sizeof error

[MarcSmith]

i was in a cpp program calling a c program where in the cpp program
had defined a 80 character string passing to the c program where sizeo
had reported it was 4 when it passed the answer back it was wrong. thi
may be off-topic. all i want to know is why did it happen? or is anyon
else having this problem? tkanks for your patience and understandin
...most of my answers are off-topic sorry about that
-
MarcSmit
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[Jens.Toerring]

MarcSmith <Ma**************@mail.codecomments.com> wrote:

i was in a cpp program calling a c program where in the cpp program i
had defined a 80 character string passing to the c program where sizeof
had reported it was 4 when it passed the answer back it was wrong. this
may be off-topic. all i want to know is why did it happen? or is anyone
else having this problem? tkanks for your patience and understanding
..most of my answers are off-topic sorry about that.

That's because you were applying 'sizeof' to a pointer and not to
an array. What you passed to the C function was a pointer to the
array (or its first element to be precise), not the array itself.
So what sizeof tells you is the size of the pointer. When you pass
an array to a function and you need want to know its size within
the function you have to pass the size together with the pointer
to the (first element of the) array.

Regards, Jens
--
\ Jens Thoms Toerring ___ Je***********@physik.fu-berlin.de
\__________________________ http://www.toerring.de

[Erik de Castro Lopo]

MarcSmith wrote:

i was in a cpp program calling a c program where in the cpp program i
had defined a 80 character string passing to the c program where sizeof
had reported it was 4 when it passed the answer back it was wrong.
The answer may have been correct. Your understanding of what was
going on may have been wrong.
this may be off-topic.
Nope, definitely on topic.
all i want to know is why did it happen?

You didn't provide any code. However, the small program below
may demonstrate what you were doing wrong.

#include <stdio.h>
#include <string.h>

int main (void)
{
char str1 [] = "12345678" ;
char *str2 = "This is a string" ;

printf ("str1, %d %d\n", sizeof (str1), strlen (str1));
printf ("str2 %d %d\n", sizeof (str2), strlen (str2));

return 0;
}

On my machine (whcih is correct) it outputs:

str1 9 8
str2 4 16

For strings, you should probably use strlen instead of sizeof.
Erik
--
+-----------------------------------------------------------+
Erik de Castro Lopo no****@mega-nerd.com (Yes it's valid)
+-----------------------------------------------------------+
"When your hammer is C++, everything begins to look like a thumb."
-- Steve Haflich, in comp.lang.c++

[pete]

MarcSmith wrote:

i was in a cpp program calling a c program where in the cpp program i
had defined a 80 character string passing to the c program
where sizeof had reported it was 4 when it passed the answer
back it was wrong. this
may be off-topic. all i want to know is why did it happen?

Object types have sizes. Strings have lengths.
#include <string.h> and use strlen() to find string length.
The length of a string does not include the null byte.

The type of "\0\0\0\0\0\0" is (char[7]).

sizeof (char[7]) is seven.
sizeof "\0\0\0\0\0\0" is seven.
strlen("\0\0\0\0\0\0") is zero.

--
pete

[Default User]

pete wrote:

MarcSmith wrote:

i was in a cpp program calling a c program where in the cpp program
i had defined a 80 character string passing to the c program
where sizeof had reported it was 4 when it passed the answer
back it was wrong. this
may be off-topic. all i want to know is why did it happen?

Object types have sizes. Strings have lengths.
#include <string.h> and use strlen() to find string length.
The length of a string does not include the null byte.

You didn't really answer his question, nor is your solution necessarily
correct. The string may not be what is desired at all, as the function
may be looking to append to the string. In that case, the size of the
buffer is desired. Not only that, but arrays do have sizes that can be
reported correctly by sizeof, just not in the case that was mentioned.
It wouldn't have mattered whether it was a string, an array of double,
or any other sort of array.

The reason that the OP encountered that condition is that arrays can't
be passed to functions. The name of the array is converted to a pointer
to the first element. So sizeof reported back the sizeof a char *,
which on that system happened to be 4.

Brian