Help with multithreads in C

Here is my current code. I also have text file with the following sentence 3 times, It is a "test" file, and it is very large. And it can have one single line or multiple lines.

When i run and compile my code, It will ask me to input a number of threads. When i enter a number, the it outputs the following: len = 107, size = 27, segmentation fault = 11.

However when, running the code this is what should be outputted: a3 and 6 can 3 file 3 have 3 is 6 it 9 large 3

Thanks for any help, it will be greatly appreciated.

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
struct thread_data
{
    FILE *fp;
    long int offset;
    int start;
    int blockSize;
};
int words=0;
void *countFrequency(void* data)
{
    struct thread_data* td=data;
    char *buffer = malloc(td->blockSize);
    int i,c;
    i=0;c=0;
    enum states { WHITESPACE, WORD };
    int state = WHITESPACE;
    fseek(td->fp, td->offset, td->start);
    char last = ' ';
    while ((fread(buffer, td->blockSize, 1, td->fp))==1)
    {
	    if ( buffer[0]== ' ' || buffer[0] == '\t' )
	    {
		    state = WHITESPACE;
	    }
	    else if (buffer[0]=='\n')
	    {
		    state = WHITESPACE;
	    }
	    else
	    {
		    if ( state == WHITESPACE )
		    {
			    words++;
		    }
		    state = WORD;
	    }
	    last = buffer[0];
    }
    free(buffer);
    pthread_exit(NULL);
    return NULL;
}
int main(int argc, char **argv)
{
    int nthreads, x, id, blockSize,len;
    FILE *fp;
    pthread_t *threads;
    struct thread_data data[nthreads];
    if (argc < 2)
    {
	    fprintf(stderr, "Usage: ./a.out <file_path>");
	    exit(-1);
    }
    if((fp=fopen(argv[1],"r"))==NULL)
    {
	    printf("Error opening file");
	    exit(-1);
    }
    printf("Enter the number of threads: ");
    scanf("%d",&nthreads);
    threads = malloc(nthreads*sizeof(pthread_t));
    fseek(fp, 0, SEEK_END);
    len = ftell(fp);
    printf("len= %d\n",len);
    blockSize=(len+nthreads-1)/nthreads;
    printf("size= %d\n",blockSize);
    for(id = 0; id < nthreads; id++)
    {
	    data[id].fp=fp;
	    data[id].offset = blockSize;
	    data[id].start = id*blockSize+1;
    }
    data[nthreads-1].start=(nthreads-1)*blockSize+1;
    for(id = 0; id < nthreads; id++)
	    pthread_create(&threads[id], NULL, &countFrequency,&data[id]);
    for(id = 0; id < nthreads; id++)
	    pthread_join(threads[id],NULL);
    fclose(fp);
    //free(threads);
    //pthread_exit(NULL);
    printf("%d\n",words);
    return 0;
}