C programming - pthread_create() structure as parameter - c

Question

I have been working out an example to demonstrate multithreading using POSIX on processing operation of an image. The encoding was done with lodepng library. I wanted the code to break a color inverse process of any loaded image into 4 equal threads. Although I have checked repeatedly, I still cant find out why the 4th thread starting and ending values are getting modified inside the threadFunc2() function. Its the same code used by other 3 threads getting correct start and end values. I have used a structure to pass start, end and thread number data into function. For example, if I load a 10x10 image which results in 100 pixels (each with 4 values for R,G, B and Transparency),it requires a 400 element array to store each color value. The 4th thread should start with 300 and 399. Its correctly calculated in the main() function before passing to function, but ends up as being 5 and 6 within the threadFunc2() at start.
#include <pthread.h>
#include "lodepng.h"
#include <stdio.h>
#include <stdlib.h>
#include "lodepng.c"
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
unsigned int error;
unsigned int encError;
unsigned char* image;
unsigned int width;
unsigned int height;
int Arraysize;
const char* filename = "10x10Circle.png";
const char* newFileName = "10x10Result.png";
struct Markers{int start;int end;int no};
void *threadFunc(void *arg){ // function to load the image into an array
pthread_mutex_lock(&mutex);
error = lodepng_decode32_file(&image, &width, &height, filename);
if(error){
printf("error %u: %s\n", error, lodepng_error_text(error));
}
Arraysize = 4*height*width;
printf("arraysize:%d, %d \n",sizeof(image)/sizeof(image[0]),Arraysize);
pthread_mutex_unlock(&mutex);
return NULL; }
void *threadFunc2(void *arg){ //function to apply inverse process on loaded data
struct Markers*vars= (struct Markers*) arg;
printf("Thread %d start|start-%d,end-%d\n",vars->no,vars->start,vars->end);
for( int i = vars->start; i<(vars->end); i=i+4){
pthread_mutex_lock(&mutex);
image[0+i]= 255-image[0+i];
image[1+i]= 255-image[1+i];
image[2+i]= 255-image[2+i];
image[3+i]= 255-image[3+i];// can be ignored
printf("Thread: %d,round:%d\n",vars->no,i);// debug line
pthread_mutex_unlock(&mutex);
}
// printf("Thread: %d,after end:%d\n",vars->no,i);
printf("**************************\n");
printf("Thread end\n");
return NULL;
}
void *encodeprocessed(){
encError = lodepng_encode32_file(newFileName, image, width, height);
if(encError){
printf("error %u: %s\n", error, lodepng_error_text(encError)); }
free(image); }
Following is the main function
int main(void){
pthread_t pth,pth0, pth1, pth2, pth3;
pthread_create(&pth,NULL,threadFunc,NULL);
pthread_join(pth, NULL );
struct Markers Positions[3];
Positions[0].start = 0;
Positions[0].end = Arraysize/4 -1;
Positions[0].no = 1;
Positions[1].start =Arraysize/4;
Positions[1].end = Arraysize/2 -1;
Positions[1].no = 2;
Positions[2].start =Arraysize/2;
Positions[2].end = Arraysize*3/4 -1;
Positions[2].no = 3;
Positions[3].start =(Arraysize*3)/4;
Positions[3].end = Arraysize -1;
Positions[3].no = 4;
//debug line
printf("%d,%d,%d,%d,%d,%d,%d,%d,%d\n",Arraysize,Positions[0].start,Positions[0].end,
Positions[1].start,Positions[1].end,Positions[2].start,Positions[2].end,
Positions[3].start,Positions[3].end);
pthread_create(&pth0,NULL,threadFunc2,&Positions[0]);
pthread_create(&pth1,NULL,threadFunc2,&Positions[1]);
pthread_create(&pth2,NULL,threadFunc2,&Positions[2]);
pthread_create(&pth3,NULL,threadFunc2,&Positions[3]);
pthread_join(pth0, NULL );
pthread_join(pth1, NULL );
pthread_join(pth2, NULL );
pthread_join(pth3, NULL );
encodeprocessed();
return 0;}
The code is running without any errors. Image gets inverted only 75% and gets saved. Anyone who can give me a clue is much appreciated.