C: valgrind shows invalid write for double pointer enum in struct-CodePudding

In my test.c, I have:

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

enum MyEnum{
    H1 = '#',
    H2 = '$'
};

typedef struct {
    enum MyEnum** myenum;
    int myint;
    char *s;
} MyStruct;

MyStruct* somefunction(const char* fp){
    FILE* file = fopen(fp, "r");
        if(!file){
        fprintf(stderr, "Error %s not found", fp);
        exit(-1);
    }
    char line[100] = {0};
    int i = 0;
    fgets(line, 100, file);
    sscanf(line, "%i", &i);
    MyStruct* strct = (MyStruct *) malloc(sizeof(strct));
    strct->myint = i;
    printf("%i\n", strct->myint);

    fclose(file);
    free(strct);
    return strct;
}


int main(int argc, char** argv){
  char* fp = argv[1];
  somefunction(fp);
  bool run = true;
  while(run){
    char entry = fgetc(stdin);
    
    switch(entry){ 
      case 'q' :{
        run = false;
        break;                      
      }
    }
  }
}

My test.txt contains:

1234
5678

Running Valgrind with give:

1234
q
==2030==
==2030== HEAP SUMMARY:
==2030== HEAP SUMMARY:
==2030==     in use at exit: 0 bytes in 0 blocks
==2030==   total heap usage: 5 allocs, 5 frees, 6,704 bytes allocated
==2030==
==2030== All heap blocks were freed -- no leaks are possible
==2030==
==2030== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)
==2030==
==2030== 1 errors in context 1 of 2:
==2030== Invalid read of size 4
==2030==    at 0x108AA9: somefunction (in /root/SokobanTechDev/test)
==2030==    by 0x108B03: main (in /root/SokobanTechDev/test)
==2030==  Address 0x51de2f8 is 0 bytes after a block of size 8 alloc'd
==2030==    at 0x4C2D0AF: malloc (vg_replace_malloc.c:381)
==2030==    by 0x108A93: somefunction (in /root/SokobanTechDev/test)
==2030==    by 0x108B03: main (in /root/SokobanTechDev/test)
==2030==
==2030==
==2030== 1 errors in context 2 of 2:
==2030== Invalid write of size 4
==2030==    at 0x108AA2: somefunction (in /root/SokobanTechDev/test)
==2030==    by 0x108B03: main (in /root/SokobanTechDev/test)
==2030==  Address 0x51de2f8 is 0 bytes after a block of size 8 alloc'd
==2030==    at 0x4C2D0AF: malloc (vg_replace_malloc.c:381)
==2030==    by 0x108A93: somefunction (in /root/SokobanTechDev/test)
==2030==    by 0x108B03: main (in /root/SokobanTechDev/test)

Why is valgrind giving me an invalid write after defining enum MyEnum? In my struct, if I comment enum MyEnum** myenum;, those contexts disappears. I theorize that he is allocating 2 pointers (of 8 bytes each) to H1 and H2, but isn't the malloc supposed to take care of that ?

I tried to look for some answers like here and here but I can't seem anything related to double pointers.

Thank you in advance

CodePudding user response：

There are at least two bugs within the function.

The first one is an incorrect memory allocation

MyStruct* strct = (MyStruct *) malloc(sizeof(strct));

You need to write either

MyStruct* strct = (MyStruct *) malloc(sizeof(*strct));

MyStruct* strct = (MyStruct *) malloc(sizeof(MyStruct));

And returning the pointer with an invalid value after call free

    //...
    free(strct);
    return strct;
}

does not make a sense.