Memory leak in C when appending to string

Question

Valgrind is saying that there is a memory leak, but I can't figure out what it is. I tried putting free's in the append function, freeing the string at the end of main and it still complains.

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

void append(char* string, char c)
{
    int stringlength = strlen(string);
    char* tmp = realloc(string, sizeof(char) * (stringlength + sizeof(char) + sizeof(char)));
    if(tmp == NULL)
    {
        free(tmp);
        free(string);
        printf("Error appending character\n");
        exit(1);
    }

    string = tmp;
    string[stringlength] = c;
    string[stringlength+1] = '\0';
}

int main()
{
    char* string = malloc(sizeof(char));
    string[0] = '\0';

    printf("string before appending: %s\n", string);
    append(string, 'c');
    printf("string after appending: %s\n", string);
    free(string);
    return 0;
}

Here is the output from Valgrind: https://pastebin.com/dtXFm5YC (Its in Pastebin so Stack Overflow would actually let me post this question

Answer 1

Your main problems are not the memory leak but the invalid access :

Invalid read of size 1
    ...
Address 0x49d0028 is 0 bytes inside a block of size 1 free'd

and invalid free :

Invalid free() / delete / delete[] / realloc()
...
Address 0x49d0028 is 0 bytes inside a block of size 1 free'd

because they have undefined behavior

• when you do char* tmp = realloc(string, sizeof(char) * (stringlength + sizeof(char) + sizeof(char))); that freed string coming from main, so when you printf it in main you access to a freed block. Note that the fact realloc freed the block is not mandatory

• because you do not free tmp newly allocated by realloc in append you create your memory leak

• then you free again string already freed by realloc

Also sizeof(char) * (stringlength + sizeof(char) + sizeof(char)) is strange, you have change sizeof(char) is by definition 1, can be just stringlength + 2

You want that :

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

void append(char ** string, char c)
{
   size_t stringlength = strlen(*string);

  *string = realloc(*string, stringlength + 2);

  if (*string == NULL)
  {
     fprintf(stderr, "cannot allocate memory\n");
     exit(1);
  }

  (*string)[stringlength] = c;
  (*string)[stringlength+1] = '\0';
}

int main()
{
    char* string = malloc(1);

    string[0] = '\0';

    printf("string before appending: '%s'\n", string);
    append(&string, 'c');
    printf("string after appending: '%s'\n", string);
    free(string);
    return 0;
}

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -g -pedantic -Wextra m.c
pi@raspberrypi:/tmp $ valgrind ./a.out
==17097== Memcheck, a memory error detector
==17097== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==17097== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==17097== Command: ./a.out
==17097== 
string before appending: ''
string after appending: 'c'
==17097== 
==17097== HEAP SUMMARY:
==17097==     in use at exit: 0 bytes in 0 blocks
==17097==   total heap usage: 3 allocs, 3 frees, 1,027 bytes allocated
==17097== 
==17097== All heap blocks were freed -- no leaks are possible
==17097== 
==17097== For counts of detected and suppressed errors, rerun with: -v
==17097== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)