Memory Leak Valgrind

Question

I wrote a program to chunk a string five by five. This is my program.

struct list
{
    char *str;
    struct list* next;
};

struct list* head = NULL;

void insert(char *cont)
{
    struct list* temp = (struct list*)malloc(sizeof(struct list));

    size_t len = strlen(cont);
    char *heapString = (char*)malloc(len);
    strcpy(heapString,cont);

    temp->str = heapString;
    temp->next = NULL;

    if(head == NULL)
    {
        head = temp;
        return ;
    }

    temp->next = head;
    head = temp;
}
void print()
{
    struct list* temp = head;
    while(temp != NULL)
    {
        printf("%s\n",temp->str);
        temp = temp->next;
    }
}
void clearmem()
{
    struct list* temp = head;
    while(temp != NULL)
    {
        free(temp->str);
        free(temp);
        temp = temp->next;
    }
}
int main()
{
    char text[] = "abcdefghijklmno";
    size_t len = strlen(text);
    while(len !=0)
    {
        char *temp;
        temp = text ;
        temp = temp + len - 5;

        insert(temp);
        *(text+len-5) = '\0';
        len = strlen(text);
        free(temp);
    }
    print();
    clearmem();
}

My program is working fine. But when I try to run this program through Valgrind, I got the following messages. It says there are 12 errors.

==2055== Invalid write of size 1
==2055==    at 0x4C32E0D: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2055==    by 0x10888C: insert (in /home/infant/Documents/Sample_codes/a.out)
==2055==    by 0x1089BD: main (in /home/infant/Documents/Sample_codes/a.out)
==2055==  Address 0x522d095 is 0 bytes after a block of size 5 alloc'd
==2055==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2055==    by 0x108875: insert (in /home/infant/Documents/Sample_codes/a.out)
==2055==    by 0x1089BD: main (in /home/infant/Documents/Sample_codes/a.out)
==2055== 
==2055== Invalid free() / delete / delete[] / realloc()
==2055==    at 0x4C30D3B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2055==    by 0x1089EB: main (in /home/infant/Documents/Sample_codes/a.out)
==2055==  Address 0x1fff00030a is on thread 1's stack
==2055==  in frame #1, created by main (???:)
==2055== 
==2055== Invalid read of size 1
==2055==    at 0x4C32D44: __strlen_sse2 (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2055==    by 0x4EBC9D1: puts (ioputs.c:35)
==2055==    by 0x1088FC: print (in /home/infant/Documents/Sample_codes/a.out)
==2055==    by 0x1089FC: main (in /home/infant/Documents/Sample_codes/a.out)
==2055==  Address 0x522d1d5 is 0 bytes after a block of size 5 alloc'd
==2055==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2055==    by 0x108875: insert (in /home/infant/Documents/Sample_codes/a.out)
==2055==    by 0x1089BD: main (in /home/infant/Documents/Sample_codes/a.out)
==2055== 
abcde
fghij
klmno
==2055== Invalid read of size 8
==2055==    at 0x108947: clearmem (in /home/infant/Documents/Sample_codes/a.out)
==2055==    by 0x108A06: main (in /home/infant/Documents/Sample_codes/a.out)
==2055==  Address 0x522d188 is 8 bytes inside a block of size 16 free'd
==2055==    at 0x4C30D3B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2055==    by 0x108942: clearmem (in /home/infant/Documents/Sample_codes/a.out)
==2055==    by 0x108A06: main (in /home/infant/Documents/Sample_codes/a.out)
==2055==  Block was alloc'd at
==2055==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2055==    by 0x108855: insert (in /home/infant/Documents/Sample_codes/a.out)
==2055==    by 0x1089BD: main (in /home/infant/Documents/Sample_codes/a.out)
==2055== 
==2055== 
==2055== HEAP SUMMARY:
==2055==     in use at exit: 0 bytes in 0 blocks
==2055==   total heap usage: 7 allocs, 10 frees, 1,087 bytes allocated
==2055== 
==2055== All heap blocks were freed -- no leaks are possible
==2055== 
==2055== For counts of detected and suppressed errors, rerun with: -v
==2055== ERROR SUMMARY: 12 errors from 4 contexts (suppressed: 0 from 0)

Even though I cleared all the Memory in the Heap, I am getting 12 errors from 4 contexts. What is my error here?

Answer 1

Step by step.

Invalid write of size 1

Your malloc() does not allocate space for the string terminator, but strcpy() tries to write it. Use

char *heapString = malloc(len + 1);

instead. (Note: no need to cast void* to char*!). For simplicity, you may also try to use the (non-standard) strdup(cont).

Invalid free() / delete / delete[] / realloc()

Your temp points to a char in text. It makes no sense to use free(), as there's nothing allocated there. Remove that call.

Invalid read of size 1

This should be related with the first error. It is interesting to note how, in print(), at compile time printf("%s\n",temp->str) is translated to a (faster) puts(temp->str). That's why Valgrind is complaining about a call to puts.

Invalid read of size 8

In

free(temp);
temp = temp->next;

you read temp after it has been freed.