what does print /s do?

what does print /s do?

[dave_rodgman]

Hi,

I'm having trouble understanding exactly what print /s is intended to
do. I've read the manual, and it says:

(from http://sourceware.org/gdb/current/onlinedocs/gdb_10.html#SEC70)

> Regard as a string, if possible. With this format, pointers to single-byte data are displayed as null-terminated strings and
> arrays of single-byte data are displayed as fixed-length strings. Other values are displayed in their natural types.
>
> Without this format, GDB displays pointers to and arrays of char, unsigned char, and signed char as strings. Single-byte
> members of a vector are displayed as an integer array.

I have two questions:

1. What is the distinction between "pointer to single-byte data" and
"pointers to and arrays of char, unsigned char, and signed char"?
Or is this just poorly worded?

2. Under what circumstances does supplying or not supplying /s make a
difference to the output? I can't find a case where it makes a
difference. For example, when printing a pointer-to-char (here, "string"
is a char array)

(gdb) p /s string
$14 = "abc"
(gdb) p string
$15 = "abc"

If I attempt to do the inverse, i.e., use /s to force interpreting a
non-ptr-to-char as a string, /s is ignored:

(gdb) p /s (int *)string
$16 = (int *) 0x804a018
(gdb) p (int *)string
$17 = (int *) 0x804a018

thanks

Dave
-- 
  
  dave_rodgman@fastmail.co.uk

Re: what does print /s do?

[Daniel Jacobowitz]

On Fri, Nov 21, 2008 at 05:10:10PM +0000, dave_rodgman@fastmail.co.uk wrote:
> I have two questions:
> 
> 1. What is the distinction between "pointer to single-byte data" and
> "pointers to and arrays of char, unsigned char, and signed char"?
> Or is this just poorly worded?

char, unsigned char, and signed char are examples of single byte data.
Members of a vector are also examples of single byte data.  For
instance, the contents of an AltiVec or SSE vector register can be
interpreted as strings using print /s.

> 2. Under what circumstances does supplying or not supplying /s make a
> difference to the output? I can't find a case where it makes a
> difference. For example, when printing a pointer-to-char (here, "string"
> is a char array)

Try it with a vector type.  /s only affects single byte data,
and (int *) in your example is four-byte data.

-- 
Daniel Jacobowitz
CodeSourcery