String Extraction and Insertion Functions in C? - c

Question

I need to write these two functions:
Precondition: hMy_string is the handle to a valid My_string object.
Postcondition: hMy_string will be the handle of a string object that contains
the next string from the file stream fp according to the following rules.
1) Leading whitespace will be ignored.
2) All characters (after the first non-whitespace character is obtained and included) will be added to the string until a stopping condition
is met. The capacity of the string will continue to grow as needed
until all characters are stored.
3) A stopping condition is met if we read a whitespace character after
we have read at least one non-whitespace character or if we reach
the end of the file.
Function will return SUCCESS if a non-empty string is read successfully.
and failure otherwise. Remember that the incoming string may aleady
contain some data and this function should replace the data but not
necessarily resize the array unless needed.
Status my_string_extraction(MY_STRING hMy_string, FILE* fp);
Precondition: hMy_string is the handle to a valid My_string object.
Postcondition: Writes the characters contained in the string object indicated by the handle hMy_string to the file stream fp.
Function will return SUCCESS if it successfully writes the string and
FAILURE otherwise.
Status my_string_insertion(MY_STRING hMy_string, FILE* fp);
However, I am getting a segmentation fault with my current code:
#include <stdio.h>
#include <stdlib.h>
#include "my_string.h"
Status my_string_extraction(MY_STRING hMy_string, FILE *fp)
{
string *pString = (string *) hMy_string;
int lws = 0;
int exit = 0;
int nws = 0;
int i;
int count = 0;
while(fp != NULL && exit == 0) {
if(pString->size >= pString->capacity) {
char *t_data = (char *)malloc(sizeof(char) * pString->capacity * 2);
if(t_data == NULL) {
return FAILURE;
}
for(i = 0; i < pString->size; i++) {
t_data[i] = pString->data[i];
}
free(pString->data);
pString->data = t_data;
pString->capacity *= 2;
if(getc(fp) == ' ' && lws == 0) {
lws++;
} else if(getc(fp) == ' ' && lws == 1) {
exit++;
} else if(getc(fp) == ' ' && nws > 0) {
exit++;
} else {
pString->data[count] = getc(fp);
count++;
pString->size++;
nws++;
}
fp++;
}
return SUCCESS;
}
Status my_string_insertion(MY_STRING hMy_string, FILE *fp)
{
int i;
string *pString = (string *) hMy_string;
for(i = 0; i < pString->size; i++) {
putc(pString->data[i], fp);
}
if(fp == NULL) {
return FAILURE;
}
return SUCCESS;
}

Answer 1

I am getting a segmentation fault with my current code
Why do you do this:
fp++; // incrementing pointer does not get you the next character in the file
When you try to get next character via getc(fp) that is enough to cause the crash . fp will be invalid pointer at that moment.
Also, I see no restrictions for crossing data boundary:
pString->data[count] = getc(fp);
count++; // count is not restricted from growing