Linux Programming Introduction

1
Linux Programming Introduction

• CS 230
• ???

2
The Unix System - History

• first developed by Ken Thompson (Bell Lab)
• on PDP-7 (16 bit machine)
• Dennis Ritchie joined (MULTICS experience)
• C was developed for Unix
• two ATT organizations
• Bell Labs Research
• 8th edition, 9th edition, Plan 9
• stream IO, RFS,
• USG-gtUSDL-gtATTIS-gtUSO-gtUSL
• System V
• UC Berkeley
• add virtual memory to the 7th edition
• DARPA funding started for a standard Unix
• TCP/IP, The Internet
• influenced SunOS significantly (4.3BSD)

3
Commercial Unix

• Sun
• SunOS from BSD
• Solaris from System V
• DEC
• Ultrix, Digital Unix
• OSF/1 (from Mach)
• MS
• Xenix -gt NT
• IBM
• AIX

4
Unix Lineage
1st ed.
USG -gt USL
Bell Labs
UCB
6th ed.
1BSD
7th ed.
3BSD
8th ed.
Sys III
4.2BSD
Mach
10 ed.
SunOS 3, 4
SVR 3
4.3BSD
OSF/1
Plan 9
SVR 4
Solaris
4.4BSD
Solaris 2
5
Design Principles

• time sharing
• supports multiple processes
• easy process creation
• simple, replaceable user interface
• shell
• file system
• multilevel tree structrure
• IO devices are treated like files
• details are hidden in low level -kernel (device
  drivers)
• not so much goals in early design
• OS for programmer, not for production
• simple and easy (not performance and
  functionality)
• interactive (no batch processing)
• C from the original programmers preference
• easy porting -gt wide spread of Unix
• small hardware platform
• entails elegance

6
System Calls

• Defines the programmer interface to UNIX
• how to use files
• how to manage processes
• .
• Details of syscall mechanism are hidden from app.
  programmers
• syscalls are provided as C functions
• classification of syscalls
• process control
• file manipulation
• information manipulation
• device manipulation (similar to files)

7
Compile and Link Review

• gcc -c -02 -D NDEBUG file_name.cpp
• NDEBUG decides the level of debugging (assert
  statement)
• -c option produces object file, not an executable
• -02 defines the level of optimization
• more optimized code is more difficult to debug
• info gcc for more information about the compiler
• g -o test main.o test.o L
  /usr/local/lib/pam lpam
• g for linking c code (gcc for C code)
• g searches /lib and /usr/lib specify other
  locations with L
• g add lib prefix and .a suffix pam
  becomes libpam.a

8
Makefile

• reciprocal main.o reciprocal.o
• g (CFLAGS) -o reciprocal main.o reciprocal.o
• main.o main.c reciprocal.hpp
• gcc (CFLAGS) -c main.c
• reciprocal.o reciprocal.cpp reciprocal.hpp
• g (CFLAGS) -c reciprocal.cpp
• clean
• rm -f .o reciprocal
• make CFLAG-02
• make clean
• info make

9
Debugging with gdb

• compile with g option
• gdb reciprocal
• (gdb) run
• Starting program reciprocal
• Program received signal SIGSEGV, Segmentation
  fault.
• __strtol_internal (nptr0x0, endptr0x0,
  base10, group0)
• at strtol.c287
• 287 strtol.c No such file or directory.
• (gdb)
• other commands
• where shows where the program stops
• up 2 go up 2 levels of stack
• print - shows the value of a variable
• break proc - stops at the 1st line of the
  procedure proc
• next - proceeds one more line

10
Execution Environment

• you must have learnt
• argv and argc to the main program
• stdout and stdin
• GNU Coding Standards
• ls - s /usr/lib
• ls - -size /usr/lib
• info (standards)User Interfaces
• getopt_long( )
• a lib function to parse command line options
• const char const short_options hov
• const struct option long_options
• help, 0, NULL, h ,
• output, 1, NULL, o ,
• verbose, 0, NULL, v ,
• NULL, 0, NULL, 0 / Required at end of
  array. /
• each call returns a short option letter
• o option(followed by ) requires and argument

11
Standard IO

• provided by standard C lib
• stdin, stdout, stderr
• can be accessed as a file descriptor
• 0, 1, 2
• can be used for redirections
• program gt output_file.txt 2gt1 stderr is
  merged with stdout
• program 2gt1 filter order gt,
  2gt1, filter
• stdout is buffered while stderr is NOT
• fflush(stdout)

12
The Environment

• Examples USER, HOME, PATH, DISPLAY, EDITOR,
• set an environment variable
• EDITORemacs
• export EDITOR
• environ variable (defined in GNU C lib) points to
  all environment variables

13
Use of Environment in a Program
14
temporary files

• You need them to store data or to pass the data
  to other programs.
• more than one processes may access the same file
• file permission should set properly
• file name should be esoteric
• mkstemp
• char temp_filename /tmp/temp_file.XXXXXX
• int fd mkstemp (temp_filename)
• unlink (temp_filename)
• replaces XXXXXX with unique characters
• opens a file for r/w
• the file is not deleted automatically
• unlink makes the file to be deleted when the
  program exits
• fd tmpfile()
• creates a temporary file for internal use only
• deletes the file when the program exits

15
Defensive Coding

• assert
• C macro expands to C code runtime overhead
• compile with NDEBUG will remove this statement
• assert(some_function() 0) this may not be
  executed
• r some_function() this is always
  executed
• assert(r 0)
• argument Boolean expression
• assert(status0)
• nice places to use
• check null pointer
• check parameter values
• system call may fail due to unexpected system
  failures
• run out of resources
• permission
• wrong parameters to the calls
• hardware may not work as you expected
• your code should cope with such failures

16
Defensive Coding (2)

• Error codes from syscall
• errno contains the error number denoted by
  EINVAL, EACCES, ..
• strerror returns the string from errono
• Blocking syscall
• returns EINTR when a signal is received while
  being blocked

17
Defensive Coding - Example
rval chown (path, user_id, -1) if (rval ! 0)
/ Save errno because its clobbered by the
next system call. / int error_code
errno assert (rval -1) switch
(error_code) case EPERM / Permission
denied. / case EROFS / PATH is on a
read-only file system. / case ENAMETOOLONG /
PATH is too long. / case ENOENT / PATH does
not exit. / case ENOTDIR / A
component of PATH is not a directory. / case
EACCES / A component of PATH is not
accessible. / fprintf (stderr, error chown of
s s\n, path, strerror (error_code)) break
/ continue ? / case EFAULT / PATH contains
an invalid memory address. / abort () case
ENOMEM / Ran out of kernel memory. / fprintf
(stderr, s\n, strerror (error_code)) exit
(1) default / Some other, unexpected, error
code. / abort ()
18
Archives

• an archive is a collection of object files
• creating an archive
• ar cr libtest.a test1.o test2.o
• shared library
• library that is linked dynamically
• is a single object file containing all the
  functions defined
• shared by many processes
• it should be position independent code (PIC)
• creation and usage
• gcc -c -fPIC test1.c creates PIC
• gcc -shared -fPIC -o libtest.so test1.o test2.o
  creates slib
• gcc -o app app.o -L. -ltest link with
  libtest.so
• lib searching order can be seen with ldd command
• the static option will search only for static
  archive (xxx.a)
• linker does not provide the path name of the
  archive to the runtime
• the runtime searches only /lib and /usr/lib
• use Wl , -rpath options or LD_LIBRARY_PATH to
  provide more paths to the runtime

19
Archives(2)

• gcc automatically link programs with libc (g,
  with libstdc)
• not with libm
• when a lib is dependent on other shared libs,
  they are linked automatically
• not with static libs
• comparison with slib
• slib supports independent upgrade on the lib
• slib makes the program image smaller
• dynamic loading (lazy linking)
• RTLD_LAZY defines how to bind symbols (resolve
  them as code executes unlike RTLD_GLOBAL which
  resolves at dlopen() time)
• dlsym() fetches a pointer to a static variable