[Solved] C popen command parsing from user

Dummas · 2011-01-20 11:15:35

Hello devs,

I've lack of programing experience with popen(); so I want to ask a question.

I'm trying to build up a simple server with C, to have a simple "remote shell". A C program listens to a connection ( e.g. "telnet 225.225.225.221 6000" ) and then it arrives, gives a user a possibility to enter a command ( e.g. "ls -al" ) and then server gives the output of this command to the user.

Now I have problem with popen. The construction of this function is like this:

FILE *popen(const char *command, const char *type);

Then I'm reading the user input, I store everything in a char buffer, but popen is accepting only const char pointers, so I do

command = ( const char * ) buffer;

Here is a sample code:

....
char buffer[255];
const char *command;
...

n = write ( client_socket, "> ", 2 );
n = read ( client_socket, buffer, 256 );

command = ( const char * ) buffer;

if ( ! ( fpipe = ( FILE *) popen ( command, "r" ) ) )
  {
    perror ( "Piping error" );
    error(1);
  }
...

But then I fire-up server and give a command to a server ( e.g. "ls" ) it returns

: command not found

So, I suppose this means, that popen(); sends nothing to a shell, although, then I do:

printf("%s\n", command);

I clearly see the sending command.

The

...
if ( ! ( fpipe = ( FILE *) popen ( "ls -al", "r" ) ) )
  {
    perror ( "Piping error" );
    error(1);
  }
...

Works just fine

How to correctly convert char buffer to a const char pointer, what popen will understand my sending command ?

Thank You

Last edited by Dummas (2011-01-21 07:10:06)

Dummas · 2011-01-20 16:07:54

Ok, solution found. Did not expect this kind of char transfer from TCP.

Solution was simply to add:

buffer[n-2] = '\0';

Close/Delete this topic please.

Inxsible · 2011-01-20 16:18:04

Just mark it [SOLVED] by editing your first post and prepending it to the subject header.

stqn · 2011-01-20 16:22:58

Do you really need to use the "command" variable? Just because popen()'s prototype says "const char *" doesn't mean it won't accept a "char *", just that it won't modify the data being pointed to.

I have no experience with popen(), but I get the impression that the contents of buffer[] must be modified somewhere... Does it really print the correct command when you insert the printf() just before the popen() call?

Or maybe it's a codepage problem?

Also you've got a 255 bytes buffer but you tell read() to read up to 256 bytes...

Dummas · 2011-01-21 07:20:58

@stqn, firslty, I was thinking that this is casting problem, that's why I was using different variable ( e.g. command ); In my final version, there is no command variable.

Yes, then I pass modified buffer ( see the solution ) to a printf(); and popen(); everything works fine.

This is raw code: https://gist.github.com/789362

( give_command is required by my task, because user passes everything with white spaces, e.g. cat / etc / rc.conf and server must make it like cat /etc/rc.conf )

Last edited by Dummas (2011-01-21 07:24:57)

stqn · 2011-01-21 08:30:45

Be careful with the return value of give_command(). This function returns a pointer to a local variable. This variable could be overwritten by any function call as soon as you're no longer inside give_command().

I'm almost sure that if you move "char command[255];" to the very beginning of give_command() then it will be overwritten. As is, it is "protected" by part[60] which will be overwritten first.

A solution would be to copy the contents of command[] into buffer inside give_command(), instead of returning a pointer to command.

I'd also check the return value of read() (you're in trouble if it returns 0 .)

Arch Linux

#1 2011-01-20 11:15:35

[Solved] C popen command parsing from user

#2 2011-01-20 16:07:54

Re: [Solved] C popen command parsing from user

#3 2011-01-20 16:18:04

Re: [Solved] C popen command parsing from user

#4 2011-01-20 16:22:58

Re: [Solved] C popen command parsing from user

#5 2011-01-21 07:20:58

Re: [Solved] C popen command parsing from user

#6 2011-01-21 08:30:45

Re: [Solved] C popen command parsing from user

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