My code is:
#include <stdio.h>
#include <string.h>
void main()
{
char string[10];
int A = -73;
unsigned int B = 31337;
strcpy(string, "sample");
// printing with different formats
printf("[A] Dec: %d, Hex: %x, Unsigned: %u\n", A,A,A);
printf("[B] Dec: %d, Hex: %x, Unsigned: %u\n", B,B,B);
printf("[field width on B] 3: '%3u', 10: '%10u', '%08u'\n", B,B,B);
// Example of unary address operator (dereferencing) and a %x
// format string
printf("variable A is at address: %08x\n", &A);
I am using the terminal in linux mint to compile, and when I try to compile using gcc I get the following error message:
basicStringFormatting.c: In function ‘main’:
basicStringFormatting.c:18:2: warning: format ‘%x’ expects argument
of type ‘unsigned int’, but argument 2 has type ‘int *’ [-Wformat=]
printf("variable A is at address: %08x\n", &A);
All I am trying to do is print the address in memory of the variable A.
2 Answers
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 D
unsigned int D_address = *((unsigned int*) &D); // D address for %08x without warning
*D = 75; // D value
printf("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 D
unsigned long D_address = *((unsigned long*) &D); // D address for %016lx without warning
*D = ULONG_MAX; // D value
printf("variable D is at address: %p / 0x%016lx with value: %lu\n", D, D_address, *D);
