27:32 UTC, SvGaming wrote:
[...]

 It works if that code is the main function. But in the full 
 program that is only one of the functions and is not the main 
 function.


Until you post your full programm ideally in a reduced form [1] 
everybody who is willing to help must guess wildly what the 
unposted parts of your program does and how it may cause the read 
to be seemingly skipped.

```
import std.stdio;
import std.process;

void foo ()
{
    writeln("Here is a list of your mounted drives: ");
    auto mounts = executeShell("cat /proc/mounts | grep media");
    writeln(mounts.output);
    writef("Type the path to your USB drive: ");
    string cont = readln;
    writeln(cont); // there was no variable named "a"
    writeln("A file selection menu will now appear so you can 
select an ISO to write.");
//   auto seliso = executeShell("zenity --file-selection");
//   writeln(seliso.output);
}

int main ()
{
    foo;
    return 0;
}
```

Also works as expected.

What does `strace` report? And what compiler/version do you use?

[1] https://stackoverflow.com/help/minimal-reproducible-example
May 15 2022
parent reply SvGaming <sven.kovac1 outlook.com> writes:
On Sunday, 15 May 2022 at 20:06:20 UTC, kdevel wrote:

 On Sunday, 15 May 2022 at 15:27:32 UTC, SvGaming wrote:
 [...]

 [...]


 Until you post your full programm ideally in a reduced form [1] 
 everybody who is willing to help must guess wildly what the 
 unposted parts of your program does and how it may cause the 
 read to be seemingly skipped.

 [...]


I use dmd but I tried befor with another compiler and got the 
same result. Not sure if it was GDC or LDC.
May 16 2022
parent reply SvGaming <sven.kovac1 outlook.com> writes:
On Monday, 16 May 2022 at 16:40:59 UTC, SvGaming wrote:

 On Sunday, 15 May 2022 at 20:06:20 UTC, kdevel wrote:

 On Sunday, 15 May 2022 at 15:27:32 UTC, SvGaming wrote:
 [...]

 [...]


 Until you post your full programm ideally in a reduced form 
 [1] everybody who is willing to help must guess wildly what 
 the unposted parts of your program does and how it may cause 
 the read to be seemingly skipped.

 [...]


 I use dmd but I tried befor with another compiler and got the 
 same result. Not sure if it was GDC or LDC.


Here is the repository on GitHub i just made for the code(i know 
it is quite messy, I am a begginer in D):
May 16 2022
parent reply SvGaming <sven.kovac1 outlook.com> writes:
On Monday, 16 May 2022 at 16:44:13 UTC, SvGaming wrote:

 On Monday, 16 May 2022 at 16:40:59 UTC, SvGaming wrote:

 On Sunday, 15 May 2022 at 20:06:20 UTC, kdevel wrote:

 On Sunday, 15 May 2022 at 15:27:32 UTC, SvGaming wrote:
 [...]

 [...]


 Until you post your full programm ideally in a reduced form 
 [1] everybody who is willing to help must guess wildly what 
 the unposted parts of your program does and how it may cause 
 the read to be seemingly skipped.

 [...]


 I use dmd but I tried befor with another compiler and got the 
 same result. Not sure if it was GDC or LDC.


 Here is the repository on GitHub i just made for the code(i 
 know it is quite messy, I am a begginer in D):


https://github.com/svgaming234/ezusb
forgot to post it in the previous reply, I am kind of stupid
May 16 2022
parent reply kdevel <kdevel vogtner.de> writes:
On Monday, 16 May 2022 at 16:53:15 UTC, SvGaming wrote:
[...]

 https://github.com/svgaming234/ezusb
 forgot to post it in the previous reply, I am kind of stupid


In main your program reads an integer:

```
    int n;
    writef("Pick an option: ");
    readf(" %s", &n);
```

but your console/Terminal is line buffered, i.e. you type 1 plus 
a newline ("\n"). Only after the newline the OS transfers control 
back to your program. Unfortunately your program does not consume 
the newline, as ltrace reveals the newline is put back to the 
input buffer with ungetch. Therefore the next read command

```
    string a;
    a = readln();
    writeln(a);
```

consumes that newline from your first input resulting in variable 
a containing the empty string ending with a newline. Following 
change fixes the issue:

```
    int n;
    writef("Pick an option: ");
    readf(" %s\n", &n); // explicitly read the newline
```

I am not sure, why %s fits here. I would have expected a %d 
format for parsing the integer.
May 16 2022
next sibling parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 5/16/22 15:25, kdevel wrote:


     string a;
     a = readln();
     writeln(a);



 consumes that newline from your first input resulting in variable a
 containing the empty string ending with a newline.


Great catch! I should put this combination in my chapter.


 I am not sure, why %s fits here. I would have expected a %d format for
 parsing the integer.


I learned to read %s as "whatever the type of the argument is" (not 
"string").

Ali
May 16 2022
prev sibling next sibling parent SvGaming <sven.kovac1 outlook.com> writes:
On Monday, 16 May 2022 at 22:25:23 UTC, kdevel wrote:

 On Monday, 16 May 2022 at 16:53:15 UTC, SvGaming wrote:
 [...]

 [...]


 In main your program reads an integer:

 ```
    int n;
    writef("Pick an option: ");
    readf(" %s", &n);
 ```

 [...]


Thanks for the help!
May 16 2022
prev sibling parent SvGaming <sven.kovac1 outlook.com> writes:
On Monday, 16 May 2022 at 22:25:23 UTC, kdevel wrote:

 On Monday, 16 May 2022 at 16:53:15 UTC, SvGaming wrote:
 [...]

 [...]


 In main your program reads an integer:

 ```
    int n;
    writef("Pick an option: ");
    readf(" %s", &n);
 ```

 [...]


Just tried the solution and it works perfectly!
Thanks, Again!
May 17 2022
prev sibling parent SvGaming <sven.kovac1 outlook.com> writes:
On Sunday, 15 May 2022 at 11:10:41 UTC, SvGaming wrote:

 I want to ask the user to select their USB drive by typing the 
 mount location. For some reason that does not work and just 
 skips over the user input part.
 ```d
 void writeusb() {
 	writeln("Here is a list of your mounted drives: ");
 	auto mounts = executeShell("cat /proc/mounts | grep media");
 	writeln(mounts.output);
 	writef("Type the path to your USB drive: ");
 	string cont = readln;
 	writeln(a);
 	writeln("A file selection menu will now appear so you can 
 select an ISO to write.");
 	auto seliso = executeShell("zenity --file-selection");
 	writeln(seliso.output);
 }
 ```
 I am clueless as to what I need to do here? Can anyone help?


Just noticed i forgot to put the proper variable name to the 
input. Should not matter though. Also when I change it nothing 
changes.
May 15 2022