CODEWITHSUNDEEP
carrayspointers
user8081549
user8081549

Reputation:

Not getting the expected results at pointer increment

Pointer increment not giving the right value, it points to indeterminate address.

Want to know why *p++= <value> not pointing to the value I want instead it points to something else

My code:

void run_exe() {
    int i;
    uint8_t buf[20] = {"0"};
    uint8_t *p = NULL;
    uint8_t tx_buf[4] = {1,1,1,1};

    p = buf;
    *p++ = 0x14;        
    *p++ = 0x20;        
    *p++ = 0x30;        
    *p++ = 0x40;        
    *p++ = 0x50;        
    *p++ = 0x60;        
    memcpy( p+6, tx_buf, sizeof(tx_buf));
   // ARRAY_SIZE: is the macro just my array length.

    for (i = 0; i < ARRAY_SIZE(buf); i++) {
        printf("%d ", *(p+i));
    }

}

I'm printing the decimal value of HEX but at least I'm expecting the correct value but I get indeterminate value.

I'm expecting the output like this.

14 20 30 40 50 60 1 1 1 1 0 0 0 0 0 0 0 0 0

Output:

0 0 0 0 0 0 1 1 1 1 0 0 0 0 255 127 0 0 0

Upvotes: 0

Views: 253

Answers (1)

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53006

You need an auxiliary pointer,

uint8_t *ptr;

p = buf;
ptr = p;

*p++ = 0x14;

and so on, because the ++ will modify the pointer's value.

Then,

for (i = 0; i < ARRAY_SIZE(buf); i++) {
    printf("%d ", ptr[i]);
}

The ++ post-increment or pre-increment operator, makes the pointer p point to p + 1 after the expression is evaluated, so your p is NOT pointing to buf when you attempt to print it.

EDIT: Of course you can just do this too,

for (i = 0; i < ARRAY_SIZE(buf); i++) {
    printf("%d ", buf[i]);
}

but I think the original answer explains the reason which is the important thing.

EXTRA: Also, I am 99.9% sure that this is wrong

uint8_t buf[20] = {"0"};

you meant,

uint8_t buf[20] = {0};

your compiler might be showing you a warning for this initialization which is faulty, because "0" has pointer type and even though it's convertible to a integer type, it mostly certainly doesn't fit a uint8_t so you have a overflow issue there.

Upvotes: 4

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