Adding structs to an array in C - c

Question

I am having a problem in adding structs to an array in C-language. I think it's stemming from the fact that I might not be using correctly the pointers.
I have some struct with the following syntax:
struct account
{
int num;
char* fname;
char* lname;
char* pin;
double bal;
};
and in my main function I want to have a for loop that adds a struct to an array after going through and setting the variables of my struct to something. This is my full code so far:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct account
{
int num;
char* fname;
char* lname;
char* pin;
double bal;
};
void split_str (struct account *a,char *line)
{
int i = 0; //incrementer to tell what we are looking at: num, fname, lname, pin, or bal
int acc_num; //temp vals to store acc_num casting and balance casting
double bal;
char * str;
str = strtok(line, " ");
while (str!= NULL){
switch(i){
case 0:
acc_num = atoi(str);
a->num = acc_num;
acc_num = 0;
break;
case 1:
a->fname = str;
break;
case 2:
a->lname = str;
break;
case 3:
a->pin = str;
break;
case 4:
bal = atof(str);
a->bal = bal;
bal = 0;
break;
}
str = strtok (NULL, " ");
i++; //increment because we want to look through all 5 possibilities of vars
}
}
int main()
{
FILE *fh = fopen("account_info.txt", "r");
struct account accts[100];
if (fh != NULL)
{
char line[256];
for (int i=0; fgets(line, sizeof line, fh) != NULL; i++)
{
split_str(&accts[i], line);
}
}
fclose(fh);
for (int i=0; i<3; i++)
{
printf("%s %s index: %i\n", accts[i].fname, accts[i].lname, i);
}
}
This code is used with an account_info.txt file. All that file contains is one "account" per line delimited with a space " " char. Example:
123456 Jane Doe 1234 250.50
123457 John Smith 2222 12.34
123458 Sally Jones 9999 321.79
The problem with my current code is that it does seem to enter a struct in to the array, but it does it incorrectly. Here is what my code spits out at me when I run it with the print statement in the second for loop of my code:
Sally Jones index: 0
Sally index: 1
Sally Jones index: 2
Any help at all for why this is acting this way would be much appreciated.

Answer 1

In split_str() you are assigning pointers (locations) to
the local strings in the accounts that are dangling after
the function has been left (because the str pointer in
split_str is de-allocated and possibly reused at any
other point in the code later on...).
The make sure that the information gathered in the
split_str survives the function calls, you need
to allocate memory to fname (or lname or pin or any
other pointer) in the function, and then use some memcpy
or other methods to copy the information that str is pointing
to into the structure pointed to by a.

Answer 2

The problem is that there is only one instance of line in main so all the array pointers are pointing to that one line and whatever it contains after the last fgets.
sscanf could be used to parse the integer and double. Using the %n specifier, the starting and stopping indexes for names and pin can be captured. The %*s specifier will read and discard the strings as their length is not yet known. Once the start and stop indexes are known, memory can be allocated and the strings copied using memcpy.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct account
{
int num;
char* fname;
char* lname;
char* pin;
double bal;
};
int split_str (struct account *a,char *line)
{
int startfn = 0;
int stopfn = 0;
int startln = 0;
int stopln = 0;
int startpin = 0;
int stoppin = 0;
if ( ( 2 == sscanf ( line, "%d %n%*s%n %n%*s%n %n%*s%n %lf"
, &(a->num)
, &startfn
, &stopfn
, &startln
, &stopln
, &startpin
, &stoppin
, &(a->bal)))) {
if ( ( a->fname = malloc ( ( stopfn - startfn) + 1)) == NULL) {
fprintf ( stderr, "problem malloc\n");
exit ( 1);
}
memcpy ( a->fname, &line[startfn], stopfn - startfn);
a->fname[stopfn - startfn] = '\0';
if ( ( a->lname = malloc ( ( stopln - startln) + 1)) == NULL) {
fprintf ( stderr, "problem malloc\n");
exit ( 1);
}
memcpy ( a->lname, &line[startln], stopln - startln);
a->lname[stopln - startln] = '\0';
if ( ( a->pin = malloc ( ( stoppin - startpin) + 1)) == NULL) {
fprintf ( stderr, "problem malloc\n");
exit ( 1);
}
memcpy ( a->pin, &line[startpin], stoppin - startpin);
a->pin[stoppin - startpin] = '\0';
return 1;
}
return 0;
}
int main()
{
FILE *fh = fopen("account_info.txt", "r");
struct account accts[100];
int loaded = 0;
if (fh != NULL)
{
char line[256];
for (int i=0; fgets(line, sizeof line, fh) != NULL; i++)
{
if ( split_str(&accts[i], line)) {
loaded++;
}
}
}
fclose(fh);
for (int i=0; i<loaded; i++)
{
printf("%s %s index: %i\n", accts[i].fname, accts[i].lname, i);
}
for (int i=0; i<loaded; i++)//free allocated memory
{
free ( accts[i].fname);
free ( accts[i].lname);
free ( accts[i].pin);
}
}