CSE2031 lecture 12

SystemCalls

• Check out these books
  
  • Stevens Advanced Programming in the unix Environment
    
  • Robins&Robins – unix systems programming
    
  • 
    
• Processes
  
  • Fork
    
  • Exec….
    

  
   

• Files
  
  • Open
    
  • Close
    
  • Read
    
  • Write
    
  • Lseek
    
  • Dup
    
  • Dup2
    

 

PerProcess file Descriptor table	System wide open file table	I-Nodes
File descriptors	Flags	Disk / device
0	Offset
1	To i-node
2
..

• Write(fd,buf,50)
  
  • Fd
    
    • File descriptor
      
  • Buf
    
    • Void* char
      
  • 50
    
    • #bytes
      
• Write(fd,"Hello",6)
  
  • If this was typed after the first line there would be an offset of 56
    
• Lseek
  
  • Lseek(fd,offset,WHENCE)
    
    • Fd
      
      • File descriptor
        
    • Offset
      
    • WHENCE
      
      • SEEK_SET
        
      • SEEK_CUR
        
      • SEEK_END
        
• Dup
  
  • Duplicates a file descriptor using another file descriptor
    
    • Int dup(int oldfd);
      
    • Int dup2(int oldfd, int newfd)
      
  • Why would you want to do that?
    
    • Because you would never want to do this
      
      • Dup2(fd,1);
        
        • First argument is one you want copied
          
        • Second is what you want it copied on
          
      • Close(fd);
        
      • Printf("hello\n";
        
      • This would write at a file temp
        

int a[15]

 

 

int (*a)[15] /** a is a pointer to an array of 15 ints**/

 

int fd

fd = open("temp", _|_|_, 0660)

/*for the last input '0660' there should always be a leading 0 because it is an octal*/

/*_| represents space for flags*/

 

#define    O_WRONLY    1<<4

    O_TRUNK    1<<2

---------------------------------------

fd = open("temp", ..flag..)

write(fd,...)

close(fd)x

---------------------------------------

 

always check the return value from a system acall

 

if((fd = open("temp", ..flags..))<0){perror("...."); exit(1)}

 

for every file there exists an 'I-Node"

along with an I-Node there is a system-wide openfiletable

and with that there is a perprocess file descriptor table