malloc a char pointer member of a struct-CodePudding

How can I allocate memory for a char pointer member of a non pointer struct? Let me explain: Here is a small program:

typedef struct student {
    char *name;
} student;
void main()
{
    /* If I declare a normal struct */
    student Josef;
    /* This line doesn't work */  
    Josef->name = malloc(20*sizeof(char));
    /* But if I declare a pointer to struct student */
    student *PJosef;
    /* This line works */
    PJosef->name = malloc(20*sizeof(char));
}

Could you tell me why? And by the way, I saw in some tutorials, a line that allocates memory for that struct pointer like:
PJosef = malloc(sizeof(student->PJosef)); // (or should I write "struct" before student, even if I typedef-ed?) Is this line necessary? The program works without it.

Thanks ahead.

CodePudding user response：

The second example is invalid. As you do not allocate memory for the struct itself you dereference the dangling pointer and invoke a UB.

It should be

student *pJosef = malloc(sizeof(*pJosef));
if(pJosef)
   pJosef -> name = malloc(20 * sizeof(pJosef -> name[0])); // here also check if allocation failed. 
else { /* error handling */}

Note usage of the objects not types in sizeofs

CodePudding user response：

Using the -> operator in:

student Josef;
Josef->name = malloc(20*sizeof(char));

Will not work because Josef is not a pointer to a struct.

The -> operator can only be used on pointers to type struct. Using the -> operator like you have, is equivalent to doing something like this:

*(Josef).name

This is completely wrong.

Instead you should use the . operator:

student Josef;
Josef.name = malloc(20*sizeof(char));

Note:

student *PJosef;
PJosef->name = malloc(20*sizeof(char));

Will not work because PJosef is a uninitialized pointer and accessing it invokes undefined behaviour.

Instead you must first allocate memory for a struct:

student *PJosef = malloc(sizeof(struct student)); 

if (PJosef == NULL)
{
    fprintf(stderr, "malloc failed");
    // error procedure
}

PJosef->name = malloc(20*sizeof(char));