passing command line argument in c as parameters - function

Question

I'm trying to make a function to assign a structure members a value.
#include <stdio.h>
#include <string.h>
typedef struct
{
int id;
char *data;
}person_t;
person_t person_build(int id, char *data);
int main (int argc, char *argv[])
{
person_t person = person_build(atoi(argv[1]), argv[2]);
return 0;
}
person_t person_build(int id, char *data)
{
person_t person;
person.id = id;
strcpy(person.data, data);
return person;
}
This program compiled successfully.
I run that program and give command line arguments as parameters
to person_build() function as parameters.
>struct5.exe 4 Something
operating system(windows 7) give me a warning this program has stopped working
but when run without any command line argument (changing the person_build() parameter other than command line arguments) that program works.
can someone explain why this behaviour happen?

Answer 1

Your program is not working because you are accessing memory structures that you have not initialized. Specifically:
typedef struct
{
int id;
char *data;
}person_t;
This creates a structure that has a char * as a member. That char * allocates no actual memory, it simply reserves a member in the structure that can hold a memory address that should point to a value. Later, you:
strcpy(person.data, data);
You are now copying data into the memory location that person.data points at even though you have never allocated memory or initialized person.data.
You could take this approach:
person_t person_build(int id, char *data)
{
person_t person;
person.id = id;
person.data = malloc(sizeof(char) * strlen(data) + 1);
if(person.data != NULL) strcpy(person.data, data);
return person;
}
This allocates memory of the proper size, accounting for null termination at the end of the string, verifies that the allocation was successful and only then will it attempt to copy into that memory.
This is far from complete. I think you may have many more obstacles yet to overcome!