Malloc problem with fread() to read file to structure in C Post: 302887799

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Top Forums Programming Malloc problem with fread() to read file to structure in C Post 302887799 by Don Cragun on Monday 10th of February 2014 10:46:04 PM

02-10-2014

Registered User

Now that you have finally shown us the output from the first part of my code, it is obvious that something is overwriting the roll_num member of struct student (after the first element in the linked list) with a pointer. If you convert the bad values in roll_num from decimal to hexadecimal you'll note that the value is between the values of the pointers assigned to temp->name and temp->next.

For example 24736352(decimal) == 1797260(hex) and the pointer assigned to head is 17297250(hex) and the pointer assigned to head->name is 1797270(hex).

This should make absolutely no difference, but please try changing:

Code:

                if(head == NULL)
                        // Create 1st element in the linked list.
                        temp = head = calloc(1, sizeof(struct student));
                else    // Add new element to the end of the linked list.  Note
                        // that the temp->next in the next statement is in the
                        // previous element in the linked list and then temp is
                        // set to a pointer to the (new) current element.
                        temp = temp->next = calloc(1, sizeof(struct student));

to:

Code:

                if(head == NULL) {
                        // Create 1st element in the linked list.
                        head = calloc(1, sizeof(struct student));
                        temp = head;
                } else {// Add new element to the end of the linked list.  Note
                        // that the temp->next in the next statement is in the
                        // previous element in the linked list and then temp is
                        // set to a pointer to the (new) current element.
                        temp->next = calloc(1, sizeof(struct student));
                        temp = temp->next;
                }

Assuming that doesn't make any difference, while you're exploring gdb watch what gets assigned to temp->roll_num.

Don Cragun

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LEARN ABOUT BSD

fread

FREAD(3S)																 FREAD(3S)

NAME

       fread, fwrite - buffered binary input/output

SYNOPSIS

       #include <stdio.h>

       fread(ptr, sizeof(*ptr), nitems, stream)
       FILE *stream;

       fwrite(ptr, sizeof(*ptr), nitems, stream)
       FILE *stream;

DESCRIPTION

       Fread reads, into a block beginning at ptr, nitems of data of the type of *ptr from the named input stream.  It returns the number of items
       actually read.

       If stream is stdin and the standard output is line buffered, then any partial output line will be flushed before any  call  to  read(2)	to
       satisfy the fread.

       Fwrite  appends	at  most  nitems  of data of the type of *ptr beginning at ptr to the named output stream.  It returns the number of items
       actually written.

SEE ALSO

       read(2), write(2), fopen(3S), getc(3S), putc(3S), gets(3S), puts(3S), printf(3S), scanf(3S)

DIAGNOSTICS

       Fread and fwrite return 0 upon end of file or error.

4th Berkeley Distribution					   May 15, 1985 							 FREAD(3S)

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