Strange error in C with a malloc of pointer in a pointer

Question

Here my example code which works fine:

//define struct
struct myStruct {
   char     *arrChar1;
   char     *arrChar2;
}
// code in Main
struct myStruct *structDeep1 = malloc( sizeof( struct myStruct *) );
structDeep1->arrChar1 = NULL;
structDeep1->arrChar2 = NULL;
structDeep1->arrChar1 = malloc( sizeof(char ) * 2); //2 char

if( structDeep1->arrChar1 == NULL)  puts("trace error");
else                                    puts("trace ok")
//stdout -> trace OK

No problem.

Now here my example with strange error:

// define a second struct
struct myStructDeep2{
    struct myStruct     *structDeep1;
}
// next code in Main
struct myStructDeep2 structDeep2 = malloc( sizeof( struct myStructDeep2*) );
structDeep2->structDeep1 = malloc( sizeof( struct myStruct *) );
structDeep2->structDeep1->arrChar1 = NULL;
structDeep2->structDeep1->arrChar2 = NULL;
structDeep2->structDeep1->arrChar1 = malloc( sizeof(char ) * 2); //2 char

if( structDeep2->structDeep1->arrChar1 == NULL)     puts("trace error");
else                                            puts("trace ok")
//stdout -> trace error

It's seem the malloc function write/crush in the second pointer. I don't understand where is the problem in my code, it's very strange.

Yes it's bad rewrite of example I corrected that for all "struct"malloc. Thx — cconigli
– cconigli, Commented May 9, 2018 at 13:58
You allocate the size for the struct not to the struct pointer. — danglingpointer
– danglingpointer, Commented May 9, 2018 at 14:29

Paul Ogilvie · Accepted Answer · 2018-05-09 14:02:26Z

2

When you do:

structDeep2->structDeep1 = malloc( sizeof( struct myStruct *) );

you allocate the size of a pointer, but you want to allocate a structure, so you must do:

structDeep2->structDeep1 = malloc( sizeof( struct myStruct) );

Now (assuming malloc succeeds), you can safely do:

structDeep2->structDeep1->arrChar1 = malloc( sizeof(char ) * 2);

answered May 9, 2018 at 14:02

Paul Ogilvie

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1 Comment

David C. Rankin Over a year ago

Don't worry, it won't be the last time that happens.... (and... do not assume malloc succeeds -- it has a return -- check it -- every time...)

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