Dynamically malloc'd array of structs

[Grant Austin]

What would be the correct syntax for setting up
a dynamic array of structs?

Suppose you have a struct declared:

struct relation {
FILE * binFile;
unsigned int numAttrs;
struct attrList * relAttrs; /* definition shown at end of post */
};
typedef struct relation relHeader;

The array only has to be sized once. The size is read from a text
file.

Currently I have something like this:

#include <stdio.h>
int main(void){

unsigned int relCount = 2;
relHeader Headers[relCount-1];

return 0;
}

I would prefer to be able to define Headers as:
relHeader * Headers;

And then malloc space for whatever size I need.
When I try:

Headers = malloc(sizeof(*Headers) * (relCount-1))

I get a warning and a syntax error as follows:

p3.c:26: warning: assignment makes pointer from integer without a cast
p3.c:26: syntax error before "Headers"

What I have will work well enough for my purposes. If it's something
simple I'm missing I'd like to fix it.

Regards,
Grant


struct attribute {
struct attribute * next;
unsigned int attrLen;
char type;
char * attrName;
};
typedef struct attribute attrList;

 

[Mark A. Odell]

I would prefer to be able to define Headers as:
relHeader * Headers;

And then malloc space for whatever size I need.
When I try:

Headers = malloc(sizeof(*Headers) * (relCount-1))

I get a warning and a syntax error as follows:

p3.c:26: warning: assignment makes pointer from integer without a cast
p3.c:26: syntax error before "Headers"

Because there is no prototype for malloc() and functions without
prototypes default to returning an 'int' type. Since Headers is a pointer
this warning is very helpful. This is *precisely* why we don't cast
malloc's return in C. Simply #include <stdlib.h> and the warning will go
away along with the bug you would have had regarding this.

 

[Leor Zolman]

What would be the correct syntax for setting up
a dynamic array of structs?

You're quite close.

Suppose you have a struct declared:

struct relation {
FILE * binFile;
unsigned int numAttrs;
struct attrList * relAttrs; /* definition shown at end of post */

Because attrList is already a typedef for a struct (well, it will be), this
isn't quite right. See my re-organized code below.

};
typedef struct relation relHeader;

The array only has to be sized once. The size is read from a text
file.

Currently I have something like this:

#include <stdio.h>
int main(void){

unsigned int relCount = 2;

Does "2" really mean 2 structs you'll be reading? If so, why are you
subtracting 1 below? Are you possibly confusing "size" with subscripting
expressions?

relHeader Headers[relCount-1];

return 0;
}

I would prefer to be able to define Headers as:
relHeader * Headers;

No prob.

And then malloc space for whatever size I need.
When I try:

Headers = malloc(sizeof(*Headers) * (relCount-1))

If you're malloc-ing relHeader objects, you just need to specify the size
of a relHeader object (see code below). And again, I suspect you don't want
to be subtracting that 1.

I get a warning and a syntax error as follows:

p3.c:26: warning: assignment makes pointer from integer without a cast
p3.c:26: syntax error before "Headers"

Looks like you didn't include a required header file. See my code below.

What I have will work well enough for my purposes. If it's something
simple I'm missing I'd like to fix it.

Regards,
Grant


struct attribute {
struct attribute * next;
unsigned int attrLen;
char type;
char * attrName;
};
typedef struct attribute attrList;

Here's the rolled-up version:

#include <stdio.h>
#include <stdlib.h> /* for malloc */

struct attribute {
struct attribute * next;
unsigned int attrLen;
char type;
char * attrName;
};

typedef struct attribute attrList;

struct relation {
FILE * binFile;
unsigned int numAttrs;
/* struct attrList * relAttrs; /* definition shown at end of post */
attrList * relAttrs; /* note: no 'struct' */
};
typedef struct relation relHeader;

int main(void){

unsigned int relCount = 2;

relHeader * Headers;

/* Headers = malloc(sizeof(*Headers) * (relCount-1)) */
Headers = malloc(sizeof(relHeader) * relCount);


return 0;
}

HTH,
-leor


Leor Zolman
BD Software
(e-mail address removed)
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

 

[Grant Austin]

Thanks,

Lesson learned.

-Grant

 

[Leor Zolman]

If you're malloc-ing relHeader objects, you just need to specify the size
of a relHeader object (see code below). And again, I suspect you don't want
to be subtracting that 1.

Sorry, I went brain-dead there. For some reason the * before Headers didn't
register to me. I'd usually put a space before that *, anyway...
-leor

Leor Zolman
BD Software
(e-mail address removed)
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

 

[Grant Austin]

Sorry, I went brain-dead there.

No worries, I am brain-dead most of the time.

You're right about the -1. I was thinking subscripts.

-g