Basic process control system calls

1
Basic process control system calls

• COMS W4118
• Henning Schulzrinne

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Process model for shell
shell (pid 10)
fork()
foreground (pid 12)
background (pid 14)
background (pid 15)
child
child
child
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Execute program
include ltunistd.hgt int execve(const char
filename, char const argv , char const
envp)

• Example execve(./a.out, argv, envp)
• Replaces existing code with program
• on success, does not return
• process exits instead
• on error, returns -1 (see errno)

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Forking

• fork creates a child process that differs from
  the parent process only in its PID and PPID, and
  in the fact that resource utilizations are set to
  0.
• Returns
• on success, PID of child is returned to parent
• 0 is returned to child
• on failure, return -1 (and errno)

if ((pid fork()) 0) / heres the child
/ exec() else / heres the parent /
wait for child(pid)
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Wait for child process
include ltsys/types.hgt include
ltsys/wait.hgt pid_t wait(int status) pid_t
waitpid(pid_t pid, int status, int options)

• Can either wait for process (synchronous)
• or poll (WNOHANG)
• use with SIGCHLD signal handler
• while ((c_pid waitpid()) gt 0)
• Returns process ID of process that exited

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I/O redirection - dup
include ltunistd.hgt int dup(int oldfd) int
dup2(int oldfd, int newfd)

• dup and dup2 create copies of a file descriptor
• both are the same
• dup2 makes newfd a copy of oldfd, closing newfd
  first
• I.e., operations on newfd are done on oldfd
  instead
• Send output to outfd instead of standard output
• dup2(outfd, STDOUT_FILENO)
• For shell, redirect I/O in child, then exec()

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Error handling
include ltstdio.hgt void perror(const char s)

• errno - integer
• perror() prints string followed by last error
  message