asked on

signal trapping in linux

Consider the following code
void alter();
main()
{
int i;
signal(SIGINT,alter);
for(i=0;i<1000;i++)
printf("Uninterrupted\n");
}
void alter()
{
printf("Interrupted");
sleep(2);
}
When I try to beak using Ctrl+C it does not go into the alter sub function.
However if use a sleep function within the for loop it connects to the alter sub function.
How can I make the SIGINT signal to be trapped in the first case.

sobhan102398

ASKER

I tried it again today.
I observed it closely and this is what i got,
When I press Ctrl + C the signal is being generated but very late and printf("Interrupted) print's so fast that is difficult to track. I used the following
void alter()
{
printf("Interrupted\n");
sleep(2);
}
The sleep doesn't work and printf statement is executed like as I have stated above.

Also when I try out this for the signal signal(SIGPWR,alter1);
void alter()
{
create("file",0700);
exit(1);
}
I switch off the computer and when I on it again the file is not created i.e SIGPWR is not trapped.Why is it so.

One last question regd signal trapping
can i trap the signal for the following code
while((no=read(f,buff,1024))>0)
write(f1,buff,no);
I used sleep(1) like this
while((no=read(f,buff,1024))>0)
{
write(f1,buff,no);
sleep(1);
}
Now I was able to trap it.
How can I trap the signal in the first case without using sleep.
Is read and write non blocking.

ozo

What are you trying to do?
I'd expect
for(i=0;i<1000;i++) printf("Uninterrupted\n");
to run so fast that the ouput buffer is full before you can type a Ctrl+C
and even if you can type Ctrl+C fast enough, the buffer won't be flushed before the next "\n" is printed.

sobhan102398

ASKER

I am trying to copy one file to another. When I interrupt the process, I should save till that point by calling alter where I write some code for that. However I am unable to trap the signal SIGINT within this loop
while((no=read(f,buff,1024))>0)
write(f1,buff,no);
I have to use sleep inorder to capture the signal.I want my program to trap the signal without using sleep

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rdelfino

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sobhan102398

ASKER

Also we connect from dos to linux using a ncsa rlogin program. Whenever we press Alt+a we get a new session . Or we press Alt+x we close the session.
i.e The program is written such that when a person presses Alt+a he goes to a particular function. So they are trapping keys at any point.
How can I simulate this.
I need to go to a particular function from whatever state I am when I press a selected hot key. Using this I can bypass the above problem(printf pob) by using another set of keys
other than Ctlr+C.
How do I do this