Hi, im just learning about multithreading for a program i am writing that ne
eds to read from multiple camreas attached to the computer at the same time.

I know how to create threads and pass around info, and i know a bit about mu
texes but this is not where the problem resides.
Every time the thread goes to open the device at /dev/video* the thread just
stops running as far as i can see.
i put printf's all over the place and it prints right before the open call o
n the camera, then the next thread kicks in and does the same thing.
I know the camreas work cuse when i go back to the non-multithreaded code, e
verything works just fine.
Also if i try the open call outside the thread, then pass the camrea into th
e thread, they open fine but when the read() call comes, then the thread die
s in the same way.
Does anyone have any ideas whats going on?
Here is the code for the program.
Thanks in advance

#include "ifqcam.h"
#include "error.h"
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

#include <sys/ioctl.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>
#include <linux/videodev.h>

#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xos.h>
#include <X11/Xatom.h>

#define SETWIDTH 320
#define SETHEIGHT 240
#define SETDEPTH 3
#define SETFOV 0.1
#define SETZDIST 50
#define WINDOWWIDTH 980
#define WINDOWHEIGHT 480
#define MAXVIEWS 15


// Globals that all files can see
int imgwidth = SETWIDTH;
int imgheight = SETHEIGHT;
int imgdepth = SETDEPTH;


extern Display *display;
extern Window win;

typedef unsigned char * charp;

pthread_mutex_t a_mutex = PTHREAD_MUTEX_INITIALIZER;


struct thread_data
{
  char deviceName[40];
  int device;
  char name[200];
  int maxFrames;
  int xOffset;
  int yOffset;
  char experiment;
  char nopng;
  int cam;
  char isbottom;
  pthread_mutex_t the_mutex;
};

void * cam_read( void * theInfo )
{
  int width, height, depth;
  int i, result;
  unsigned char * image;
  struct thread_data *data;
  data = (struct thread_data *)theInfo;

  char prefix[200];
  int rc;

  fflush(stdout);
  printf( "%s\n", data->deviceName );
/*   rc = pthread_mutex_lock( &a_mutex ); */
/*   if( !rc ) */
/*     { */
/*       printf( "Mutex locked in cam_open\n" ); */
/*     }   */
/*   else */
/*     { */
/*       printf( "Cant lock mutex in cam_open\n" ); */
/*     } */
  //data->device = open( data->deviceName, O_RDWR);
  data->device = open_camera( data->deviceName, a_mutex );  
/*   if( !rc ) */
/*     { */
/*       printf( "Mutex unlocked in cam_open\n" ); */
/*     } */
  fflush(stdout);
  printf( "%s %d\n", "hi", data->device );
  

  for( i = 0; i < data->maxFrames; i++ )
    {
      printf( "%s %i\n", "READ FRAME:", i );
      // read the cam image
      result = qcam_read( data->device, &width, &height, &depth,
			  &image, a_mutex );
      printf( "%d\n", result );
      if( !result )	
	{
	  printf( "%s %i\n", "BLEAH:", i );
	  get_actual_image( image, width, height, depth,
			    data->xOffset, data->yOffset, data->isbottom, 1 );
	}
      
      if( !data->nopng )
	{
	  if( !data->experiment )
	    {
	      sprintf( prefix, "../var/%s_%d/%s_%d %03d.png", data->name, data->cam, data->name, data->cam, i );
	    }
	  else if ( data->experiment )
	    {
	      sprintf( prefix, "../var/experiments/image_sequences/%s/%s_%d/%s_%d %03d.png", data->name, 
		       data->name, data->cam, data->name, 
		       data->cam, i );
	    }
	  // Ends up looknig like "dirname_cam/frame.png"
	  png_write_qcam( prefix, width, height, i, image, data->isbottom );
	  //fprintf( stdout, "%s\n", prefix );
	}
      free( image );
    }
  
  close_camera( data->device );
  pthread_exit(0);
}



int main (int argc, char ** argv) 
{
  charp image[MAXVIEWS];
  int width[MAXVIEWS], height[MAXVIEWS], depth[MAXVIEWS];
  int xOffSet[MAXVIEWS], yOffSet[MAXVIEWS];
  int error;
  int i, j;
  int gzpx[MAXVIEWS], gzpy[MAXVIEWS], gzpz[MAXVIEWS];
  double pitch[MAXVIEWS], roll[MAXVIEWS], yaw[MAXVIEWS];
  int isbottom[MAXVIEWS];
  int devices[MAXVIEWS]; // Store the camera device(s)
  FILE *stereo; // stereo.dat file pointer
  FILE *directory[MAXVIEWS]; // File pointer for the flatview info in each dir
  int numDirs = 0; // Holds the number of directorys we will be looking at, must be less than MAXVIEWS.
  int maxFrames, currentFrame;
  int viewNum; // Stores the current view being looked at.
  char name[125] = {};// Stores the name of the capture we are working on
  char filename[125];
  FILE *fp;
  char prefix[125];
  
  int nopng = 0; // if 1 no png will be writen.
  int up_down = 0;
  int ud_cam = 0;
  int go = 0;
  int experiment = 0; // if 1 this is capturing an experiment and will create the corect directories accordingly.
  
  pthread_t threads[MAXVIEWS] = {};
  struct thread_data *info[MAXVIEWS] = {};
  
  
  // Get the program flags
  for( i = 0; i < argc; i++ )
    {
      if( !strcmp( argv[i], "-npng" ))
	nopng = 1;
      if( !strcmp( argv[i], "-name" ))
	strcpy( name, argv[++i] );
      if( !strcmp( argv[i], "-exp" ))
	experiment = 1;
      if( !strcmp( argv[i], "-cams" ))
	numDirs = argv[++i];
      if( !strcmp( argv[i], "-u" ))
	up_down = 1;
      if( !strcmp( argv[i], "-go" ))
	go = 1;
    }
  
  if( !go )
    {
      // First get all the info from the user about what we will be caputring
      if( !strcmp( name, "" ) && !nopng)
	{
	  fprintf( stdout, "What will this test be called? " );
	  fscanf( stdin, "%s", name );
	}
      
      if( numDirs == 0 )
	{
	  fprintf( stdout, "How many cameras will we be capturing from? " );
	  fscanf( stdin, "%d", &numDirs );
	}
      
      if( up_down )
	{
	  fprintf( stdout, "What camera is upside down? " );
	  fscanf( stdin, "%d", &ud_cam );
	}
    }
  for( i = 0; i < numDirs; i++ )
    {
      if( !nopng && !go )
	{
	  // Get the x, y, z, yaw, pitch, roll for each camera
	  fprintf( stdout, "cam %d X pos: ", i+1 );
	  fscanf( stdin, "%d", &gzpx[i] );
	  fprintf( stdout, "cam %d Y pos: ", i+1 );
	  fscanf( stdin, "%d", &gzpy[i] );
	  fprintf( stdout, "cam %d Z pos: ", i+1 );
	  fscanf( stdin, "%d", &gzpz[i] );
	  
	  fprintf( stdout, "cam %d Yaw: ", i+1 );
	  fscanf( stdin, "%lf", &yaw[i] );
	  fprintf( stdout, "cam %d Pitch: ", i+1 );
	  fscanf( stdin, "%lf", &pitch[i] );
	  fprintf( stdout, "cam %d Roll: ", i+1 );
	  fscanf( stdin, "%lf", &roll[i] );
	  
	  fprintf( stdout, "If the camera is upside down enter 1, otherwise enter 0. " );
	  fscanf( stdin, "%d", &isbottom[i] ); 
	  
	  // Make the directories that the pics will be stored in
	  {
	    if( !experiment )
	      {
		sprintf( filename, "mkdir ../var/%s_%d", name, i ); 
		//printf( "%s\n", filename );
	      }
	    else if ( experiment )
	      {
		sprintf( filename, "mkdir ../var/experiments/image_sequences/%s/%s_%d", name, name, i );
	      }
	    system( filename );
	  }
	}
      else
	{
	  gzpx[i]      = 0;
	  gzpy[i]      = 0;
	  gzpz[i]      = 0;
	  yaw[i]       = 0;
	  pitch[i]     = 0;
	  roll[i]      = 0;
	  if( up_down == 1 && (ud_cam - 1) == i )
	    isbottom[i]  = 1;
	  else
	    isbottom[i] = 0;
	}
    }
  
  if( !nopng )
    {
      fprintf( stdout, "How many frames will we capture? " );
      fscanf( stdin, "%d", &maxFrames );
    }
  else
    {
      maxFrames = 5000;
    }
  
  
  // init the display
  init_display(argv[0],WINDOWWIDTH,WINDOWHEIGHT);
  
  
  // Setup the x and y offset for the images 
  for( i = 0; i < numDirs; i++ )
    {
      if( i < 2 )
	{
	  xOffSet[i] = ( i % 3 ) * 320;
	  yOffSet[i] = ( i / 2 ) * 240;
	}
      if( i >= 2 )
	{
	  xOffSet[i] = (( i + 1 ) % 3 ) *  320;
	  yOffSet[i] = (( i + 1 ) / 2 ) + 240;
	}
    }
  
  // Write the .dat file for each directory
  if( !nopng )
    {
      for( i = 0; i < numDirs; i++ )
	{
	  if( !experiment )
	    {
	      sprintf( filename, "../var/%s_%d.dat", name, i );
	      sprintf( prefix, "%s_%d/%s_%d", name, i, name, i );
	    }
	  else if ( experiment )
	    {
	      sprintf( filename, "../var/experiments/image_sequences/%s/%s_%d.dat", name, name, i );
	      sprintf( prefix, "experiments/image_sequences/%s/%s_%d/%s_%d", name, name, i, name, i );
	    }
	  fp = fopen( filename, "w" );
	  fprintf( fp, "%s\n%d\n%d\n%d\n%d %d %d\n%lf %lf %lf\n", prefix, 0,
		   currentFrame, 3, gzpx[i], gzpy[i], gzpz[i],
		   yaw[i], pitch[i], roll[i] );
	  fclose( fp );
	}
    }
   
  
  // Make all the threads
  for( i = 0; i < numDirs; i++ )
    {
      info[i] = ( struct thread_data* )malloc( sizeof( struct thread_data ));
      sprintf( info[i]->deviceName, "/dev/video%d", i );
      strcpy( info[i]->name, name );
      info[i]->maxFrames   = maxFrames;
      info[i]->xOffset     = xOffSet[i];
      info[i]->yOffset     = yOffSet[i];
      info[i]->nopng       = nopng;
      info[i]->experiment  = experiment;
      info[i]->cam         = i;
      info[i]->isbottom    = isbottom[i];
      //info[i]->the_mutex   = a_mutex;

      pthread_create( &threads[i], 0,
		      cam_read,
		      (void *)info[i]);
    }
    
/*   for( i = 0; i < numDirs; i++ ) */
/*     { */
/*       pthread_join(info[i], 0); */
/*     } */
  
  
}

Why does the camera open need the mutex? Maybe it doesn't release the mutex so the code never gets in a second time?

Maybe the mutex starts out busy so no one can get it?

try running two single threaded programs at the same time. see if two different cameras can be accessed at the same time.

Why does the camera open need the mutex? Maybe it doesn't release the mutex so the code never gets in a second time?

Maybe the mutex starts out busy so no one can get it?

i agree. maybe pthread_mutex_unlock function will help?

The open does not need a mutex, i just threw it in there to see if it made a difference.
Also, when the program is run with trying to only access one camera the problem still appears. And yes, you can run two programs that both access cams at the same time.

I don't see a call to pthread_mutex_init(). Did I just miss it? You need to call that on the mutex before you start, don't you?

What's in the "ifqcam.h" file? Did you write the qcam_ functions? If so, maybe the problem is in that code.

"In order to create a mutex, we first need to declare a variable of type pthread_mutex_t, and then initialize it. The simplest way it by assigning it the PTHREAD_MUTEX_INITIALIZER constant. So we'll use a code that looks something like this:

pthread_mutex_t a_mutex = PTHREAD_MUTEX_INITIALIZER;"

I have that at the top of my program. so its initialized and ready to go.

What's in the "ifqcam.h" file? Did you write the qcam_ functions? If so, maybe the problem is in that code.

the only code in that file im using is the open_camera() and the qcam_read() functions.

int open_camera(const char *devicename, pthread_mutex_t mutex)
{
  int rc;
  rc = pthread_mutex_lock( &a_mutex );
  if( !rc )
    {
      printf( "Mutex locked in cam_open\n" );
    }  
  else
    {
      printf( "Cant lock mutex in cam_open\n" );
    }
  int device = open(devicename, O_RDWR);
  pthread_mutex_unlock( &a_mutex );
  if( !rc )
    {
      printf( "Mutex unlocked in cam_open\n" );
    }  
  if(device <= 0)
    {
      printf("Device %s couldn't be opened\n", devicename);
      return 0;
    }
  //return 1;
  return device;
}

and the qcam_read function the thread never even makes it to at all.

the only code in that file im using is the open_camera() and the qcam_read() functions.

int open_camera(const char *devicename, pthread_mutex_t mutex)
{
int rc;
rc = pthread_mutex_lock( &a_mutex );
if( !rc )
{
printf( "Mutex locked in cam_open\n" );
} 
else
{
printf( "Cant lock mutex in cam_open\n" );
}
int device = open(devicename, O_RDWR);
pthread_mutex_unlock( &a_mutex );
if( !rc )
{
printf( "Mutex unlocked in cam_open\n" );
} 
if(device <= 0)
{
printf("Device %s couldn't be opened\n", devicename);
return 0;
}
//return 1;
return device;
}

and the qcam_read function the thread never even makes it to at all.

The problem could be that you release your mutex too soon. The program is calling pthread_mutex_unlock() right after open(). Try releasing the mutex after the close() function.

Can you show us your qcam_read() code?

The thing is it does not even get to the point where it unlocks the mutex. if i comment out the mutex stuff, and put a print before and after the open() call, it will print before, but not after. its just dying on the open().

Here is the qcam_read()

int qcam_read( int device, int *imgwidth, int * imgheight, int * imgdepth,	
       unsigned char ** image, pthread_mutex_t the_mutex) {

  int rc;
  ioctl(device, VIDIOCGCAP, &vidcap);
  ioctl(device, VIDIOCGWIN, &vidwin);
  ioctl(device, VIDIOCGPICT, &vidpic);
  *imgwidth= vidwin.width;
  *imgheight= vidwin.height;
  *imgdepth= vidpic.depth/8;
  *image = malloc(vidcap.maxwidth * vidcap.maxheight * 3);
  rc = pthread_mutex_lock(&a_mutex);
  printf( "device %d\twidth %d\theight %d\n", device, vidcap.maxwidth, vidcap.maxheight );
  printf( "%x\n", *image );
  if( !rc )
    {
      printf( "Mutex locked in qcam_read\n" );
    }
  else
    {
      printf( "Cant lock mutex in cam_read\n" );
    }  
  read(device, *image, (vidcap.maxwidth * vidcap.maxheight * 3));
  rc = pthread_mutex_unlock(&a_mutex);
  if( !rc )
    {
      printf( "Mutex unlocked in qcam_read\n" );
    }  

  printf( "Qcam_read %d\n",  device );
  return 1;
}

Thx for you help :)

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