How to print using g++ with printf correctly?

Question

I am compiling this code with g++:

#include <pthread.h>
#include <iostream>
#include <cstdlib>
#include <string>
#include <stdio.h>

using namespace std;

#define num_threads 3
#define car_limit 4

pthread_mutex_t mutex; // mutex lock
pthread_t cid;         // thread id
pthread_attr_t attr;   // thread attrubutes

void *OneCar(void *dir);
void ArriveBridge(int *direction);
void CrossBridge();
void ExitBridge(int *direction);


int main()
{

    int dir[3] = {0,1,1};
    pthread_mutex_init(&mutex, NULL);
    pthread_attr_init(&attr);

    //cout<< "Pthread Create" << endl;
    printf("Pthread Create\n");

    for(int i = 0; i < num_threads; i++)
    {
        pthread_create(&cid, &attr, OneCar, (void *)&dir[i]);
    }


return 0;
}


void ArriveBridge(int *direction)
{
    //cout<<"Arrive"<<*direction << endl;
    int dr;

    if(*direction == 0)
        dr=0;
    else
        dr=1;

    printf("Arrive%d", dr);
}

void CrossBridge(int *dir)
{   
    char d;

    if(*dir == 0)
        d = 'N';
    else
        d = 'S';

    //cout<<"Crossing Bridge going:"<<d<<endl;
    printf("Crossing Bridge going %c", d);
}

void ExitBridge(int *direction)
{
    //cout<<"Exit" <<*direction<<endl;
    int dr;

    if(*direction == 0)
        dr=0;
    else
        dr=1;

    printf("Exit%d\n", dr);
}

void *OneCar(void *dir)
{
    int *cardir;
    cardir = (int *) dir;

    //cout<<*cardir;

    ArriveBridge(cardir);
    CrossBridge(cardir);
    ExitBridge(cardir);

    return 0;
}

and I am expecting this result printed to the screen:

> Pthread Create
> Arrive0Crossing Bridge going NExit0
> Arrive1Crossing Bridge going SExit1
> Arrive1Crossing Bridge going NExit1

But i get this instead:

Pthread Create
Arrive0Crossing Bridge going NExit0

Why doesnt it print the rest out?

score 3 · Answer 1 · answered Feb 25 '11 at 20:53

3

Have you tried joining your threads at the end of main? It could be that the program is terminating before the other threads are completely finished.

answered Feb 25 '11 at 20:53

Jeremiah Willcock

27,461
5
68
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score 3 · Accepted Answer · answered Feb 25 '11 at 21:15

You need to use "pthread_join" in main to wait for all threads to exit before your program terminates. You should also use an array to hold the id of each thread that you create:

pthread_t cid[num_threads];  // thread id`

You'll then want to call join on every thread you create:

for(int i = 0; i < num_threads; i++)
{
    pthread_create(&cid[i], &attr, OneCar, (void *)&dir[i]);
}


for(int i = 0; i < num_threads; ++i)
{
   pthread_join(cid[i], NULL);
};

Running the modified code now gives:

Pthread Create
Arrive0Crossing Bridge going NExit0
Arrive1Crossing Bridge going SExit1
Arrive1Crossing Bridge going SExit1

score 1 · Answer 3 · answered Feb 25 '11 at 20:55

1

You missed the newlines ("\n"):

printf("Arrive%d\n", dr);
printf("Crossing Bridge going %c\n", d);

Because of that, the streams are probably not flushed. Additionally, if you don't wait for your threads (pthread_join) your program will exit before the threads could do their work.

answered Feb 25 '11 at 20:55

DarkDust

85,893
19
180
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`\n` won't cause a stream to be flushed. – rubenvb Feb 25 '11 at 21:22
@rubenvb: Yes, it does. See for example [C primer plus](http://books.google.com/books?id=iRa-3L4jLhEC&pg=PA84&lpg=PA84&dq=printf+buffer+flush+new+line&source=web&ots=dXfybt2e9I&sig=2jZHctY3fcmVz1zI4y9JuE8NeWE&hl=en&sa=X&oi=book_result&resnum=6&ct=result#v=onepage&q&f=false). See also [this question](http://stackoverflow.com/questions/1716296/why-does-printf-not-flush-after-the-call-unless-a-newline-is-in-the-format-string) which cites the relevant C99 section in one of the answers. – DarkDust Feb 26 '11 at 00:48
@DarkDust: Will the same behavior be present in C++? I was confusing this with `std::cout`, which doesn't flush without a call to `flush()` (either through a direct call or `std::endl`. – rubenvb Feb 26 '11 at 13:09
@rubenvb: According to [this doc](http://www.cplusplus.com/reference/iostream/manipulators/endl/): yes, `std::endl` does also flush the buffered stream. Interestingly, in [this question](http://stackoverflow.com/questions/213907/c-stdendl-vs-n) they say that for `std::cout`, the `\n` does *not* flush the stream, so `endl` is like `\n` plus `flush`. – DarkDust Feb 26 '11 at 18:00
@DarkDust: that's what I'm saying. I wonder how this works for `printf` in c++? – rubenvb Feb 26 '11 at 22:51
@rubenvb: According to the C99 standard, the `\n` in a `printf` yields an implicit flush for *line buffered* streams (which `stdout` is by default). – DarkDust Feb 27 '11 at 13:03
@DarkDust: yes, but the question is explicitely about C++, not C99. There *is* a difference. – rubenvb Feb 27 '11 at 13:38
@rubenvb: But he's using `printf`, it doesn't matter whether he is calling it from C or C++. His output bug stems from that. He has some commented out lines which use `cout` and `endl`, but it's the same problem here: if he omits the `endls` and thus the implicit flush, he's getting the bug. – DarkDust Feb 27 '11 at 14:40

How to print using g++ with printf correctly?

3 Answers3