赋值从指针进行整数运算而无需强制转换

Question

struct libimg 
{
   Elf32_Phdr ph;
};

struct libimg limg = {
   {
      p_type: PT_LOAD,
      p_memsz: 2 * PAGE_SIZE
   }
};

static void makelib(void *r)
{
    limg.ph.p_vaddr = r;
}

And Elf32_Phdr is defined in linux/elf.h

typedef struct elf32_phdr{
   Elf32_Addr    p_vaddr;
   ....
 } Elf32_Phdr;

I want to assign p_vaddr value from the argument. But I get this warning assignment makes integer from pointer without a cast . I use gdb to check and print r shows (void *)0x08040000

Answer 1

The compiler is warning you that an assignment caused an implicit cast. It does so because that might have been unintentional, and could potentially have undesirable effects. You could rectify that by using the correct type in the first place:

static void makelib(Elf32_Addr r)
{
    limg.ph.p_vaddr = r;
}

That might cause other code lines to generate warnings, and you should use a cast in these places, for example:

makelib((Elf32_Addr)someVariable);

The idea is to cast at the place where you change the meaning of a value from type A to type B, where it's a concise, deliberate decision. You should only do that (casting) if you can't use the proper type in the first place, that is, if someVariable can't be Elf32_Addr .

Answer 2

In Linux/arch/powerpc/boot/elf.h

 94 typedef struct elf32_phdr {
 95         Elf32_Word p_type;
 96         Elf32_Off p_offset;
 97         Elf32_Addr p_vaddr;
 98         Elf32_Addr p_paddr;
 99         Elf32_Word p_filesz;
100         Elf32_Word p_memsz;
101         Elf32_Word p_flags;
102         Elf32_Word p_align;
103 } Elf32_Phdr;

Where Elf32_Addr

typedef unsigned int Elf32_Addr;

Finally you are assigning a void * to a unsigned int

Cast it as unsigned int or better to Elf32_Addr to avoid warning:

limg.ph.p_vaddr = (Elf32_Addr)r;