Thread: Pthreads

Hello to the community!
I have a problem about parallel C, I want to read a tga colored image, and convert it to grayscale, and subsequently, I want to convert the grayscale to a black&white.
I have done it serially, and now I want to do this in a parallel way -with pthreads.
I am working in a 4core laptop, I use ubuntu (through VMware) and gcc compiler.
My code is the above:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <pthread.h>


#define NUM_THREADS 4


typedef struct
{
    char  idlength;
    char  colourmaptype;
    char  datatypecode;
    short int colourmaporigin;
    short int colourmaplength;
    char  colourmapdepth;
    short int x_origin;
    short int y_origin;
    short width;
    short height;
    char  bitsperpixel;
    char  imagedescriptor;
} HEADER;


typedef struct
{
    unsigned char r;
    unsigned char g;
    unsigned char b;
} PIXEL;


void CreateHeader(HEADER, FILE *);


HEADER header;
FILE *fp, *fp2;
clock_t cl_diff[NUM_THREADS] = {0};


void *Gray(void *id)
{
    clock_t cl_mid1, cl_mid2;
    PIXEL pixel;
    float gray;
    int thread_id = *((int*)id);
    int i;
printf("\n--START: THREAD %d--\n", thread_id);


    int apo = thread_id * header.width * header.height / NUM_THREADS;
    int ews = apo + header.width * header.height / NUM_THREADS;


    for (i = apo; i < ews; i++)
    {
        pixel.b = fgetc(fp);
        pixel.g = fgetc(fp);
        pixel.r = fgetc(fp);


        cl_mid1 = clock();


        gray = 0.114 * pixel.b + 0.587 * pixel.g + 0.299 * pixel.r;


        cl_mid2 = clock();
        cl_diff[thread_id] += cl_mid2 - cl_mid1;


        putc(gray,fp2);
        putc(gray,fp2);
        putc(gray,fp2);


    }
printf("\n--END: THREAD %d--\n", thread_id);
}


main(int argc, char *argv[])
{
    clock_t start = clock(), cl_totdiff;
    int i, j;
    char filename[25];
    char graystr[] = "_gray.tga";
    pthread_t thread[NUM_THREADS];


    if (argc != 2)
    {
        printf("\nGive tga image - bye--\n");
        exit(-1);
    }


    fp = fopen(argv[1],"r");


    // Header filling
    header.idlength = getc(fp);
    header.colourmaptype = getc(fp);
    header.datatypecode = getc(fp);
    fread(&header.colourmaporigin,2,1,fp);
    fread(&header.colourmaplength,2,1,fp);
    header.colourmapdepth = fgetc(fp);
    fread(&header.x_origin,2,1,fp);
    fread(&header.y_origin,2,1,fp);
    fread(&header.width,2,1,fp);
    fread(&header.height,2,1,fp);
    header.bitsperpixel = fgetc(fp);
    header.imagedescriptor = fgetc(fp);


    for (i = 0; argv[1][i] != '\0'; i++)
        filename[i] = argv[1][i];
    filename[i] = '\0';


    for (j = 0; j < 10; j++)
        filename[i-4+j] = graystr[j];
    
    fp2 = fopen(filename,"w");


    CreateHeader(header, fp2);


    for (i = 0; i < NUM_THREADS; i++)
    {
        pthread_create(&thread[i], NULL, Gray, &i);
printf("\nthread:%d\n",i);
        //pthread_join(thread[i], NULL);
    }


    for (i = 0; i < NUM_THREADS; i++)
        pthread_join(thread[i], NULL);


    printf("\n---GRAYSCALE PIC: SUCCESS---\n");


    for (i = 0; i < NUM_THREADS; i++)
        cl_totdiff += cl_diff[i];


    printf("Time elapsed: %f sec\n", ((double)clock() - start) / CLOCKS_PER_SEC);


    fclose(fp);
    fclose(fp2);
}


void CreateHeader(HEADER header, FILE *ptr)
{
    putc(0,ptr);
    putc(0,ptr);
    putc(2,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc(0,ptr);
    putc((header.width & 0x00FF),ptr);
    putc((header.width & 0xFF00) / 256,ptr);
    putc((header.height & 0x00FF),ptr);
    putc((header.height & 0xFF00) / 256,ptr);
    putc(24,ptr);
    putc(0,ptr);
}

I know that my problem is around pthread_join(), but I can't solve it. If I use pthread_join() immediately after pthread_create() (not a parallel way of course, but to check), I can see the threads (thread numbers: 0, 1, 2, 3) create and end one after another normally, until the picture is completed.
But when I use the above way, with the many pthread_join() after the pthread_create(), I have problems, like thread numbers 0, 0, 1, 3, etc..

I will deeply appreciate your help, as I have stuck in this many hours..

-I have left the calculation of the procedure time for the time being, as I try to solve this..

The problem is that you pass the same "id" pointer to every thread (&i), and then modify the variable i while they are still running. You need to pass a different pointer to every thread, and you should not modify the pointed data until the threads have finished.