CODEWITHSUNDEEP
linuxmultithreadingterminal
Kh40tiK
Kh40tiK

Reputation: 2336

linux: Multithreaded, block terminal for one thread

I want to write a program running 2 threads. While main thread busy doing its job, the other thread act as an interactive cmdline, reads user input, then print something to terminal.

My code looks like this now:

#include <pthread.h>

//Needed for pthread
#ifndef _REENTRANT
#define _REENTRANT
#endif

#include "whatever_u_need.h"
bool g_isDone = false;

void* cmdMain( void* ) {
    static char* buf;
    buf = (char*)malloc( 257 );
    buf[256]=0;
    size_t size = 256;

    while(!g_isDone) {
        printf( "> " );
        getline( &buf, &size, stdin );
        if( buf[0] == 'q' ) { 
            g_isDone  =true;
            break;
        }
        //echo
        puts(buf);
    }
    free( buf );
    pthread_exit(NULL);
}

pthread_t g_cmd_thread;

int main() {
    pthread_create( &g_cmd_thread, NULL, cmdMain, NULL );
    while(1) {
        //non-interactive jobs
    }
    pthread_cancel( g_cmd_thread );

    return 0;
}

The problem is, when executing getline(), I hit ENTER, then the terminal moves 2 lines down. Definitely both threads have recieved the "ENTER message". How can I turn off the terminal I/O for the main thread but keeping the other thread's command-line functionality?

I'm using Ubuntu with bash shell.

Upvotes: 0

Views: 400

Answers (2)

Duck
Duck

Reputation: 27552

getline retains the newline from when you hit enter. You then puts that buffer and puts adds a newline. Thus the terminal moves down two lines.

from man (3) getline:

getline() reads an entire line from stream, storing the address of the buffer containing the text into *lineptr. The buffer is null-terminated and includes the newline character, if one was found.

from man (3) puts:

puts() writes the string s and a trailing newline to stdout.

Upvotes: 2

Kh40tiK
Kh40tiK

Reputation: 2336

I think I've got the bug. By default, puts() adds a trailing trailing newline to the stdout, which produces "double newline" effect in the above code. The bug has nothing to do with multithreading at all.

Guess I'll double-check the man page next time.

Upvotes: 0

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