Segmentation Fault at Do/while loop

Question

I have a function to add a new contact and store it in a file, also it will be stored in a linked list. I have a do/while loop to check if the phone Number that user entered is already available but I'm having a segmentation fault at this loop

void addContact(contactPtr *sPtr)
{
    FILE *fptr;
    contactPtr checkPtr = *sPtr;
    contactPtr crntPtr=checkPtr;
    contactPtr prevPtr;
    contactPtr newPtr;
    int check = 0;

    newPtr = (Contact *)malloc(sizeof(Contact));
    printf("Enter contact name: ");
    fflush(stdin);
    gets(newPtr->name);

    do
    {
        checkPtr=crntPtr;
        check=0;
        printf("Enter contact number: ");
        fflush(stdin);
        gets(newPtr->phoneNum);
        for (checkPtr; checkPtr != NULL; checkPtr = checkPtr->next)
        {
            if (strcmp(checkPtr->phoneNum, newPtr->phoneNum) == 0)
            {
                printf("Phone No is already available\n");
                check = 1;
                break;
            }
        }
    } while (check);

    fprintf(fptr, "\n%s\n", newPtr->name);
    fprintf(fptr, "%s\n", newPtr->phoneNum);

    checkPtr = crntPtr;
    while (checkPtr != NULL)
    {
        crntPtr = checkPtr;
        checkPtr = checkPtr->next;
    }
    while (crntPtr != NULL && (strcmp(newPtr->name, crntPtr->name) > 0))
    {
        prevPtr = crntPtr;
        crntPtr = crntPtr->next;
    }
    if (prevPtr == NULL)
    {
        newPtr->next = *sPtr;
        *sPtr = newPtr;
    }
    else
    {
        prevPtr->next = newPtr;
        newPtr->next = crntPtr;
    }
}

Can you run your code inside a debugger to see at which _line_ it segfaults? (and then check the values of all involved variables) — CherryDT, Jul 19 '20 at 14:38
About using gets: https://stackoverflow.com/questions/1694036/why-is-the-gets-function-so-dangerous-that-it-should-not-be-used — JCWasmx86, Jul 19 '20 at 14:39

RobertS supports Monica Cellio · Accepted Answer · 2020-07-19T15:07:47.297

0

FILE *fptr;

You never opened a stream to a file, where fptr is pointing to.

Using:

fprintf(fptr, "\n%s\n", newPtr->name);
fprintf(fptr, "%s\n", newPtr->phoneNum);

invokes undefined behavior then.

Open a file with fopen() and check whether the returned value is a null pointer in which case we have an error routine:

FILE *fptr = fopen("file.txt", "w");
if (!fptr)
{
    perror("fopen() failed");
    exit(EXIT_FAILURE);
}

Furthermore you need to close a stream by fclose(fptr); when no longer needed.

To flush stdin with fflush(stdin); invokes under the most implementations also undefined behavior.

Using fflush(stdin) with especially this answer explains it perfectly.

gets() should never be used. It is dangerous. Use fgets() instead.

Why is the gets function so dangerous that it should not be used?

edited Jul 19 '20 at 15:07

answered Jul 19 '20 at 14:42

RobertS supports Monica Cellio

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Thank you. I'll check it. Is there any way to get rid of '\n' after using fgets for most cases ? – chro1104 Jul 19 '20 at 14:55
@chro1104 [here](https://stackoverflow.com/a/28462221) - This is the best approach to remove the newline from the input. – RobertS supports Monica Cellio Jul 19 '20 at 14:58

Segmentation Fault at Do/while loop

1 Answers1