Write own printf function, trying to include %p.

Question

So I'm writing my own printf function so I'm using stdarg.h and the system call write(). But I have no idea on how to incorrect %p into my function. Would %X produce the same result?

I have %x done as so:

'x': x=va_arg(argp,argp, unsigned int); 
     char *temp = convert(x,16);
     write(1, temp, lengthOFtemp);
     break;

char *convert(unsigned int, int)
{
static char buf[33];
char *ptr;

ptr=&buf[sizeof(buff)-1];
*ptr='\0';
do
{
*--ptr="0123456789abcdef"[num%base];
num/=base;
}while(num!=0);

return(ptr);
}

Related: http://stackoverflow.com/questions/2369541/where-is-p-useful-with-printf — John Carter, Feb 13 '13 at 04:45
You need to select an integral type that can hold a pointer and cast your pointer to it. This is inherently non-portable. On a 64-bit system you may have to use `unsigned long long int` and `%llX` or similar.On a 32-bit system, `unsigned int` and `%X` may work. — n. 1.8e9-where's-my-share m., Feb 13 '13 at 04:53
@n.m.: For systems which support ``, including C99 compliant systems and MSVC, you can use `uintptr_t`. This is portable. — Dietrich Epp, Feb 13 '13 at 07:19
Your sample code doesn't compile, and isn't even close. You have parameters without names, mix of `buf` and `buff`, and more and more. — ugoren, Feb 13 '13 at 08:23
@DietrichEpp: yes, but what format character corresponds to it? — n. 1.8e9-where's-my-share m., Feb 13 '13 at 10:47
@n.m.: the `PRIxPTR` macro from ``, but I don't think that's relevant because the asker is implementing `printf()` and is writing the conversion conversion code rather than relying on library functions. — Dietrich Epp, Feb 13 '13 at 11:17
@DietrichEpp: I mean, he cannot portably use an existing integer format of *his own* printf function, or at least it is not trivial. — n. 1.8e9-where's-my-share m., Feb 13 '13 at 12:24

score 0 · Answer 1 · answered Feb 13 '13 at 04:45

0

The difference between %p and %X is that pointer sizes are platform specific - 8 bytes on 64-bit platforms, 4 bytes on 32-bit platforms. Thus, you must use an 'unsigned long' variable to handle the pointer argument from va_args, not 'unsigned int' which remains 4 bytes on some 64-bit platforms.

Otherwise yes, very much like the %X code you have above.

answered Feb 13 '13 at 04:45

PQuinn

1,002
6
11

Thanks. how would I check the system platform (X-bit) and then run %p? – Morki Feb 13 '13 at 04:58
No check needed, just use 'unsigned long'. It will size accordingly for ILP32 and LP64 (32-bit and 64-bit) systems. – PQuinn Feb 13 '13 at 05:04
So p=va_arg(argp,argp, unsigned long); will work for both? Then I just have to just do convert(p,16)? – Morki Feb 13 '13 at 05:09
You must make sure you read the right number of bytes from the stack with va_arg. Thus, if the parameter is %X, you are expecting an integer which is 4 bytes. If the parameter is %p you are expecting a pointer, which is as I explained. Thus you need different calls to va_arg for each. – PQuinn Feb 13 '13 at 05:15
Use `intptr_t` or `uintptr_t` to store a pointer to void (`void *`) as an integer value in a portable manner. – alk Feb 13 '13 at 07:03

alk · Answer 2 · 2013-02-13T07:29:13.413

Write own printf function, trying to include %p.

2 Answers2