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I have ran into an issue that has made me realise I don't know as much about pointers as I thought I did.

test.h

#ifndef TEST_H_INCLUDED
#define TEST_H_INCLUDED


void getDataAlt(char** dst);

#endif // TEST_H_INCLUDED

test.c

#include "test.h"

void getDataAlt(char** dst) {
    char* tmp = *dst;
    tmp = malloc(sizeof(char) * 50);
    sprintf(tmp, "This is a test.\r\n");
    *dst = tmp;
}

main.c

#include <stdio.h>
#include <stdlib.h>
#include "test.h"

#define enableOther

void getData(char** dst);

int main()
{
    char* anOther;
    getData(&anOther);
    printf("Hello %s\n", anOther);
    free(anOther);
    #ifdef enableOther
    char * anOtherAlt;
    getDataAlt(&anOtherAlt);
    printf("Hello Alt %s\n", anOtherAlt);
    free(anOtherAlt);
    #endif
    return 0;
}

void getData(char** dst) {
    char* tmp = *dst;
    tmp = malloc(sizeof(char) * 50);
    sprintf(tmp, "This is a test.\r\n");
    *dst = tmp;
}

This is my code. I am trying to let the function do the allocating etc. and modify the char* that is passed to it.

The local getData works without issue, the one inside the seperate .h /.c file works up till the point of the free(getDataAlt) and it spits out a "double free" error, and I can't for the life of me figure it out.

Running code blocks 20.03 on windows 11, MingW compiler version 8.1.0

New Screenshots:

I have ran into an issue that has made me realise I don't know as much about pointers as I thought I did.

test.h

#ifndef TEST_H_INCLUDED
#define TEST_H_INCLUDED


void getDataAlt(char** dst);

#endif // TEST_H_INCLUDED

test.c

#include "test.h"

void getDataAlt(char** dst) {
    char* tmp = *dst;
    tmp = malloc(sizeof(char) * 50);
    sprintf(tmp, "This is a test.\r\n");
    *dst = tmp;
}

main.c

#include <stdio.h>
#include <stdlib.h>
#include "test.h"

#define enableOther

void getData(char** dst);

int main()
{
    char* anOther;
    getData(&anOther);
    printf("Hello %s\n", anOther);
    free(anOther);
    #ifdef enableOther
    char * anOtherAlt;
    getDataAlt(&anOtherAlt);
    printf("Hello Alt %s\n", anOtherAlt);
    free(anOtherAlt);
    #endif
    return 0;
}

void getData(char** dst) {
    char* tmp = *dst;
    tmp = malloc(sizeof(char) * 50);
    sprintf(tmp, "This is a test.\r\n");
    *dst = tmp;
}

This is my code. I am trying to let the function do the allocating etc. and modify the char* that is passed to it.

The local getData works without issue, the one inside the seperate .h /.c file works up till the point of the free(getDataAlt) and it spits out a "double free" error, and I can't for the life of me figure it out.

Running code blocks 20.03 on windows 11, MingW compiler version 8.1.0

New Screenshots:

• c
• pointers
• mingw
• codeblocks

Share Improve this question Follow edited Jan 31 at 14:56 John Bollinger 183k1111 gold badges9696 silver badges189189 bronze badges asked Jan 31 at 10:43 BagOfTricksBagOfTricks 1344 bronze badges 24

• 1 If you are calling free(getDataAlt); it is no surprise it crashes -- getDataAlt is a function pointer, not a pointer to heap allocated memory. However, the code you show doesn't have that problem and should work fine. Your issue may be that you're not compiling what you think you are. Maybe you have an old test.o you're getting and not recompiling test.c – Chris Dodd Commented Jan 31 at 10:59
• 3 In test.c you do not include stdio.h. Using malloc without including that header should cause a warning in your compiler for implicitely declaring a function. Then, the compiler assumes malloc would return an int which is not correct. – Gerhardh Commented Jan 31 at 11:00
• 3 There is no point initializing tmp to *dst in getDataAlt and getData when the code assigns a new value to tmp immediately afterwards. – Ian Abbott Commented Jan 31 at 11:10
• 2 @Gerhardh I think you meant stdlib.h for malloc, but it does also need stdio.h for sprintf. – Ian Abbott Commented Jan 31 at 11:16
• 1 What happens when the project is compiled like recommended here: What compiler options are recommended for beginners learning C? – Lundin Commented Jan 31 at 11:25

 |  Show 19 more comments

1 Answer 1

Sorted by: Reset to default 2

There's a bug in the leak checker you're using.

If I run this code (with the leak checker) under valgrind I get the following output:

==19372== Memcheck, a memory error detector
==19372== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==19372== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==19372== Command: ./main
==19372== 
Hello This is a test.

Hello Alt This is a test.

WARNING:: (main.c:20) Double free detected
/*========= SUMMARY =========*/
  Total allocations      1  
  Total Free             2  
  Total Memory allocated 50 bytes 
  Total Memory freed     50 bytes 
  Memory Leaked          0 bytes 
==19372== 
==19372== HEAP SUMMARY:
==19372==     in use at exit: 0 bytes in 0 blocks
==19372==   total heap usage: 2 allocs, 2 frees, 100 bytes allocated
==19372== 
==19372== All heap blocks were freed -- no leaks are possible
==19372== 
==19372== For lists of detected and suppressed errors, rerun with: -s
==19372== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

It's clear both from looking at the code and the valgrind output that there's no double-free going on. So the leak checker is buggy.

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