System Interface

1
System Interface

• Interface that provides services from the OS
  (Higher than BIOS)
• Memory
• Scheduler
• File/Storage System
• Inter-process Communication and Network, etc

program
libc
Operating System
BIOS
2
File System

• A popular but also complex subsystem
• File descriptors (pseudo files, like stdin and
  stdout)
• Low level IO
• File Management
• Examples

file pointer
/
usr
bin
home
file
EOF
3
A little about File Attributes

• File attributes (srwxrwxrwx)
• read, write and execute
• an octal digit, rwx, desribes the permission to a
  set of users
• Three sets of differents users, owner, group and
  others
• Some unix commands umask, chmod, etc to change
  the attributes

4
File Accesses

• Read and Write
• getc() and putc() getchar() and putchar()
• int read(int fd, char buf, int n)
• int write(int fd, char buf, int n)

include syscalls.h / Should be stdio.h / int
main () char buf1024 int n
while ((n read(0, buf, 1024)) gt 0)
write ( 1, buf, n) return 0
include syscalls.h / Should be stdio.h / int
getchar () char c return (read(0, c,
1) 1 ) ? c EOF
5
File Accesses -- continued

• Open, creat, close and unlink
• int open(char name, int flags, int perms)
• Persm can be O_RONLY, O_RDWR, O_WRONLY and others
• int creat(char name, int perms) / within open,
  O_CREAT /
• int close(int fd)
• int unlink(char path)

include stdio.h / A copy program/ int main
() char f1 file1, f2file2 int
fd1, fd2, n if (fd1 open(f1, O_RDONLY, 0)
-1) usage(uanble to opening source
file s\n, f1) if (fd2 creat(f2, 0666)
-1) usage(unable to creating new file s
\n, f2) while ((n read(fd1, buf, 1024) gt
0) write ( fd2, buf, n) return 0
include ltstdio.hgt void usage(char fmt, )
va_list args va_start(args, fmt)
fprintf(stderr, error ) vfprintf(stderr,
fmt, args) va_end(args)
fprint(usage srcfile destfile\n)
6
File seek -- access with fix location

• lseek
• long lseek(int fd, long offset, itn origin)
• Origin 0, 1, 2, determines where to start
  offset.
• 0 beginning 1 current location 2 the end.

include stdio.h / A copy program/ int main
() char f1 file1, f2file2 int
fd1, fd2, n if (fd1 open(f1, O_RDONLY, 0)
-1) usage(uanble to opening source
file s\n, f1) if (fd2 creat(f2, 0666)
-1) usage(unable to creating new file s
\n, f2) else lseek(fd2, 0L, 2)
while ((n read(fd1, buf, 1024) gt 0)
write ( fd2, buf, n) return 0
7
Recap on lab 4

• File access alternatives
• File fp fopen()
• fgets()
• Int fd open(name, flags, perms)
• read(fd, buff, n)

include ltstdio.hgt / lab 4 / int main ()
char maibox argv1 int fd File
fp if 1 fp fopen(mailbox,
r) else fd open (mailbox,
O_RDONLY, 0) endif for ( ) / keep
reading every line and process the
HEADER and BODIES / return 0