How to printf a memory address in C
Use the format specifier %p
:
printf("variable A is at address: %p\n", (void*)&A);
The standard requires that the argument is of type void*
for %p
specifier. Since, printf
is a variadic function, there's no implicit conversion to void *
from T *
which would happen implicitly for any non-variadic functions in C. Hence, the cast is required. To quote the standard:
7.21.6 Formatted input/output functions (C11 draft)
p The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printing characters, in an implementation-defined manner.
Whereas you are using %x
, which expects unsigned int
whereas &A
is of type int *
. You can read about format specifiers for printf from the manual. Format specifier mismatch in printf leads to undefined behaviour.
A workaround to use %x
with length specifier to print an int
or unsigned int
without compiler complaining about casting would be to use malloc:
unsigned int* D = malloc(sizeof(unsigned int)); // Allocates Dunsigned int D_address = *((unsigned int*) &D); // D address for %08x without warning*D = 75; // D valueprintf("variable D is at address: %p / 0x%08x with value: %u\n", D, D_address, *D);
Alternatively you can compile with gcc -w
flag to suppress those casting warnings.
Edit for 64-bit addresses:
unsigned long* D = malloc(sizeof(unsigned long)); // Allocates Dunsigned long D_address = *((unsigned long*) &D); // D address for %016lx without warning*D = ULONG_MAX; // D valueprintf("variable D is at address: %p / 0x%016lx with value: %lu\n", D, D_address, *D);