Why Does my program have Memory Leaks?

Question

I am trying to create a small c program that will read a string with arbitrary size, without having any memory leaks.

According to my research, the function malloc can be used to allocate a number of bytes for whatever data we want to store.

In my program, I start by allocating space for 0 characters, and I make the pointer word point to it. Then whenever I read a single character, I make a pointer oldWord that points to word , which frees the old memory location once I allocate a larger memory location for the new character.

My research shows that the function free can be used to free an old memory location that is no longer needed. However, I am not sure where I am going wrong. Below you can see my code.

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

int main(void){
    char *word = malloc(0);

    printf("Enter name: ");
    readWord(word);
    printf("Your name is: %s\n", word);

    free(word);

    word = realloc(0);

    printf("Enter name: ");
    readWord(word);
    printf("Your name is: %s\n", word);

    free(word);

    return 0;
}

void readWord(char *word){
    int i = 0;
    char *oldWord, c = getchar();

    while(c != ' ' && c != '\n'){
        oldWord = word;
        word = realloc(word, i + 1);
        free(oldWord);
        word[i++] = c;
        c = getchar();
    }

    oldWord = word;
    word = realloc(word, i + 1);
    free(oldWord);
    word[i] = '\0';
}

Answer 1

The problem as I see it here is with

   free(oldWord);

without checking the failure of realloc() . In case realloc() is success, passing the same pointer to free() causes undefined behavior .

That said, some more notes

• a syntax like

   word = realloc(word, i + 1); 

  is dangerous, in case realloc() fails, you'll lose the actual pointer, too. You should use a temporary pointer to hold the return value of realloc() , check for success and only then, assign it back to the original pointer, if you need.

• In your code, c is of char type, which may not be sufficient to hold all the possible values returned by getchar() , for example, EOF. You should use an int type, that is what getchar() returns.

Answer 2

There are multiple problems in your code:

• you free the pointer you passed to realloc() . This is incorrect as realloc() will have freed the memory already if the block was moved. Otherwise the pointer is freed twice.

• The pointer reallocated bu readWord() is never passed back to the caller.

• Allocating a 0 sized block has unspecified behavior: it may return NULL or a valid pointer that should not be dereferenced but can be passed to free() or realloc() .

• You do not test for end of file: there is an infinite loop if the file does not have a space nor a linefeed in it, for example if the file is empty.

• you do not have a prototype for readWord() before it is called.

Here is an improved yet simplistic version:

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

char *readWord(void);

int main(void) {
    char *word;

    printf("Enter name: ");
    word = readWord();
    if (word == NULL) {
        printf("Unexpected end of file\n");
    else
        printf("Your name is: %s\n", word);

    free(word);
    return 0;
}

char *readWord(void) {
    int c;
    size_t i = 0;
    char *word = NULL;

    while ((c = getchar()) != EOF && !isspace(c)) {
        word = realloc(word, i + 2);
        if (word == NULL)
            break;
        word[i++] = c;
        word[i] = '\0';
    }
    return word;
}