How to copy file in buffer without a string

Question

I would like to copy a file in a buffer without a certain string

Here's the code

void remove_line(FILE *fp, char *string, char *output) {
    char buff[6];
    output = malloc(500);
    int i = 0;
    while (fgets(buff, 6, fp) != NULL) {
        char temp[6];
        strcpy(temp, buff);
        temp[strcspn(temp, "\n")] = 0;
        if (strcmp(temp, string) != 0) {
            memcpy(output + i * 6, buff, 6);
            i++;
        }
    }
    printf("%s", output);
}

int main(int argc, char *argv[])
{
    FILE *f;
    f = fopen("file", "a+");
    char *output = NULL;
    char line[] = "hello";
    remove_line(f, line, output);
    fclose(f);
}

and the file contains :

hello
world

My goal is to only have "world" in buffer, but currently I have nothing.

Thanks you !