Seg Fault, nl replication

Question

I have gotten this error that has prevented me from continuing my code and testing other parts of the program much at all. The task is to replicate what the nl command does along with the a, n, and t arguments for -b as well as the option to replace the tab normally placed between the number of the line and the line itself with whatever we would like. My code is far from complete but so far the seg fault is hindering me from moving on to test much else and I don't know what else to do.

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

#define MAX_LINE_LENGTH 101

void process_stream(FILE *fpntr, char style, char *string);
char *fgetline(FILE *fpntr);


int main(int argc, char *argv[]) 
{
char *string = "\t";
char style = 't';
int opt;


//printf("Hello from main\n");

while((opt = getopt(argc, argv, "b:s:")) != -1)
    switch (opt)
    {
        case 'b':
            style = optarg[0];
            //printf("Hello from case b\n");
            break;
        case 's':
            string = optarg;
            break;
        case '?':
            break;
    }


for(int i=1; i<argc; i++)
    {
        FILE *fpntr = fopen(argv[1], "r");
        //printf("Hello from main for loop\n");
        process_stream(fpntr, style, string);
    }
 return EXIT_SUCCESS;
}


void process_stream(FILE *fpntr,char style,char *string)
{
int cnt = 1;
char *line;

while ((line = fgetline(fpntr)) != NULL)
{
    if(style == 'a')
    printf("%6d%s%s", cnt++, string, line);

    else if(style == 't' && *line != '\n')
    printf("%6d%s%s", cnt++, string, line);

    else if(style == 'n')
    printf("      %s%s", string, line);

}
fclose(fpntr);
}


char *fgetline(FILE *fpntr)
{
static char line_buff[MAX_LINE_LENGTH];
int next, pos = 0;


while((next = fgetc(fpntr)) != EOF && next != '\n')
{
    line_buff[pos++] = next;
}
 line_buff[pos] = '\0';
if(next==EOF && pos == '\0')
return NULL;
else return line_buff;


}