Segmentation fault when trying to create multiple threads for file IO: - c

Question

I am facing a segmentation fault when I run the following program . I
am trying to make a multithreading program which performs 100 times :
Open the file.
Read the last line of the file (each line contains a number).
Add 3 to the number.
Append the new number at the end of the file.
Close the file.
The number of threads is specified by the user.
When I create 2 threads, this code runs correctly,
but when I try to create 1 or a number different from 2,
it shows segmentation fault.
I am using a semaphore for synchronization among threads.
Do you have any idea why this happens ?
Part of main:
for(i=0;i<NumberOfThreads;i++) {
if (pthread_create( pid[i], NULL, subthread, NULL ) != 0) {
fprintf(stderr, "pthread_create failed with i = %d. errno = %d, %s\n",i, errno, strerror(errno));
break;
}
}
for(i=0;i<NumberOfThreads;i++) {
pthread_join(*pid[i], NULL);
}
return 0;
}
void *subthread(void) {
int i ;
int temp ;
char ch;
int offset =0 ;
FILE * fd1 ;
FILE * fd2 ;
for(i=0 ;i<100;i++) {
sem_wait(&sem1);
fd1 = fopen (NameOfFile,"r");
fseek (fd1,-2,SEEK_END);
offset=-2;
while((ch=getc(fd1)) !='\n')
{
offset=offset-1;
fseek (fd1,offset,SEEK_END);
}
fscanf(fd1,"%d",&temp);
fclose(fd1);
fd2 = fopen (NameOfFile,"a");
temp=temp+3;
fprintf(fd2,"\n%d",temp);
printf("%d\n",temp);
fclose(fd2);
sem_post(&sem1);
}
pthread_exit(0);
}
This is just a part of the code.
When I run the code and give 1 thread as an input, it shows an segmentation fault. For 2 threads the execution is correct.
For more than 2 threads it shows again segmentation fault
When I try to debug, to see what is causing the segmentation fault, the log says that the function PTHREAD_CREATE causes it.

Answer 1

For pthread_create() the pointer passed as first argument must point to a valid location where pthread_create() writes thre thread id.
But as you have said in a comment, pid is defined as
pthread_t * pid[NumberOfThreads+1];
which is an array of pointers that do not point to a valid location initially. They might be NULL or undetermined depending on how the array is defined.
I would suggest you change the definition to
pthread_t pid[NumberOfThreads+1];
and call pthread_create(pid+i,....) or pthread_create(&pid[i],...); and use pid[i] instead of *pid[i] for other pthread_...() calls