CODEWITHSUNDEEP
cfreerealloc
Adrian Laufer
Adrian Laufer

Reputation: 35

Error with free() in C (invalid next size) after realloc

I've been trying to figure out what the problem is for hours and can't get it right. Here is the code, which is of course a lot longer but I've reduced it to the problem itself.

#define BUFFER_SIZE 60
char *str;

void readText() {
    char read_char;
    int i = 0;
    str = (char *) calloc(BUFFER_SIZE, sizeof(char));
    while ((read_char = getchar()) != EOF) { /* user hits ctrl+d */
        *(str+i) = read_char;
        i++;
        if (i % BUFFER_SIZE == 0) {
            str = (char *) realloc(str, BUFFER_SIZE * sizeof(char));
        }
    }
    textSize = i;
    /* Here I print the text... same error printing or not printing */
    free(str);
}

}

I only get the error when the input text exceeds the buffer size.

(edited: if (i % BUFFER_SIZE == 0) so it makes it every time it get to 60, but the error is the same )

Thanks

Upvotes: 0

Views: 140

Answers (1)

David Ranieri
David Ranieri

Reputation: 41017

That's because you are reallocating with the same size, you need to use the new size when the string grows up:

while ((read_char = getchar()) != EOF) { /* user hits ctrl+d */
    if (i >= BUFFER_SIZE) {
        str = realloc(str, i);
    }
    *(str+i) = read_char;
    i++;
}

Also, it seems that you forget to set the trailing NUL, you need it in order to build a valid (printable) string, switch to

    if (i >= BUFFER_SIZE - 1) {

and

str[i] = '\0';

after the while loop.

Finally, prefer

size_t i = 0; // or better yet size_t len = 0;

over

int i = 0;

to pass the size to realloc and friends.

Upvotes: 1

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