Problem with structs and strings in C

[Bleakcabal]

Keep in mind my program is written in C ( not C++ ).

I have a function which takes two structs as parameters is supposed to
put the values of the source struct in the destination struct. Only
the first character is put into the destination struct for each field.
void AssignValues(in fo *source, info *destination)
{
*destination->name = *source->name;
*destination->class = *source->class;
*destination->alignment = *source->alignment;
}
Name, class and alignment are defined as
char variableName[20];
In a struct called info.

If my source is : test, TEST example
The destination is : t, T, e

Why is that ?

Thanks for your help.

[Stig Brautaset]

Bleakcabal <bl********@gdn mail.net> wrote:

Keep in mind my program is written in C ( not C++ ).

I have a function which takes two structs as parameters is supposed to
put the values of the source struct in the destination struct. Only
the first character is put into the destination struct for each field.

Use strcpy() or similar to copy strings.

Stig

--
brautaset.org

[pete]

Bleakcabal wrote:

Keep in mind my program is written in C ( not C++ ).

I have a function which takes two structs as parameters is supposed to
put the values of the source struct in the destination struct. Only
the first character is put into the destination struct for each field.

void AssignValues(in fo *source, info *destination)
{
*destination->name = *source->name;
*destination->class = *source->class;
*destination->alignment = *source->alignment;
}

void AssignValues(in fo *source, info *destination)
{
*destination = *source;
}

--
pete

[Leor Zolman]

On 9 Feb 2004 05:12:27 -0800, bl********@gdnm ail.net (Bleakcabal)
wrote:

Keep in mind my program is written in C ( not C++ ).

I have a function which takes two structs as parameters is supposed to
put the values of the source struct in the destination struct. Only
the first character is put into the destination struct for each field.
void AssignValues(in fo *source, info *destination)
{
*destination->name = *source->name;
*destination->class = *source->class;
*destination->alignment = *source->alignment;
}
Name, class and alignment are defined as
char variableName[20];
In a struct called info.

If my source is : test, TEST example
The destination is : t, T, e

Why is that ?

A few solutions have been posted, and they're correct given your
context so far [just be careful: if you use strcpy, make sure the
source strings have been initialized properly; if you use structure
assignment, keep in mind that it will cease to work correctly if you
were to change the type of any of your data members from arrays to
pointers...and that may very well end up being your next step ;-) ]

Another hint: if code involving arrays or pointers doesn't work right,
carefully check the /types/ of the expressions involved. In this case,
all six main expressions are of the form:
* array
Remember that array names used in /most/ expressions "decay" to being
a pointer to their first element, so type-wise that equates to:
* (char *)
The *'s cancel leaving "char" as the type. Hence the char-to-char
assignment.

Rule of thumb: There's no single C assignment expression that can
directly copy an array (you'd have to cheat and embed the array in a
structure and copy the structure, which is why Pete's solution works
so nicely in this case.)
-leor

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Michael]

Hi Pete,
this seems to be similar to a
question I posted earlier. But
won't

*destination = *source;

just copy the adresses of the array,
s.th like a non-deep copy? So both
structs share the strings?
Kind Regards,
Michael

[Leor Zolman]

On Mon, 09 Feb 2004 15:49:13 +0100, Michael <mi****@gmx.net > wrote:

Hi Pete,
this seems to be similar to a
question I posted earlier. But
won't

*destination = *source;

just copy the adresses of the array,
s.th like a non-deep copy? So both
structs share the strings?

Nope, there's no problem here because we're copying values, not
addresses. If the data members of the structure were pointers, /then/
we'd be copying addresses and inviting all sorts of undefined behavior
down the road when the program begins freeing them (or at least chaos
before that if the program expects the data being pointed to by the
two separate pointers to be distinct.)
-leor

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[CBFalconer]

Michael wrote:

this seems to be similar to a
question I posted earlier. But
won't

*destination = *source;

just copy the adresses of the array,
s.th like a non-deep copy? So both
structs share the strings?

You removed context quotations, including all definitions. IIR
you defined the fields within the structures as arrays, not as
pointers, so there is nothing deep to copy.

--
Chuck F (cb********@yah oo.com) (cb********@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

[Al Bowers]

CBFalconer wrote:

Michael wrote:

this seems to be similar to a
question I posted earlier. But
won't

*destination = *source;

just copy the adresses of the array,
s.th like a non-deep copy? So both
structs share the strings?

You removed context quotations, including all definitions. IIR
you defined the fields within the structures as arrays, not as
pointers, so there is nothing deep to copy.

The op definitions are a little unclear. There is some evidence
that the members are typedef char array of 20 characters. If this
is the case, struct assignment will be safe as will using function
strcpy.

Example:

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

typedef char variableName[20];
typedef struct info
{
variableName name;
variableName class;
variableName alignment;
}info;

info init = {"No data","No data", "No data"};

void AssignValues1(c onst info *source, info *destination)
{
*destination = *source;
}

void AssignValues2(c onst info *source, info *destination)
{
strcpy(destinat ion->name,source->name);
strcpy(destinat ion->class, source->class);
strcpy(destinat ion->alignment,sour ce->alignment);
return;
}

void PrintInfo(const info *p)
{
char *s = "No Data";
printf("Name: %s\nClass: %s\nAlignment: %s\n\n",
p->name,p->class,p->alignment);
}

int main(void)
{
info myinfo = {"George Washington","Po litical Science","Junio r"};
info newinfo = init;

puts("struct newinfo contents");
PrintInfo(&newi nfo);

newinfo = myinfo;
puts("Using simple assignment");
PrintInfo(&newi nfo);
newinfo = init;

puts("Using function AssignValues1") ;
AssignValues1(& myinfo, &newinfo);
PrintInfo(&newi nfo);
newinfo = init;

puts("Using function AssignValues2") ;
AssignValues2(& myinfo, &newinfo);
PrintInfo(&newi nfo);

return 0;
}
--
Al Bowers
Tampa, Fl USA
mailto: xa******@myrapi dsys.com (remove the x to send email)
http://www.geocities.com/abowers822/