Pointer-to-pointer-to-pointer question

M

masood.iqbal

The code example below shows the dynamic allocation of a 2D array. I
must admit that it took quite a while for me to get there (I already
have another posting to that effect), but I am glad that I finally got
it working. Now here's the problem:

I am able to get the 2D array dynamically allocated correctly as long
as I am doing it "in-line" (i.e. without invoking any function). The
moment I try to do it in another function, I get a core dump. Any help
will be appreciated.

Since this function is expected to update a pointer-to-pointer type, I
am actually passing the pointer-to-pointer-to-pointer type to the
function. What am I missing here?

You can see that the source code works correctly when I am perform 2D
array initialization "in-line" (i.e. by not invoking a function
call) simply by un-commenting the line

/* #define INLINE_INIT */

Masood

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

/*#define INLINE_INIT*/

#define MAXROWS 3
#define MAXCOLS 5


void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
/* C++: *tblPtr = new (int)[numCols]; */
}


main()
{
int startVal = 5;

int **tbl;

#ifdef INLINE_INIT
tbl = (int **)malloc(MAXROWS*sizeof(int*));
/* C++ : tbl = new (int*)[MAXROWS]; */

for(size_t i = 0; i < MAXCOLS; i++)
tbl = (int *)malloc(MAXCOLS*sizeof(int));
/* C++: tbl = new (int)[MAXCOLS]; */
#else
buildTbl(&tbl, MAXROWS, MAXCOLS);
#endif

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
tbl[row][col] = startVal++;

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
printf("Row: %d, Col: %d => %d\n",
row, col, tbl[row][col]);
return 0;
}
 
T

Tobias Oed

Masood

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

/*#define INLINE_INIT*/

#define MAXROWS 3
#define MAXCOLS 5


void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
Click to expand...


(*tblPtr) = ...

Note that x = malloc(n * sizeof *x);
is preferred to x = (type *) malloc(n * sizeof(type);
You should check that malloc doesn't return NULL.
You could change the return type of buildTbl to int ** and
return the array that way instead of through the parameter.

Do I remember that stuff Ok? Nice to see most of you are
still here!

Tobias
/* C++: *tblPtr = new (int)[numCols]; */
}


main()
{
int startVal = 5;

int **tbl;

#ifdef INLINE_INIT
tbl = (int **)malloc(MAXROWS*sizeof(int*));
/* C++ : tbl = new (int*)[MAXROWS]; */

for(size_t i = 0; i < MAXCOLS; i++)
tbl = (int *)malloc(MAXCOLS*sizeof(int));
/* C++: tbl = new (int)[MAXCOLS]; */
#else
buildTbl(&tbl, MAXROWS, MAXCOLS);
#endif

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
tbl[row][col] = startVal++;

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
printf("Row: %d, Col: %d => %d\n",
row, col, tbl[row][col]);
return 0;
}
Click to expand...
 
M

Michael Mair

The code example below shows the dynamic allocation of a 2D array. I
must admit that it took quite a while for me to get there (I already
have another posting to that effect), but I am glad that I finally got
it working. Now here's the problem:

I am able to get the 2D array dynamically allocated correctly as long
as I am doing it "in-line" (i.e. without invoking any function). The
moment I try to do it in another function, I get a core dump. Any help
will be appreciated.

Since this function is expected to update a pointer-to-pointer type, I
am actually passing the pointer-to-pointer-to-pointer type to the
function. What am I missing here?
Click to expand...

You can get lost with the intricacies of C.
One "trick" to get around it: Use a local "pointer-to-pointer type"
variable to do as you always did and assign its value to the object
your "pointer-to-pointer-to-pointer type" argument points to.
-> Essentially the same code, one additional line.
(You perform all operations on *tblPtr which essentially means that
you could also work with a int **tbl and put *tblPtr=tbl at the end
if everything worked out)
If you follow Tobias's advice, it becomes even easier.
You can see that the source code works correctly when I am perform 2D
array initialization "in-line" (i.e. by not invoking a function
call) simply by un-commenting the line

/* #define INLINE_INIT */

Masood
Click to expand...
[snip: Code]

Tobias Oed has already mentioned everything I can see.
Follow _every_ point of his advice regarding malloc().

BTW: Thank you for providing a good minimal example!


Cheers
Michael
 
C

Christian Kandeler

void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
Click to expand...

The condition should be i < numRows.
*tblPtr = (int *)malloc(numCols*sizeof(int));
Click to expand...


This will access memory not belonging to tblPtr if i is greater than 0.
Change it to the following:
(*tblPtr) = malloc(numCols*sizeof(int));
It's also a good idea to check the return value of malloc().


Christian
 
A

Amit

/* #define INLINE_INIT */

Masood
Click to expand...
void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
Click to expand...

(*tblPtr) = (int *)malloc(numCols*sizeof(int)); <------
/* C++: *tblPtr = new (int)[numCols]; */
}
Click to expand...


I am not a master of C, but what i can see is the problem of precedence.
And yes you should check the return value of malloc also.
 
J

Jonathan Burd

The code example below shows the dynamic allocation of a 2D array. I
must admit that it took quite a while for me to get there (I already
have another posting to that effect), but I am glad that I finally got
it working. Now here's the problem:

I am able to get the 2D array dynamically allocated correctly as long
as I am doing it "in-line" (i.e. without invoking any function). The
moment I try to do it in another function, I get a core dump. Any help
will be appreciated.

Since this function is expected to update a pointer-to-pointer type, I
am actually passing the pointer-to-pointer-to-pointer type to the
function. What am I missing here?

You can see that the source code works correctly when I am perform 2D
array initialization "in-line" (i.e. by not invoking a function
call) simply by un-commenting the line

/* #define INLINE_INIT */

Masood

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

/*#define INLINE_INIT*/

#define MAXROWS 3
#define MAXCOLS 5


void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
Click to expand...

Return an int** instead.
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
/* C++: *tblPtr = new (int)[numCols]; */
}
Click to expand...


Instead of allocating memory so many times, a better method
would be to call malloc only once to allocate all the
memory you need. This would improve the efficiency of
your code. (See example below.)
main()
{
Click to expand...

int main (void)... is better.
int startVal = 5;

int **tbl;

#ifdef INLINE_INIT
tbl = (int **)malloc(MAXROWS*sizeof(int*));
Click to expand...

Don't cast malloc or any other function that
returns a pointer unless you really know what you are
doing.

Should be tbl = malloc(...);
/* C++ : tbl = new (int*)[MAXROWS]; */

for(size_t i = 0; i < MAXCOLS; i++)
Click to expand...

for (size_t i ... ) won't work with C'89.
Avoid it.
tbl = (int *)malloc(MAXCOLS*sizeof(int));
Click to expand...


See above.
/* C++: tbl = new (int)[MAXCOLS]; */
#else
buildTbl(&tbl, MAXROWS, MAXCOLS);
#endif

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
tbl[row][col] = startVal++;

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
printf("Row: %d, Col: %d => %d\n",
row, col, tbl[row][col]);
return 0;
}
Click to expand...


I've seen code that did not have proper braces
to block the scope of such for loops. The problem
I faced was that the formatting of the code was
lost and the code that followed that for
statement was a large "if statement".
Let me tell you, it wasn't easy to reformat that
code manually. Properly placing braces can save you a lot of time.
Use them.

Additionally, I don't see why, after you've been told before,
you don't use a proper NG client to post. It is *difficult*
to read unindented code. Please use a good client.


/* grid_alloc.c. */
#include <stdlib.h>

int **grid_alloc (size_int rows, size_int cols)
{
int **mem; /* allocated memory */
int *p; /* temporary ptr */
size_int r; /* row index */

mem = malloc (rows * cols * sizeof(int) + rows * sizeof(*mem));
if(mem != NULL)
{
/* memory layout:
* [ pointers: one pointer to each row of data ][ data ]
*/
for (r = 0, p = (int*)mem + rows; r < rows; ++r, p += cols)
{
mem[r] = p;
}
}

return mem;
}

/* ... */

This allows you to use array syntax to address elements.
Don't forget to use free() when you're done with it, however.

Regards,
Jonathan.
 
J

Jonathan Burd

Jonathan said:
The code example below shows the dynamic allocation of a 2D array. I
must admit that it took quite a while for me to get there (I already
have another posting to that effect), but I am glad that I finally got
it working. Now here's the problem:

I am able to get the 2D array dynamically allocated correctly as long
as I am doing it "in-line" (i.e. without invoking any function). The
moment I try to do it in another function, I get a core dump. Any help
will be appreciated.

Since this function is expected to update a pointer-to-pointer type, I
am actually passing the pointer-to-pointer-to-pointer type to the
function. What am I missing here?

You can see that the source code works correctly when I am perform 2D
array initialization "in-line" (i.e. by not invoking a function
call) simply by un-commenting the line

/* #define INLINE_INIT */

Masood

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

/*#define INLINE_INIT*/

#define MAXROWS 3
#define MAXCOLS 5


void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
Click to expand...


Return an int** instead.
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
/* C++: *tblPtr = new (int)[numCols]; */
}
Click to expand...



Instead of allocating memory so many times, a better method
would be to call malloc only once to allocate all the
memory you need. This would improve the efficiency of
your code. (See example below.)
main()
{
Click to expand...


int main (void)... is better.
int startVal = 5;

int **tbl;

#ifdef INLINE_INIT
tbl = (int **)malloc(MAXROWS*sizeof(int*));
Click to expand...


Don't cast malloc or any other function that
returns a pointer unless you really know what you are
doing.

Should be tbl = malloc(...);
/* C++ : tbl = new (int*)[MAXROWS]; */

for(size_t i = 0; i < MAXCOLS; i++)
Click to expand...


for (size_t i ... ) won't work with C'89.
Avoid it.
tbl = (int *)malloc(MAXCOLS*sizeof(int));
Click to expand...



See above.
/* C++: tbl = new (int)[MAXCOLS]; */
#else
buildTbl(&tbl, MAXROWS, MAXCOLS);
#endif

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
tbl[row][col] = startVal++;

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
printf("Row: %d, Col: %d => %d\n",
row, col, tbl[row][col]);
return 0;
}
Click to expand...


I've seen code that did not have proper braces
to block the scope of such for loops. The problem
I faced was that the formatting of the code was
lost and the code that followed that for
statement was a large "if statement".
Let me tell you, it wasn't easy to reformat that
code manually. Properly placing braces can save you a lot of time.
Use them.

Additionally, I don't see why, after you've been told before,
you don't use a proper NG client to post. It is *difficult*
to read unindented code. Please use a good client.


/* grid_alloc.c. */
#include <stdlib.h>

int **grid_alloc (size_int rows, size_int cols)
{
int **mem; /* allocated memory */
int *p; /* temporary ptr */
size_int r; /* row index */
Click to expand...


Typos. Should be size_int should be size_t. The editor
I was using to search-n-replace goofed. :|
mem = malloc (rows * cols * sizeof(int) + rows * sizeof(*mem));
if(mem != NULL)
{
/* memory layout:
* [ pointers: one pointer to each row of data ][ data ]
*/
for (r = 0, p = (int*)mem + rows; r < rows; ++r, p += cols)
{
mem[r] = p;
}
}

return mem;
}

/* ... */

This allows you to use array syntax to address elements.
Don't forget to use free() when you're done with it, however.

Regards,
Jonathan.
Click to expand...
 
B

Barry Schwarz

The code example below shows the dynamic allocation of a 2D array. I
must admit that it took quite a while for me to get there (I already
have another posting to that effect), but I am glad that I finally got
it working. Now here's the problem:

I am able to get the 2D array dynamically allocated correctly as long
as I am doing it "in-line" (i.e. without invoking any function). The
moment I try to do it in another function, I get a core dump. Any help
will be appreciated.

Since this function is expected to update a pointer-to-pointer type, I
am actually passing the pointer-to-pointer-to-pointer type to the
function. What am I missing here?

You can see that the source code works correctly when I am perform 2D
array initialization "in-line" (i.e. by not invoking a function
call) simply by un-commenting the line

/* #define INLINE_INIT */

Masood

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

/*#define INLINE_INIT*/

#define MAXROWS 3
#define MAXCOLS 5


void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
Click to expand...


This line is interpreted as
*(tblPtr) = ...

You want
(*tblPtr) = ...

The parentheses are not optional.
/* C++: *tblPtr = new (int)[numCols]; */
}


main()
{
int startVal = 5;

int **tbl;

#ifdef INLINE_INIT
tbl = (int **)malloc(MAXROWS*sizeof(int*));
/* C++ : tbl = new (int*)[MAXROWS]; */

for(size_t i = 0; i < MAXCOLS; i++)
tbl = (int *)malloc(MAXCOLS*sizeof(int));
/* C++: tbl = new (int)[MAXCOLS]; */
#else
buildTbl(&tbl, MAXROWS, MAXCOLS);
#endif

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
tbl[row][col] = startVal++;

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
printf("Row: %d, Col: %d => %d\n",
row, col, tbl[row][col]);
return 0;
}
Click to expand...




<<Remove the del for email>>
 
B

Barry Schwarz

The code example below shows the dynamic allocation of a 2D array. I
must admit that it took quite a while for me to get there (I already
have another posting to that effect), but I am glad that I finally got
it working. Now here's the problem:

I am able to get the 2D array dynamically allocated correctly as long
as I am doing it "in-line" (i.e. without invoking any function). The
moment I try to do it in another function, I get a core dump. Any help
will be appreciated.

Since this function is expected to update a pointer-to-pointer type, I
am actually passing the pointer-to-pointer-to-pointer type to the
function. What am I missing here?

You can see that the source code works correctly when I am perform 2D
array initialization "in-line" (i.e. by not invoking a function
call) simply by un-commenting the line

/* #define INLINE_INIT */

Masood

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

/*#define INLINE_INIT*/

#define MAXROWS 3
#define MAXCOLS 5


void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
Click to expand...


This line is interpreted as
*(tblPtr) = ...

You want
(*tblPtr) = ...

The parentheses are not optional.
/* C++: *tblPtr = new (int)[numCols]; */
}


main()
{
int startVal = 5;

int **tbl;

#ifdef INLINE_INIT
tbl = (int **)malloc(MAXROWS*sizeof(int*));
/* C++ : tbl = new (int*)[MAXROWS]; */

for(size_t i = 0; i < MAXCOLS; i++)
tbl = (int *)malloc(MAXCOLS*sizeof(int));
/* C++: tbl = new (int)[MAXCOLS]; */
#else
buildTbl(&tbl, MAXROWS, MAXCOLS);
#endif

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
tbl[row][col] = startVal++;

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
printf("Row: %d, Col: %d => %d\n",
row, col, tbl[row][col]);
return 0;
}
Click to expand...




<<Remove the del for email>>
 
B

Barry Schwarz

The code example below shows the dynamic allocation of a 2D array. I
must admit that it took quite a while for me to get there (I already
have another posting to that effect), but I am glad that I finally got
it working. Now here's the problem:

I am able to get the 2D array dynamically allocated correctly as long
as I am doing it "in-line" (i.e. without invoking any function). The
moment I try to do it in another function, I get a core dump. Any help
will be appreciated.

Since this function is expected to update a pointer-to-pointer type, I
am actually passing the pointer-to-pointer-to-pointer type to the
function. What am I missing here?

You can see that the source code works correctly when I am perform 2D
array initialization "in-line" (i.e. by not invoking a function
call) simply by un-commenting the line

/* #define INLINE_INIT */

Masood

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

/*#define INLINE_INIT*/

#define MAXROWS 3
#define MAXCOLS 5


void
buildTbl(int ***tblPtr, size_t numRows, size_t numCols)
{
*tblPtr = (int **)malloc(numRows*sizeof(int*));
/* C++ : *tblPtr = new (int*)[numRows]; */

for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
Click to expand...


This line is interpreted as
*(tblPtr) = ...

You want
(*tblPtr) = ...

The parentheses are not optional.
/* C++: *tblPtr = new (int)[numCols]; */
}


main()
{
int startVal = 5;

int **tbl;

#ifdef INLINE_INIT
tbl = (int **)malloc(MAXROWS*sizeof(int*));
/* C++ : tbl = new (int*)[MAXROWS]; */

for(size_t i = 0; i < MAXCOLS; i++)
tbl = (int *)malloc(MAXCOLS*sizeof(int));
/* C++: tbl = new (int)[MAXCOLS]; */
#else
buildTbl(&tbl, MAXROWS, MAXCOLS);
#endif

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
tbl[row][col] = startVal++;

for(size_t row = 0; row < MAXROWS; row++)
for(size_t col = 0; col < MAXCOLS; col++)
printf("Row: %d, Col: %d => %d\n",
row, col, tbl[row][col]);
return 0;
}
Click to expand...




<<Remove the del for email>>
 
C

Chris Torek

[Side note: the article to which I am posting a follow-up got posted
three times, with three different message-IDs. Software glitch, I
presume.]
for(size_t i = 0; i < numCols; i++)
*tblPtr = (int *)malloc(numCols*sizeof(int));
Click to expand...
Click to expand...


This line is interpreted as
*(tblPtr) = ...

You want
(*tblPtr) = ...

The parentheses are not optional.
Click to expand...


Right. Note, however, that if for some reason one finds the
parenthesized form objectionable, (*p) can be rewritten as
p[0]. (Likewise, p->field can be rewritten as p[0].field.)

I think this particular rewrite is misleading and would avoid
it myself. In fact, I would probably use code of the form:

nonvoid_return_type some_fn(T ***ptable, ...) {
T **table;

... do all the local work with table and table ...

*ptable = table;
return other_useful_return_value;
}

or, as in this particular case (where the function had "void" as
its return type), just return a "T **" value and eliminate the
three-levels-deep pointer argument.
 

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