Hi Can anyone help me out with this! thesis problem PLease :(

[Alan]

Hi, I m working on thesis on mechanical engieering. I m using C to
solve a particualr problem.
I have this code underneath which is giving me problems:


for (t=0;t<=maxt;t++){
printf("testing loop\n");
for (x=0;x<=maxx;x++){
for(y=0;y<=maxy;y++){
printf(".");
if (x==0){
printf("hello");
tp[t][x][y]=tp[t-1][x+1][y]; //Problem here
}
}}}

I simplified as much as possible.

the problem is at the indicated line.

tp is a 3 dimensional array, allocated using calloc().

Im filling array with some values.

When the program arrives at this line, it exits and prints only one "."
and one "hello".

But without this line, it prints "." and "hello" according to the loop
iterations.

Can anhybody indetify the problem please ?

Im using Lcc win as a compiler and the array is a pointer to pointer to
pointer i.e. --> double ***tp

Thanks!!!

Alan

 

[santosh]

Hi, I m working on thesis on mechanical engieering. I m using C to
solve a particualr problem.
I have this code underneath which is giving me problems:


for (t=0;t<=maxt;t++){
printf("testing loop\n");
for (x=0;x<=maxx;x++){
for(y=0;y<=maxy;y++){
printf(".");
if (x==0){
printf("hello");
tp[t][x][y]=tp[t-1][x+1][y]; //Problem here
}
}}}

I simplified as much as possible.

When t == 0, tp[t-1]... is accessing an area of memory which is not a
part of the array. This is probably what's causing UB.

 

[David Resnick]

Hi, I m working on thesis on mechanical engieering. I m using C to
solve a particualr problem.
I have this code underneath which is giving me problems:


for (t=0;t<=maxt;t++){
printf("testing loop\n");
for (x=0;x<=maxx;x++){
for(y=0;y<=maxy;y++){
printf(".");
if (x==0){
printf("hello");
tp[t][x][y]=tp[t-1][x+1][y]; //Problem here
}
}}}

I simplified as much as possible.

the problem is at the indicated line.

tp is a 3 dimensional array, allocated using calloc().

Im filling array with some values.

When the program arrives at this line, it exits and prints only one "."
and one "hello".

But without this line, it prints "." and "hello" according to the loop
iterations.

I'd make the "hello" line display t, x, and y. It might be
illuminating to see where/why this fails.

For example, what if x = 0 and t = 0? Looks like an out of bounds
access with tp[-1][1][y]... If maxx is also 0 the 1 is also out of
bounds.

Also, if you calloc standard doesn't guarantee that all bits 0 is a
valid float I believe, assign to 0.0 in a loop.

Also, if you calloc it this array as one big block, you can't use
multiple subscripts. See http://c-faq.com/aryptr/dynmuldimary.html.

-David

 

[Eric Sosman]

santosh wrote On 11/20/06 13:47,:

Hi, I m working on thesis on mechanical engieering. I m using C to
solve a particualr problem.
I have this code underneath which is giving me problems:


for (t=0;t<=maxt;t++){
printf("testing loop\n");
for (x=0;x<=maxx;x++){
for(y=0;y<=maxy;y++){
printf(".");
if (x==0){
printf("hello");
tp[t][x][y]=tp[t-1][x+1][y]; //Problem here
}
}}}

I simplified as much as possible.

When t == 0, tp[t-1]... is accessing an area of memory which is not a
part of the array. This is probably what's causing UB.

Another potential trouble spot is the [x+1] index,
once the program gets up to x == maxx.

Yet another potential trouble spot is the use of <=
instead of < in the loop conditions. It might be all
right, but only if the array dimensions are a little
larger than might be expected. (For example, if maxt
is 5, the leftmost dimension requires at least 6 elements:
0,1,2,3,4,5 inclusive are six distinct index values.)

 

[Martin Ambuhl]

for (t=0;t<=maxt;t++){
printf("testing loop\n");
for (x=0;x<=maxx;x++){
for(y=0;y<=maxy;y++){
printf(".");
if (x==0){
printf("hello");
tp[t][x][y]=tp[t-1][x+1][y]; //Problem here
}
}}}

In the first time through the loop t==0 so the assignment refers to
tp[-1][x+1][y], which is out of bounds.