writing to a text file in C using fprintf() - c

Question

I am working with a program to write the summation of 1-500 and 500-1000 using two separate threads. I need the output to be written in to a text file which is created by the program itself. When I run the program it creates the file according to the given name, but I am not getting the output as needed. It only writes one single line to the text file. That is the summation of 500-1000. But when I get the output using console it shows the answer as needed. How to overcome this problem. Thanks!
#include <stdio.h>
#include <pthread.h>
#include <fcntl.h>
#include <stdlib.h>
#define ARRAYSIZE 1000
#define THREADS 2
void *slave(void *myid);
/* shared data */
int data[ARRAYSIZE]; /* Array of numbers to sum */
int sum = 0;
pthread_mutex_t mutex;/* mutually exclusive lock variable */
int wsize; /* size of work for each thread */
int fd1;
int fd2;
FILE * fp;
char name[20];
/* end of shared data */
void *slave(void *myid)
{
int i,low,high,myresult=0;
low = (int) myid * wsize;
high = low + wsize;
for(i=low;i<high;i++)
myresult += data[i];
/*printf("I am thread:%d low=%d high=%d myresult=%d \n",
(int)myid, low,high,myresult);*/
pthread_mutex_lock(&mutex);
sum += myresult; /* add partial sum to local sum */
fp = fopen (name, "w+");
//printf("the sum from %d to %d is %d",low,i,myresult);
fprintf(fp,"the sum from %d to %d is %d\n",low,i,myresult);
printf("the sum from %d to %d is %d\n",low,i,myresult);
fclose(fp);
pthread_mutex_unlock(&mutex);
return;
}
main()
{
int i;
pthread_t tid[THREADS];
pthread_mutex_init(&mutex,NULL); /* initialize mutex */
wsize = ARRAYSIZE/THREADS; /* wsize must be an integer */
for (i=0;i<ARRAYSIZE;i++) /* initialize data[] */
data[i] = i+1;
printf("Enter file name : \n");
scanf("%s",name);
//printf("Name = %s",name);
fd1=creat(name,0666);
close(fd1);
for (i=0;i<THREADS;i++) /* create threads */
if (pthread_create(&tid[i],NULL,slave,(void *)i) != 0)
perror("Pthread_create fails");
for (i=0;i<THREADS;i++){ /* join threads */
if (pthread_join(tid[i],NULL) != 0){
perror("Pthread_join fails");
}
}
}

Answer 1

Its because you are opening the same file two times, one on each thread. They are overwriting each other's job.
To solve this you can:
Use the a+ mode on fopen() to append the new line to end of the existing file, or;
Open the file in main() and the threads will only fprintf() to it.