That One String Pointer Error

I've explained this error a million times to a million students and I'm sure I'll explain it a million more. I've managed to explain it everytime with the same images and explanations I wrote the first time around. This is my attempt to clean that explanation up so I can copy and paste it in the future.

Let's say you're reading a file of words and numbers and it looks something like this:

apple 5
banana 6
blueberry 9
grape 5

And you want to store all these fruits words and their lengths in separate structs that you have stored in an array for future use. The output you WANT is:

[ apple : 5 ]
[ banana : 6 ]
[ blueberry : 9 ]
[ grape : 5 ]

But the output you GET is:

[ grape : 5 ]
[ grape : 6 ]
[ grape : 9 ]
[ grape : 5 ]

Oh no! It looks like that last string duplicated to be every string! How did this happen?

I'm willing to bet you've got some code that looks something like this:

#include<stdio.h>
#include<stdlib.h>

typedef struct Fruit {
	char *word;
	int len;
} Fruit;

void main() {
	FILE *fp;
	char buf[10];
	int n, i;
	struct Fruit input[4];

	fp = fopen( "fruitfile.txt", "r" );

	i = 0;
	while( fscanf(fp, "%s %d", buf, &n) != EOF ) {
		input[i].word = buf;
		input[i].len = n;
		i++;
	}

	fclose( fp );

	for( i = 0; i < 4; i++ )
		printf("[ %s : %d ]\n", input[i].word, input[i].len);
}

Note: Only one line of this code actually matters regarding the error

So let's draw it out. We've got two main variables word and n and a big data structure input. This looks something like:

Empty Data

Every iteration of the while loop, buf and n are filled with the values from the file. These change each iteration of the loop. We then use the next lines input[i].word = buf and input[i].len to store those values into our data structure. The result after the first iteration looks like this:

apple input

So now we iterate again. We fill buf and n with our new values, then store those in our data structure. After banana our data looks like this:

banana input

Here we can start to see where the source of our error is. We changes the values of n and buf. Since n is an int, the value just gets "copied" over to the data structure. Since buf is a pointer to a string, the pointer gets "copied" to the data structure, not the actual string. At the end of the while loop, everything points to buf which stores the last input.

Here are those last two iterations:

blueberry input

grape input

How do we fix this? How do we store the actual characters into the data structure and not just copy the pointer?

Easy, we just need to malloc the necessary data and use strcpy

// Copies pointer
input[i].word = buf;

// Copies character data
input[i].word = malloc( n );
strcpy( input[i].word, buf );

This would give us the following structure. Even with one iteration, we can see that we'll avoid all the pointers pointing to the same place.

With strcpy