打印整数时fprintf的分段错误

Question

I keep getting a segmentation fault on the fprintf and from all my research, my sintax and initialisations are all okay. Any help will be greatly appreciated. NOTE: The printf("enter i") are just lines i use to check where the program fails and why.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>
#include <math.h>
#include <time.h>
#include <arpa/inet.h>
#include <stdbool.h>

void itoa(char *buf, int value);//found on stack overflow

int how_many_digits(int n)
{
int i = 0;
while(n > 10){
    n = n / 10;
    i++;
}
return i + 1;   
}

int main()
{
srand(time(NULL));
int server_port, main_socket, port;
struct sockaddr_in service; 
FILE *fileptr = NULL;
main_socket = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
service.sin_family = AF_INET;
service.sin_addr.s_addr = inet_addr("127.0.0.1");
do{
    printf("enter 5\n");
    port = rand() % 62974 + 1025;
    service.sin_port = htons(port);
    if(bind(main_socket, (struct sockaddr*) &service, sizeof(service)) == 0) break;
    } while(1); 
printf("enter 4\n");    
fopen("server_port", "w");
printf("enter 6\n");
printf("%d\n", port);
fprintf(fileptr,"%d", port);
printf("enter 7\n");
fflush(fileptr);    
fclose(fileptr);
printf("enter 8\n");
listen(main_socket, 1);
server_port = accept(main_socket, NULL, NULL);
return 0;
}

Answer 1

Your file has not been opened correctly. You are attempting to write to a NULL pointer here:

fprintf(fileptr,"%d", port);

Solution:

fileptr = fopen("server_port", "w");