CODEWITHSUNDEEP
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SGarnotel
SGarnotel

Reputation: 83

What's the difference between var and &(*var)

Some source code use the notation &(*var) where var is already a pointer, like int *var = ....

Is there a difference between this two notations ?

Is var != &(*var) ?

Example: https://github.com/FreeFem/FreeFem-sources/blob/develop/src/medit/inout_popenbinaire.c#L40

Upvotes: 8

Views: 1913

Answers (3)

chqrlie
chqrlie

Reputation: 144951

&(*var) evaluates to the same as var in most circumstances, at compile time, but note some caveats:

  • var should be a valid pointer, although there is not reason for the compiler to generate code to dereference it. Indeed C11 specifies that there is no undefined behavior for &*var even if var is a null pointer.
  • as Pascal Cuoq commented, var should not be a pointer to void, but the C Standard explicitly allows it for this very case... Shocking indeed.
  • if var is an array, with more than one element, var and &*var have a different type and sizeof(var) and sizeof(&(*var)) have a different value: the first is the size of the array while the second is the size of a pointer to the its first element.
  • As commented by Peter, if var is an uninitialised pointer, then evaluating var == &(*var) gives undefined behaviour (since evaluating either side of the == gives undefined behaviour - accessing the value of an uninitialised variable is what gives the undefined behaviour).

The example you link to in the question uses this construction for no purpose whatsoever:

#define WrdSiz 4

void getline_bin_float_vertex(int ddim, double *c, int *ref) {
    int i;
    float ff;

    for (i = 0; i < ddim; i++) {
        fread((unsigned char *)&(ff), WrdSiz, 1, stdin);
        c[i] = ff;
    }
    fread((unsigned char *)&(*ref), WrdSiz, 1, stdin);
}

The programmer is reading a number of floating point values and an integer from a file redirected into the standard input. The code is awkward and non-portable:

  • the size of float is silently assumed to be 4
  • the size of int is silently assumend to be the same 4 bytes
  • c must be a valid pointer, unless ddim is 0
  • ref must be a valid pointer as fread will try and store 4 bytes at the address it points to, unless the stream is at end of file
  • end of file and read errors are silently ignored and undefined behavior is possible.

The code can be simplified and protected this way:

#define WrdSiz 4

/* read values from stdin, return non-zero in case of failure */
int getline_bin_float_vertex(int ddim, double *c, int *ref) {
    int i;
    float ff;

    assert(sizeof float == WrdSiz);
    assert(sizeof int == WrdSiz);
    assert(dim == 0 || c != NULL);
    assert(ref != NULL);

    for (i = 0; i < ddim; i++) {
        if (fread(&ff, sizeof ff, 1, stdin) != 1)
            return -1;
        c[i] = ff;
    }

    if (fread(ref, sizeof(*ref), 1, stdin) != 1)
        return -1;
    return 0;
}

Other portions of this source code show poor or even invalid constructions, you should study this package only for examples of what not to do.

Upvotes: 10

0___________
0___________

Reputation: 67749

Probably it was a longer expression inside the parentheses but during the debugging has been reduced to this one and left in the source code . It does not make any sense

Edit - the github example it looks exactly as I expected.

Upvotes: 0

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

&(*var) might have had undefined behavior when var (a pointer variable) does not contain a valid address for old versions of C. Be scared in that case. But C11 standard §6.5.3.2 states that the compiler has to handle it as equivalent in all cases (including when var is NULL)

So var == &(*var) is true when var contains a valid address. When it contains something else (e.g. NULL), it might crash (segmentation fault) if you use a non-C11 conforming non-optimizing compiler (but most compilers, even old ones, won't generate some crashing code in that case)

But the compiler (even an old C99 one) is allowed (under as-if rule) to optimize &(*var) into var

Upvotes: 5

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