Memory Allocating problem

[boss_bhat]

Hi all ,
I am beginner to C programming. I have a defined astructure like the
following, and i am using aliases for the different data types in the
structure,

typedef struct _NAME_INFO {
struct _NAME_INFO *Next;
ULONG LastId;
ULONG Id;
PVOID Value;
BOOLEAN Used;
USHORT CodePage;
WCHAR *Name;
} NAME_INFO, *PNAME_INFO;

When I allocate memory to the defined structure, in the following way,

p = (PNAME_INFO)mal loc(sizeof(PNAM E_INFO)+ (wcslen(Name) + 1 ) *
sizeof( WCHAR ));
p->Name = (WCHAR *) malloc(( wcslen(Name) + 1 ) * sizeof( WCHAR ));
It core dumps when allocating memory to the pointer to wide character
*Name. Can somebody explain how to overcome this problem.

Thanks in Advance

Prasanna Bhat Mavinkuli

[Rajan]

I think you should replace sizeof(PNAME_IN FO) to sizeof(NAME_INF O).

[manoj1978]

bo*******@yahoo .co.in wrote:

Hi all ,
I am beginner to C programming. I have a defined astructure like the
following, and i am using aliases for the different data types in the
structure,

typedef struct _NAME_INFO {
struct _NAME_INFO *Next;
ULONG LastId;
ULONG Id;
PVOID Value;
BOOLEAN Used;
USHORT CodePage;
WCHAR *Name;
} NAME_INFO, *PNAME_INFO;

When I allocate memory to the defined structure, in the following way,

p = (PNAME_INFO)mal loc(sizeof(PNAM E_INFO)+ (wcslen(Name) + 1 ) *
sizeof( WCHAR ));
1. remove the cast and include stdlib.h
2. use p = malloc(sizeof(N AME_INFO));
3. verify p is not NULL before using.
p->Name = (WCHAR *) malloc(( wcslen(Name) + 1 ) * sizeof( WCHAR ));
It core dumps when allocating memory to the pointer to wide character
*Name. Can somebody explain how to overcome this problem.

Thanks in Advance

Prasanna Bhat Mavinkuli

[CBFalconer]

Rajan wrote:

I think you should replace sizeof(PNAME_IN FO) to sizeof(NAME_INF O).

I think you should replace your newsreader with something that can
create proper quotes, or should pay attention to my sig below.
What you wrote is useless in isolation.

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson

[CBFalconer]

bo*******@yahoo .co.in wrote:

I am beginner to C programming. I have a defined astructure like
the following, and i am using aliases for the different data types
in the structure,

typedef struct _NAME_INFO {
struct _NAME_INFO *Next;
ULONG LastId;
ULONG Id;
PVOID Value;
BOOLEAN Used;
USHORT CodePage;
WCHAR *Name;
} NAME_INFO, *PNAME_INFO;

When I allocate memory to the defined structure, in the following way,

p = (PNAME_INFO)mal loc(sizeof(PNAM E_INFO)+ (wcslen(Name) + 1 ) * sizeof( WCHAR ));
p->Name = (WCHAR *) malloc(( wcslen(Name) + 1 ) * sizeof( WCHAR ));
It core dumps when allocating memory to the pointer to wide character
*Name. Can somebody explain how to overcome this problem.

Try writing:

if (NULL == (p = malloc(sizeof *p))) {
/* take corrective action, no memory */;
}
else if (NULL ==
(p->Name = malloc((sizeof *(p->Name)) * (1+wcslen(Name) )) {
/* take corrective action, no memory */
}
else {
/* allocations successful, enjoy */
}

The only thing that has a need for a varying length is the Name
field. The only thing the casts are doing for you is suppressing
helpful compiler error messages. You didn't show the declaration
for Name, so that may also be uninitialized.

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson

[Keith Thompson]

bo*******@yahoo .co.in writes:

I am beginner to C programming. I have a defined astructure like the
following, and i am using aliases for the different data types in the
structure,

typedef struct _NAME_INFO {
struct _NAME_INFO *Next;
ULONG LastId;
ULONG Id;
PVOID Value;
BOOLEAN Used;
USHORT CodePage;
WCHAR *Name;
} NAME_INFO, *PNAME_INFO;
Why are you using all these aliases? If PVOID is supposed to be a
pointer to void, just use void*; if it isn't, the name PVOID is
misleading.

There's some debate as to whether typedefs for structs are a good
idea. Some argue that just using "struct foo" directly is clearer
than using a typedef name. A typedef hides the fact that the type is
a struct. There are cases where you want to do this (e.g., the type
FILE in <stdio.h>), but this isn't such a case.

The name _NAME_INFO is potentially a problem. Many identifiers
starting with an underscore are reserved to the implementation. (The
rule is a bit more complicated, and has to do with what character
follows the initial underscore, but it's best to avoid declaring
things with leading underscores altogether.)

If you're using a C99 compiler, you can use type "bool" if you have
"#include <stdbool.h>". Otherwise, you can define type "bool"
yourself. One approach is:

#if __STDC_VERSION_ _ >= 199901L
#include <stdbool.h>
#else
typedef int bool; /* or typedef char bool; */
#define false 0
#define true 1
#endif

So, given the above definition of "bool", and assuming I'm guessing
correctly what your aliases mean, and assuming "#include <stddef.h>"
for the definition of wchar_t, I'd declare your structure as follows:

struct Name_Info {
struct name_info *Next;
unsigned long LastId;
unsigned long Id;
void *Value;
bool Used;
unsigned short CodePage;
wchar_t *Name;
};

If you want to be able to refer to the type as "Name_Info" rather than
"struct Name_Info", you can do this:

typedef struct Name_Info {
/* member declarations as above */
} Name_Info;

Structure tags and typedef names are in separate namespaces, so
there's no need to use distinct identifiers. However, I probably
wouldn't bother with the typedef; instead, I'd use "struct Name_Info"
directly.

I definitely wouldn't create a typedef for a pointer to a struct
Name_Info. Hiding the fact that something is a pointer is likely to
cause confusion.
When I allocate memory to the defined structure, in the following way,

p = (PNAME_INFO)mal loc(sizeof(PNAM E_INFO)+ (wcslen(Name) + 1 ) *
sizeof( WCHAR ));
p->Name = (WCHAR *) malloc(( wcslen(Name) + 1 ) * sizeof( WCHAR ));
It core dumps when allocating memory to the pointer to wide character
*Name. Can somebody explain how to overcome this problem.

If you're allocating an object of type "struct Name_Info", you just
want to allocate the size of the struct. You could possibly allocate
a "struct Name_Info" and the wide string to be pointed to by the Name
member in a single chunk, but getting the alignment right would be
difficult -- and since you're allocating space for p->Name separately,
that's obviously not what you're trying to do anyway.

Casting the result of malloc() is unnecessary and can mask the error
of forgetting the "#include <stdlib.h>".

So here's how I'd do the allocations:

Name_Info *p;
p = malloc(sizeof *p);
if (p == NULL) {
/* error handling */
}
p->Name = malloc((wcslen( Name) + 1) * sizeof wchar_t);
if (p->Name == NULL) {
/* error handling */
}

In the first malloc(), note that the argument is "sizeof *p". By
referring to the pointer object and not to its type, we avoid problems
if the type is changed later on. Since the expression "*p" is the
operand of a sizeof, it's not evaluated; it's used only to determine
the size of the type p points to -- which is exactly what we want.

I'm assuming that the error handling code will bail out of whatever
context you're in, so you won't attempt to assign to p->Name unless
you know that the allocation for p has already succeeded.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[John Bode]

bo*******@yahoo .co.in wrote:

Hi all ,
I am beginner to C programming. I have a defined astructure like the
following, and i am using aliases for the different data types in the
structure,

Unless you have a really good reason for using typedef names for
primitive data types (for example, you know USHORT will *always* be a
particular size), use the regular type names, i.e., change
typedef struct _NAME_INFO {
struct _NAME_INFO *Next;
ULONG LastId;
ULONG Id;
PVOID Value;
BOOLEAN Used;
USHORT CodePage;
WCHAR *Name;
} NAME_INFO, *PNAME_INFO;

to

typedef struct NAME_INFO { // don't use leading underscore
struct NAME_INFO *Next;
unsigned long LastId;
unsigned long Id;
void *Value; // I'm assuming PVOID == void*
bool Used; // assuming bool type is available; otherwise int will
work
unsigned short CodePage;
wchar_t *Name;
} NAME_INFO;

While I'll argue with Keith over the virtue of creating a typedef name
for the struct type (I do it all the time, but I also create accessor
functions to do member assignments), I won't argue about creating a
typedef name for pointer types; don't do it. It will just cause
heartburn. I know there are a lot of examples of the practice,
especially in MS Windows code, but it winds up causing more problems
than it allegedly solves.

Just use NAME_INFO* when you need a pointer.
When I allocate memory to the defined structure, in the following way,

p = (PNAME_INFO)mal loc(sizeof(PNAM E_INFO)+ (wcslen(Name) + 1 ) *
sizeof( WCHAR ));
p->Name = (WCHAR *) malloc(( wcslen(Name) + 1 ) * sizeof( WCHAR ));
It core dumps when allocating memory to the pointer to wide character
*Name. Can somebody explain how to overcome this problem.

You're allocating p incorrectly. Use the following (assuming p is type
NAME_INFO *):

p = malloc(sizeof *p);

Note that I'm using sizeof on the actual object I'm allocating to, not
its type. Also notice I'm not casting the result of malloc(). You
don't need to cast, and doing so can mask a compile time error if you
forget to #include <stdlib.h>.

To allocate the Name member, do the following.

if (p) // make sure p was successfully allocated
{
p->Name = malloc(sizeof *(p->Name) * wcslen(Name));
}
Thanks in Advance

Prasanna Bhat Mavinkuli

[Keith Thompson]

"John Bode" <jo*******@my-deja.com> writes:
[snip]

Unless you have a really good reason for using typedef names for
primitive data types (for example, you know USHORT will *always* be a
particular size), use the regular type names, i.e., change

To expand on that a bit, USHORT is obviously an abbreviation for
"unsigned short". If it *is* unsigned short, it's much clearer to use
unsigned short directly. If it *isn't* unsigned short, the name is
grossly misleading.

If you specifically want, say, a 16-bit unsigned type, a name like
"uint16_t" is much clearer (and that's exactly the name used in C99's
<stdint.h>). But be aware that not all implementations necessarily
have a 16-bit unsigned type.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.