1. Section 3
Syscalls, I/O, Signals
February 3rd, 2017
Taught by Josh Don

2. Signals
Signal: An asynchronous notification sent to a process to notify it that an event has occurred
When a signal is sent, the OS interrupts the process’ normal flow of execution to deliver the signal
Ex: Ctrl-C sends SIGINT

3. List of Signals

4. Run default signal handler for SIGINT!
Signals
Run default signal handler for SIGINT!
Process A
Process B
SIGINT

5. func can be a custom function, or it can be: SIG_DFL: restore default
Signal redirection
Process B
Process A
SIGINT
Run !
func
signal(SIGINT, )
func
func can be a custom function, or it can be:
SIG_DFL: restore default
SIG_IGN: ignore signal

6. Demo

7. Files vs File Descriptors
File: A computer resource that allows a program to store information durably (persists even after the program finishes executing).
File Descriptor (FD): An integer value that is used to reference a particular file. A process gets a file descriptor when it OPENs a file.
FILE: A handy wrapper around a file descriptor, adding buffering and other features
Get an FD →
int open(char *filename,…)
Get a FILE →
FILE *fopen(char *filename,…)
Get FD from FILE →
int fileno(FILE *stream);

8. File descriptors When a process starts, it get FDs 0-2 opened
Kernel File Table
stdin
stdout
Process File Table
stderr FD
a.txt
When a process starts, it get FDs 0-2 opened
b.txt 1 2

9. File descriptors cont. >> open(“b.txt”, O_CREAT|O_WR)
Kernel File Table
stdin
Process File Table
stdout
>> open(“b.txt”, O_CREAT|O_WR)
stderr FD
a.txt
b.txt 1 2 3

10. dup
The dup(int oldfd) system call creates a copy of the file descriptor oldfd, using the lowest-numbered unused file descriptor for the new descriptor.

11. Now we have another reference to stdout
dup example
Kernel File Table
stdin
>> dup(1)
Now we have another reference to stdout
Process File Table
stdout
stderr FD
a.txt
b.txt 1 2 3 4

12. dup2
The dup2(int oldfd, int newfd) system call performs the same task as dup(), but instead of using the lowest-numbered unused file descriptor, it uses the file descriptor number specified in newfd. If the file descriptor newfd was previously open, it is silently closed before being reused.

13. >> dup2(3, 1) dup2 example Kernel File Table stdin stdout
Process File Table
stderr FD
a.txt
b.txt 1 2 3 4