CODEWITHSUNDEEP
cgccmemorymemory-leaksvalgrind
flamingo
flamingo

Reputation: 168

Why I am not able to free memory? Invalid free() / delete / delete[] / realloc()

I am trying to improve the ugly C code, which causes a memory leak. Valgrind points: 

==19046== 1,001 bytes in 1 blocks are definitely lost in loss record 1 of 1
==19046==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==19046==    by 0x109D0B: save_params (ugly.c:188)

save_params is very long, but after removing other parts it can be presented like this:

/* Save params to file */
int save_params(int nb_iter) {
    char *word = malloc(sizeof(char) * MAX_STRING_LENGTH + 1); // <----- ugly.c:188 line 
    FILE *fid, *fout, *fgs;

        for (a = 0; a < vocab_size; a++) {
            if (fscanf(fid,format,word) == 0) {free(word); return 1;}
            if (strcmp(word, "<unk>") == 0) {free(word); return 1;}
            fprintf(fout, "%s",word);
            //...
            if (save > 0) {
                fprintf(fgs, "%s",word);
            if (fscanf(fid,format,word) == 0) {free(word); return 1;}
        }

        if (use_unk_vec) {
            word = "<unk>";
            fprintf(fout, "%s",word);
        }

        fclose(fid);
        fclose(fout);
        if (save > 0) fclose(fgs);

    }
    free(word); <--------- that free is "invalid"
    return 0;
}

Valgrind output:

==19046== Invalid free() / delete / delete[] / realloc()
==19046==    by 0x10AAC6: save_params (ugly.c:288)
==19046== 
==19046== HEAP SUMMARY:
==19046==     in use at exit: 1,001 bytes in 1 blocks
==19046==   total heap usage: 78 allocs, 78 frees, 13,764,515 bytes allocated
==19046== 
==19046== 1,001 bytes in 1 blocks are definitely lost in loss record 1 of 1
==19046==    by 0x109D0B: save_params (ugly.c:188)

==19046== LEAK SUMMARY:
==19046==    definitely lost: 1,001 bytes in 1 blocks
==19046==    indirectly lost: 0 bytes in 0 blocks

==19046== 
==19046== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

Could you give me a suggestion what I am doing wrong? How should I deallocate memory reserved by malloc?

Upvotes: 0

Views: 839

Answers (1)

Barmar
Barmar

Reputation: 782614

You can't free it because yo udid:

word = "<unk>";

Now word no longer points to the memory that you allocated, it points to this literal string. You have a memory leak, and an error when you try to free this.

There's no need to reassign word there. Just write the literal string to the file.

if (use_unk_vec) {
    fprintf(fout, "<unk>");
}

It's also unclear why you need the use malloc() and free() in the first place. Since you're only using word in this function, you should just declare it as a local array:

char word[MAX_STRING_LENGTH + 1];

Upvotes: 3

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