Visual Studios giving me Dereferencing Null Pointer warning

Question

I am just trying to use malloc and the code runs fine but visual studios gives me issues with it. Is this how you're supposed to do these things or am I doing something wrong?

The warning shows up right on the line:

chadley->name = "chadley"

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


typedef struct {
    int age;
    char *name;
} Person;


int main()
{
    Person bradley;
    bradley.name = "bradley";
    bradley.age = 2334444;

    Person *chadley = malloc(sizeof(Person));
    chadley->name = "chadley";
    chadley->age = 234;

    printf("Person object %s of age %d", chadley->name, chadley->age);
}

Error Code: Dereferencing NULL pointer 'chadley'

Sure it runs fine under normal conditions, but what if you have *literally no memory left?* — SIGSTACKFAULT, Jul 11 '19 at 18:46
*"It isn't even a null pointer I casted it to a Person pointer!"* First, the cast isn't needed [and shouldn't be there](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). Second, the statement touches two issues that have *nothing* to do with one another. Casting has *nothing* to do with evaluating a pointer against a value, NULL or otherwise. — WhozCraig, Jul 11 '19 at 18:47
Add something like `if(chadley == NULL) return 1;` between line 16 and 17. — SIGSTACKFAULT, Jul 11 '19 at 18:48
okay I added an if(chadley) for every line of code that references it and the warning went away. This is the correct way to be making a struct with malloc and setting its members though right? — bturner1273, Jul 11 '19 at 18:48
@bturner1273 you only need to check if malloc failed once. if it did, just give up and exit. — SIGSTACKFAULT, Jul 11 '19 at 18:49
Are you using a C or C++ compiler? Different language, different warnings, errors — chux - Reinstate Monica, Jul 11 '19 at 18:55
c, visual studios just didn't like that I didn't add a null check under malloc — bturner1273, Jul 11 '19 at 18:58
Please [edit] the question and add the verbatim diagnostics message - there should also be a diagnostics code ... — Antti Haapala -- Слава Україні, Jul 12 '19 at 02:03

Thomas Jager · Answer 1 · 2019-07-11T18:53:28.273

3

A Person * can be a null pointer. It refers to the value, not the type.

If it fails to successfully allocate memory, malloc will return a NULL value. You should always check the value returned from malloc before using it.

You can check that the pointer returned is valid doing something like:

Person *chadley = malloc(sizeof(Person));

if (!chadley) {
    /* The allocation failed, don't continue. */
    return 1;
}

chadley->name = "chadley";
chadley->age = 234;

edited Jul 11 '19 at 18:53

answered Jul 11 '19 at 18:49

Thomas Jager

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1

[Lose the cast on `malloc`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) and you'll probably earn at least one uptick. – WhozCraig Jul 11 '19 at 18:50
`Person *chadley = malloc(sizeof *chadley);` would be more maintainable. – chux - Reinstate Monica Jul 11 '19 at 18:58
@chux why do you say this? is there a difference? – bturner1273 Jul 11 '19 at 19:13
Are you just saying in case I changed the name of the struct? – bturner1273 Jul 11 '19 at 19:26
1

@bturner1273: In case you change it's type, yeah. Compare `T *p = malloc( sizeof (T))` to `T *p = malloc( sizeof *p )`. In the first case, if `T` ever changes, you have to make both the declaration and `sizeof` operator match up. In the second, you only have to worry about the declaration. It also makes life easier if you have to worry about whether `T` is another pointer type, or a pointer to an array type, or whatever. – John Bode Jul 11 '19 at 19:28

score 1 · Answer 2 · answered Jul 11 '19 at 18:54

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


typedef struct {
    int age;
    char *name;
} Person;


int main()
{
    Person bradley;
    bradley.name = "bradley";
    bradley.age = 2334444;

    Person *chadley = malloc(sizeof(Person));
    if (chadley == NULL) return 1;
    chadley->name = "chadley";
    chadley->age = 234;
    printf("Person object %s of age %d", chadley->name, chadley->age);
}

works thank you!

Visual Studios giving me Dereferencing Null Pointer warning

2 Answers2