Using System Calls Unix

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Using System Calls (Unix)

• Have to tell compiler (if C/C) where to find
  the headers, etc. i.e., the include files
• May have to tell compiler where the library
  itself lives
• Use Unix man pages to tell you about this for
  each system call
• Other resources

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Recall Parameters to main in C. main(int argc,
char argv) //also char envp
.. argc Number of command line
parameters (always at least 1). argv Array of
pointers to command line parameters. Each
parameter is a null terminated string.
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Example ./a.out 1 4 main(int argc, char
argv) .. argc 3
argv points to an array with three elements, each
of which is a pointer to a null terminated
string.
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Example ./a.out 1 4
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The name of the program is always contained in
element 0 of argv.
Ex Want to print out name of program
(inefficiently) char ptr ptr argv0
while (ptr ! \0) printf(c, ptr)
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Exec Family of System Calls

• Used to begin a processes execution.
• Overwrites process (core) image of the calling
  process with that of called program.
• Several flavors, use the one most suited to
  needs.
• int execv( char path, char argvec)
• int execl(const char path, const char arg0,
  ..., const char argn, char
• / NULL/)

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exec Function calls

• int execv( char path, char argvec)
• pathname Can be an executable program in your
  directory (application code) or a system program
  such as ls, cd, date, ..
• argvec Array of pointers to NULL terminated
  strings.
• First element should always be the name of
  the
• program, last element should always be NULL.

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main (int argc, argv) int my_pid
char args3 my_pid fork() if
(my_pid 0) args0 ./a.out All
executed by args1 2 child process
args2 NULL execv(./a.out, args)
printf(OOOpppssss.\n) else printf(Not
a problem!\n) Executed by parent
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Program a.out will begin to execute and will have
the following parameters to main argc
3. argv0 ./a.out argv1 2
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Process Management System Calls

• wait() Blocks parent process until all child
  processes have terminated
• wait(pid) Blocks until a particular child process
  terminates.
• Exit() Terminate the process and de-allocate its
  resources.

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Very Simple Shell

• while(1)
• type_prompt()
• read_command(command, parameters)
• if (fork() ! 0) //I am the parent
• wait()
• else
• execv(command, parameters)
• printf(OOOPPPSSS\)

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File Management System Calls

• int fd open(name, how, mode)
• name is pathname
• Mode is file permissions.
• how is an integer whose bits tell the system
  call how to open the file
• O_RDONLY read only (0)
• O_WRONLY write only (1)
• O_RDWR read and write (2)
• int s close(fd)

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File Management System Calls

• int n read(int fd, void buffer, int nbytes)
• fd file descriptor
• buffer destination
• nbytes how many bytes to read
• n number read, or 0 on EOF
• file position is incremented
• int n write(int fd, void buffer, int
• nbytes)

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File Management System Calls

• pos lseek(fd,offset,whence)
• offset and pos are of type off_t (on
  Solaris), which is really a long integer
• whence determines what offset means
• SEEK_SET file position is set to offset bytes
• SEEK_CUR file position is set to current
  position offset
• SEEK_END file position is set to end of file
  offset
• pos new file position

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Reading from stdin

• scanf (char , var) reads from the terminal
  according to format string (spaces ignored).
• Example
• int j, k
• scanf(d, j) read first arg, store as int.
• scanf(d, k) // read second arg, store as
  int.

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char x, buf, buf1 buf (char ) malloc(1024)
buf1 (char ) malloc(1024) scanf(s,
buf) scanf(s, buf1)