how to write integer var to file using write() function in c

First, read the man page of write(). It writes bytes, not element types.

So, what you are trying to achieve, cannot be accomplished directly with write(). you need to use snprintf() to convert the int to char string, which you can use with write(). Please check the following code.

1. Define a char array, print the value of the int pointer to that array using snprintf().
2. Use the char array as the argument of write(). It'll work.

NOTE: It's always a best practice to add some error check to the system calls and library calls. It provides many useful information in the case they fail.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pthread.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>

void *func(void *ptr);
int file;

int main()
{
        pthread_t thread1, thread2;
        int iret1, iret2;
        int  p;
        p=1;

        file=open("file1.txt", O_CREAT | O_TRUNC | O_RDWR , 0666);
        iret1 = pthread_create(&thread1, NULL, func, (void *)&p);
        pthread_join(thread1, NULL);

        close(file);
        exit(0);
}

void *func(void *ptr)
{
        int *num;
        int i, ret = -1;
        num = (int *)ptr;
        char buf[4] = {0};                              //to use with write()

        printf("%d\n", *num);
        for (i=0; i<10; i++)
        {
                printf("%d", *num);
                memset(buf, 0, sizeof (buf));
                snprintf(buf, sizeof *num, "%d", *num);               //print to buffer 
                ret = write(file, buf, strlen(buf));    //write the buf to file
                if (ret < 0)                            //check for erroneous condition
                        printf("ret is %d, errno %d\n", ret, errno);
        }
}

write(fd, &var, sizeof(i)); is a short fix for this question.