[C][GMP] What are these trash data written on files ?

Code:

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <stdint.h>#include <gmp.h>int main(void){FILE * fr, * fw;char input[201];char output[201];unsigned long int size = 0;unsigned long int rt;unsigned char ch[5] = {112,111,119,101,114};unsigned char plus[4]={112,108,117,115};printf("Enter filename : ");fgets(input, 200, stdin);input[strlen (input) - 1] = '\0';fr = fopen(input, "rb");if (fr == NULL){    fprintf(stderr, "Error during opening of file %s\n", input);    perror("");    return 1;}fseek(fr, 0L, SEEK_END);size = ftell(fr);fseek(fr, 0L, SEEK_SET);if (size == -1){fprintf(stderr, "Size doesn't match\n'");perror("");return 1;}if ( size > SIZE_MAX){fprintf(stderr, "The size of file is greater than acceptable\n");perror("");return 1;}unsigned char * buf = NULL;buf = malloc((size_t)size);if(buf == NULL){fprintf(stderr, "Buffer failure\n");perror("");return 1;}int r;r = fread(buf, size, 1, fr);printf("%d\n", r);if (ferror(fr)){fprintf(stderr, "Error during reading from file %s\n", input);perror("");fclose(fr);return 1;}fclose(fr);fr = NULL;mpz_t x;mpz_init(x);mpz_t y;mpz_init(y);mpz_import(x, size, 1, (size_t)1u, 0, 0, buf);if (size<=15)rt = 1;else if((size>15)&&(size<=100))rt = 9;else if((size>100)&&(size<=1000))rt = 99;else if((size>1000)&&(size<=10000))rt = 999;else if((size>10000)&&(size<=100000))rt = 9999;else if((size>100000)&&(size<=1000000))rt = 99999;else if((size>1000000)&&(size<=10000000))rt = 999999;else if((size>10000000)&&(100000000))rt = 9999999;elsert = 99999999;mpz_root (y, x, rt);mpz_t root;mpz_init(root);mpz_t remain;mpz_init(remain);mpz_rootrem (root, remain, y, rt);int rootnum = 0;unsigned long int num = rt;do{num = num/10;++rootnum;}while(num != 0);printf("\nEnter compressed filename : ");fgets(output, 200, stdin);output[strlen (output) - 1] = '\0';memset (buf, 0u, size);mpz_export(buf, NULL, 1, (size_t)1u, 0, 0, y);fw = fopen(output, "wb");if (fw == NULL) {fprintf(stderr, "Error during opening of file %s\n", output);perror("");return 1;}fwrite(buf, mpz_sizeinbase(y, 10), 1, fw);if (ferror(fw)) {fprintf(stderr, "Error during writing to file %s\n", output);perror("");fclose(fw);return 1;}fclose(fw);fw = fopen(output, "ab+");if(fw == NULL)printf("File can't be written\n'");if(ferror(fw)){fprintf(stderr, "Error during writing to file %s\n", output);perror("");fclose(fw);return 1;}fwrite(ch, 5, 1, fw);fclose(fw);fw = fopen(output, "ab");if(fw == NULL)printf("File can't be written\n'");if(ferror(fw)){fprintf(stderr, "Error during writing to file %s\n", output);perror("");fclose(fw);return 1;}int w;w = fwrite(&rt, rootnum, 1, fw);printf("%d\n", w);fclose(fw);fw = fopen(output, "ab");if(fw == NULL)printf("File can't be written\n'");if(ferror(fw)){fprintf(stderr, "Error during writing to file %s\n", output);perror("");fclose(fw);return 1;}fwrite(plus, 4, 1, fw);fclose(fw);fw = fopen(output, "ab");if(fw == NULL)printf("File can't be written\n'");if(ferror(fw)){fprintf(stderr, "Error during writing to file %s\n", output);perror("");fclose(fw);return 1;}fwrite(remain, mpz_sizeinbase(remain, 10), 1, fw);fclose(fw);free(buf);mpz_clear(x);mpz_clear(y);return 0;}