My code can't reallocate memory (realloc())

Question

I've asked to write code that checks the biggest word in the text file(file1.txt) and write all the words with that size to another text file(file1a.txt) but it says that I have a realloc problem...if I write only one big word in the file1.txt it works but when I write 2 or more it wont pass the reallocation...

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

    int check_big_word(FILE *read){
        int count_letter = 0;
        int max_letter_size = 0;
        char letter;

        letter = fgetc(read);

        while(!feof(read) && letter != EOF){
            count_letter = 0;
            while(!(letter >= 'a' && letter <= 'z') || (letter >= 'A' && letter <= 'Z')){
                if (feof(read)){
                    break;
                }
                letter = fgetc(read);
            }
            while((letter >= 'a' && letter <= 'z') || (letter >= 'A' && letter <= 'Z')){
                if (feof(read)){
                    break;
                }
                count_letter++;
                letter = fgetc(read);
            }
            if( count_letter > max_letter_size ){
                max_letter_size = count_letter;
            }
        }
        rewind(read);
        return max_letter_size;
    }

    void print_all_big(FILE *read, FILE *write){
        int big_size = check_big_word(read);
        int count_letter = 0;
        int count_words = 0,i,beggining_of_string = 0;
        char letter;
        char *string_of_biggers;
        string_of_biggers = NULL;

        letter = fgetc(read);

        while(!feof(read)){
            count_letter = 0;
            while(!(letter >= 'a' && letter <= 'z') || (letter >= 'A' && letter <= 'Z')){
                if (feof(read)){
                    break;
                }
                count_words++;
                letter = fgetc(read);
            }
            while((letter >= 'a' && letter <= 'z') || (letter >= 'A' && letter <= 'Z')){
                if (feof(read)){
                    break;
                }
                count_letter++;
                letter = fgetc(read);
            }
            if(count_letter == big_size){
                if(string_of_biggers == NULL){
                    string_of_biggers = (char*)malloc(sizeof(char)*(big_size+1));
                }else{
                    //******* THE PROBLEM IS HERE *****
                    string_of_biggers = (char*)realloc(string_of_biggers, sizeof(char)*(big_size+1));
                }
                rewind(read);
                fseek(read,count_words*sizeof(char),SEEK_SET);
                for(i = 0; i<big_size;i++){
                    *string_of_biggers = fgetc(read);
                    beggining_of_string++;
                    string_of_biggers++;
                }
                *string_of_biggers = ' ';
                beggining_of_string++;
                string_of_biggers++;
            }
            count_words += count_letter;
        }
        string_of_biggers = string_of_biggers - beggining_of_string;
        fputs(string_of_biggers,write);
        fclose(read);
        fclose(write);
    }


void main(){
    FILE *r, *w;
    r = fopen("file1.txt", "rt");
    w = fopen("file1a.txt", "wt");
    print_all_big(r,w);
}

THE GIVEN ERROR IS :

* glibc detected * ./lab14_1: realloc(): invalid pointer: 0x000000000107649c *** ======= Backtrace: ========= /lib/x86_64-linux-gnu/libc.so.6(+0x7eb96)[0x7f4c0499fb96] /lib/x86_64-linux-gnu/libc.so.6(realloc+0x28e)[0x7f4c049a489e] ./lab14_1[0x400946] ./lab14_1[0x400a54] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xed)[0x7f4c0494276d] ./lab14_1[0x400689] ======= Memory map: ======== 00400000-00401000 r-xp 00000000 00:14 25693062 /home/tailedwiz/Desktop/C_programming/lab14_1 00600000-00601000 r--p 00000000 00:14 25693062 /home/tailedwiz/Desktop/C_programming/lab14_1 00601000-00602000 rw-p 00001000 00:14 25693062 /home/tailedwiz/Desktop/C_programming/lab14_1 01076000-01097000 rw-p 00000000 00:00 0 [heap] 7f4c0470b000-7f4c04720000 r-xp 00000000 08:01 11013974 /lib/x86_64-linux-gnu/libgcc_s.so.1 7f4c04720000-7f4c0491f000 ---p 00015000 08:01 11013974 /lib/x86_64-linux-gnu/libgcc_s.so.1 7f4c0491f000-7f4c04920000 r--p 00014000 08:01 11013974 /lib/x86_64-linux-gnu/libgcc_s.so.1 7f4c04920000-7f4c04921000 rw-p 00015000 08:01 11013974 /lib/x86_64-linux-gnu/libgcc_s.so.1 7f4c04921000-7f4c04ad6000 r-xp 00000000 08:01 11013953 /lib/x86_64-linux-gnu/libc-2.15.so 7f4c04ad6000-7f4c04cd6000 ---p 001b5000 08:01 11013953 /lib/x86_64-linux-gnu/libc-2.15.so 7f4c04cd6000-7f4c04cda000 r--p 001b5000 08:01 11013953 /lib/x86_64-linux-gnu/libc-2.15.so 7f4c04cda000-7f4c04cdc000 rw-p 001b9000 08:01 11013953 /lib/x86_64-linux-gnu/libc-2.15.so 7f4c04cdc000-7f4c04ce1000 rw-p 00000000 00:00 0 7f4c04ce1000-7f4c04d03000 r-xp 00000000 08:01 11013933 /lib/x86_64-linux-gnu/ld-2.15.so 7f4c04ee3000-7f4c04ee6000 rw-p 00000000 00:00 0 7f4c04eff000-7f4c04f03000 rw-p 00000000 00:00 0 7f4c04f03000-7f4c04f04000 r--p 00022000 08:01 11013933 /lib/x86_64-linux-gnu/ld-2.15.so 7f4c04f04000-7f4c04f06000 rw-p 00023000 08:01 11013933 /lib/x86_64-linux-gnu/ld-2.15.so 7fffd5c4d000-7fffd5c6e000 rw-p 00000000 00:00 0 [stack] 7fffd5c9b000-7fffd5c9d000 r-xp 00000000 00:00 0 [vdso] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] Aborted (core dumped)

Answer 1

When you call realloc, you must pass the exact pointer that was returned by a previous call to malloc or realloc. You are not doing that. You are passing the same variable, but you have changed the pointer in the variable.

Don't modify the pointer but use an index. That prevents your bug and makes the code much more readable.