Mayank Mandava

Linux System Programming by Robert Love [Chapters 5-8]

Chapter 5 - Processes

  • A binary is a compiled blob of code
  • A process is a running program, includes

    • the binary image
    • virtualized memory instance
    • kernel resources like open files
    • security context like user
    • one or more threads
    • a thread has a virtualized processor, stack, registers and instruction pointer
    • in a single threaded process, the process IS the thread
    • all threads share the same memory
  • A process has a PID that's guaranteed to be unique at a given moment
  • The idle process has PID 0
  • The first process is called the init process (pid 1)
  • The kernel tries the following init programs:

    • /sbin/init
    • /etc/init
    • /bin/init
    • /bin/sh
  • The init process handles the remainder of the boot process, launching services and a login program
  • The kernel keeps monotinically increasing the PID till it wraps around
  • Every process (child) is spawned from another process (parent)
  • Each process has a ppid(parent pid)
  • Each process inherits the parent's UID and GID
  • Processes belong to process groups. Normally children have the same group as the parent
  • when you do a ls | less both ls and less will have the same process group
  • to get your own pid:
#include <sys/types.h>
#include <unistd.h>
// returns the process ID of the invoking process
pid_t getpid (void);
// returns the process ID of the invoking process’s parent
pid_t getppid (void);
// Neither call will return an error
printf ("My pid=%jd\n", (intmax_t) getpid ());
printf ("Parent's pid=%jd\n", (intmax_t) getppid ());
// intmax_t is guaranteed to be larger than any int on the system
Running processes
  • fork creates a near duplicate of the parent process
  • exec family of calls loads a binary into memory, replacing the previous program, and executes it
  • to execute a new program, you have to fist fork, then exec
#include <unistd.h>
int ret;
  ret = execl ("/bin/vi", "vi", "/home/kidd/hooks.txt", NULL);
  // Arguments must be NULL terminated
  if (ret ==1)
    perror ("execl");
  • normally, execl does not return since there's nothing to return to
  • it clears out most attributes of the calling process including signals and locks
  • it keeps the open file ids. They should be either manually closed or using fcntl
  • In addition, we have execlp, execle, execv, execvp, execve

    • l means list, v mean array (variadic vs array args)
    • e means it takes a new environment
    • p means it seaches the PATH for the filename
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>

int main() {
    int pid = fork();
    if(pid == -1) {
        perror("fork"); return 1;
    if(pid == 0) { //child
        int ret;
        ret = execlp("ls", "ls", NULL);
        if (ret == -1) {
            perror("exec"); return 1;
    } else {