valgrind giving 160 bytes in 4 blocks though total heap usage is: 448 allocs, 448 frees, 34,684 bytes allocated - valgrind

Question

When I run my code valgrind is giving this error.
Invalid free() / delete / delete[] / realloc()
==7363== at 0x4C2A82E: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7363== by 0x4009F8: main (in /home/arihant/ELF/elf)
==7363== Address 0x51f3fa0 is 0 bytes inside a block of size 16 free'd
==7363== at 0x4C2A82E: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7363== by 0x4009B1: main (in /home/arihant/ELF/elf)
==7363==
==7363==
==7363== HEAP SUMMARY:
==7363== in use at exit: 160 bytes in 4 blocks
==7363== total heap usage: 448 allocs, 448 frees, 34,684 bytes allocated
==7363==
==7363== LEAK SUMMARY:
==7363== definitely lost: 160 bytes in 4 blocks
==7363== indirectly lost: 0 bytes in 0 blocks
==7363== possibly lost: 0 bytes in 0 blocks
==7363== still reachable: 0 bytes in 0 blocks
==7363== suppressed: 0 bytes in 0 blocks
==7363== Rerun with --leak-check=full to see details of leaked memory
==7363==
==7363== For counts of detected and suppressed errors, rerun with: -v
==7363== ERROR SUMMARY: 6 errors from 2 contexts (suppressed: 2 from 2)
why I am getting 160 bytes lost though number of allocs and free are equal
my code is
18 fp = fopen("output", "r");
19 obj_elf = (Elf32_Ehdr *)malloc(sizeof(Elf32_Ehdr));
20 fread(obj_elf, 1, sizeof(Elf32_Ehdr), fp);
21
22 if (argc < 2) {
23 print_menu();
24 free(obj_elf);
25 fclose(fp);
26 return 0;
27 }
28
29 if (argv[1][0] == '-') {
30 switch (argv[1][1]) {
31 case 'e':
32 elf_header(obj_elf); /*elf header function call*/
33 break;
34 case 's':
35 read_section_header(fp, obj_elf, obj_sect_hdr);
36 print_section_header(fp, obj_sect_hdr, obj_elf);
37
38 for (i = 0; i < obj_elf->e_shnum; i++) {
39 free(obj_sect_hdr[i]);
40 free(sec_name[i]);
41 }
42 break;
43 case 'S':
44 read_section_header(fp, obj_elf, obj_sect_hdr);
45 read_symbol_table(fp, obj_elf, obj_sect_hdr, obj_sym);
46 symbol_table(fp, obj_elf, obj_sect_hdr, obj_sym);
47
48 for (i = 0; i < obj_elf->e_shnum; i++) {
49 free(obj_sect_hdr[i]);
50 free(sec_name[i]);
51 }
52 for (i = 0; i < n_entries_sym_t; i++) {
53 free(obj_sym[i]);
54 free(symbol_name[i]);
55 }
56
In main function there are many free how will I know which free is invalid??

Answer 1

Your answer is at the top of your output:
Invalid free() / delete / delete[] / realloc()
==7363== at 0x4C2A82E: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7363== by 0x4009F8: main (in /home/arihant/ELF/elf)
==7363== Address 0x51f3fa0 is 0 bytes inside a block of size 16 free'd
==7363== at 0x4C2A82E: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7363== by 0x4009B1: main (in /home/arihant/ELF/elf)
Just matching up the number of allocations and frees is not sufficient when one or more of your calls to free is invalid. You will need to build with debugging to see which free() is the problem. It looks like it's probably something allocated in read_section_header() or something else not listed.