sem_wait seems to block after random number of iterations - c

Question

I am trying to make my aperiodic task wait for a specific key press event.My
aperiodic function waits first time for the event to occur but post that it runs 3 iterations without waiting and then waits again. I am unable to discern mistake in the code. I have initialized the array of event semaphores with value 0 for each
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <linux/input.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>
sem_t mutex[10],event[10];
sem_t mtx,mtx1,mtx2,mtx3; //These 4 mutexes and the array mutex[10] are defined for periodic task
typedef struct task
{
int taskid;
int priority;
int iter;
}task;
void *wait_for_event()
{
int fd;
fd = open("/dev/input/by-path/platform-i8042-serio-0-event-kbd", O_RDONLY);
struct input_event ev;
while (1)
{
read(fd, &ev, sizeof(struct input_event));
if(ev.type == 1) /* signal any waiting thread for keypress event */
sem_post(&event[(ev.code -1)%10]);
}
}
void *aperiodic(void *t)
{
pthread_t tid;
sem_wait(&mtx);
while(1){
sem_wait(&mtx1);
int i,j;
struct task *temp=(struct task *)t;
printf(" The running task is : %d",temp->taskid);
/*do some busy computation */
for(i=0;i<temp->iter;i++)
{
j=i+j;
}
printf("\nTOTAL iterations %d\n", j);
printf("\n ENTERED APERIODIC TASK..Enter the input event id\n");
/*wait for event with the task id temp->taskid to occur. This sem_wait waits for keypress event. But it is not waiting in every iteration of while loop. It is waiting once in 3 iterations. */
sem_wait(&event[temp->taskid]);
sem_post(&mtx1);
}
in main method i have initialized
for(i=1;i<=10;i++)
{ /*the initial value of all mutexes in event array is 0 */
sem_init(&event[i],0,0);
}