Segmentation Fault when writing to a file in C

Question

Whenever I attempt to write to a file, I am getting a segmentation fault. I don't have any access to software that can tell me where it's coming from since I'm at school. If anyone could help me out that would be great.

//OLMHash.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "OLMHash.h"

int main()
{
    createAccount();
    return 0;
}

struct account createAccount()
{


    struct account *newAccount;
    newAccount = (struct account *)malloc(sizeof(struct account));

    printf("Enter userid: ");
    scanf("%s", newAccount->userid);
    printf("\nEnter password: ");
    scanf("%s", newAccount->password);

    char *output = malloc(sizeof(char) * 255);
    strcpy(output, newAccount->userid);
    strcat(output, "\t");
    strcat(output, newAccount->password);

    FILE* fp;
    fp = fopen("accountslist.txt", "r");
    fputs(newAccount->userid, fp);

    free(output);
}

-

//OLMHash.h

struct account
{
    char userid[32];
    char password[12];  

};

struct account createAccount();

You opened the file for reading, not writing. And please *always* check the return value from `fopen()`. — Weather Vane, Sep 22 '16 at 17:14
Casts for malloc in C is a bad idea - http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — Ed Heal, Sep 22 '16 at 17:15
I think I've solved it. Weather Vane had it right. I was opening the file for reading when the file didn't even exist yet. After physically creating the file the error disappeared. Thank you — Cameron Rosier, Sep 22 '16 at 17:16
Think what will happen when the user is entering a *long* name or password... — Eugene Sh., Sep 22 '16 at 17:19
Why allocate `output` if you don't use it? Why doesn't your function return what it should return? Why didn't you compile with warnings on? — Paul Ogilvie, Sep 22 '16 at 17:40
Note too that you didn't close the file - every open has a close, every malloc has a free, etc., etc., etc. — KevinDTimm, Sep 22 '16 at 17:49

score 1 · Answer 1 · answered Sep 22 '16 at 17:22

1

You opened the file for reading instead of writing, and you did not check the action succeeded. Try

fp = fopen("accountslist.txt", "w");
if(fp == NULL) {
    // get out code
    exit(1);
}

answered Sep 22 '16 at 17:22

Weather Vane

33,872
7
36
56

score -2 · Answer 2 · answered Sep 22 '16 at 17:17

-2

When calling fopen, I was opening it for reading rather than writing. I physically created the file that way I can leave it as "reading"

answered Sep 22 '16 at 17:17

Cameron Rosier

1
1
1

And always, *always*, check the return of `scanf`. Otherwise, you have no idea if you are processing actual values or garbage. – David C. Rankin Sep 22 '16 at 17:18
1

If you want to WRITE to a file, then open it for WRITING. – Paul Ogilvie Sep 22 '16 at 17:39

Segmentation Fault when writing to a file in C

2 Answers2