Differences between fork and exec

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What are the differences between fork and exec

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I think some concepts from  Advanced Unix Programming  by Marc Rochkind were helpful in understanding the different roles of fork   exec    especially for someone used to the Windows CreateProcess   model      A program is a collection of instructions and data that is kept in a regular file on disk   from 1 1 2 Programs  Processes  and Threads          In order to run a program  the kernel is first asked to create a new process  which is an environment in which a program executes   also from 1 1 2 Programs  Processes  and Threads          It   s impossible to understand the exec or fork system calls without fully understanding the distinction between a process and a program  If these terms are new to you  you may want to go back and review Section 1 1 2  If you   re ready to proceed now  we   ll summarize the distinction in one sentence  A process is an execution environment that consists of instruction  user-data  and system-data segments  as well as lots of other resources acquired at runtime  whereas a program is a file containing instructions and data that are used to initialize the instruction and user-data segments of a process   from 5 3 exec System Calls    Once you understand the distinction between a program and a process  the behavior of fork   and exec   function can be summarized as    fork   creates a duplicate of the current process exec   replaces the program in the current process with another program    this is essentially a simplified  for dummies  version of paxdiablo s much more detailed answer

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The use of fork and exec exemplifies the spirit of UNIX in that it provides a very simple way to start new processes   The fork call basically makes a duplicate of the current process  identical in almost every way  Not everything is copied over  for example  resource limits in some implementations  but the idea is to create as close a copy as possible   The new process  child  gets a different process ID  PID  and has the PID of the old process  parent  as its parent PID  PPID   Because the two processes are now running exactly the same code  they can tell which is which by the return code of fork - the child gets 0  the parent gets the PID of the child  This is all  of course  assuming the fork call works - if not  no child is created and the parent gets an error code   The exec call is a way to basically replace the entire current process with a new program  It loads the program into the current process space and runs it from the entry point   So  fork and exec are often used in sequence to get a new program running as a child of a current process  Shells typically do this whenever you try to run a program like find - the shell forks  then the child loads the find program into memory  setting up all command line arguments  standard I O and so forth   But they re not required to be used together  It s perfectly acceptable for a program to fork itself without execing if  for example  the program contains both parent and child code  you need to be careful what you do  each implementation may have restrictions   This was used quite a lot  and still is  for daemons which simply listen on a TCP port and fork a copy of themselves to process a specific request while the parent goes back to listening   Similarly  programs that know they re finished and just want to run another program don t need to fork  exec and then wait for the child  They can just load the child directly into their process space   Some UNIX implementations have an optimized fork which uses what they call copy-on-write  This is a trick to delay the copying of the process space in fork until the program attempts to change something in that space  This is useful for those programs using only fork and not exec in that they don t have to copy an entire process space   If the exec is called following fork  and this is what happens mostly   that causes a write to the process space and it is then copied for the child process   Note that there is a whole family of exec calls  execl  execle  execve and so on  but exec in context here means any of them   The following diagram illustrates the typical fork exec operation where the bash shell is used to list a directory with the ls command    --------    pid 7      ppid 4     bash      --------              calls fork     V  --------               --------    pid 7       forks      pid 22     ppid 4   ---------- gt    ppid 7     bash                   bash      --------               --------                                     waits for pid 22       calls exec to run ls                            V                         --------                           pid 22                            ppid 7                            ls           V                   --------   --------                      pid 7                      exits   ppid 4    lt ---------------    bash      --------              continues     V

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Fork creates a copy of a calling process   generally follows the structure    int cpid   fork      if  cpid     0          child code    exit 0         parent code  wait cpid       end    for child process text code  data stack is same as calling process  child process executes code in if block   EXEC replaces the current process with new process s code data stack  generally follows the structure    int cpid   fork      if  cpid     0            child code    exec foo      exit 0            parent code  wait cpid       end    after exec call unix kernel clears the child process text data stack and fills with foo process related text data  thus child process is with different code  foo s code  not same as parent

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fork   splits the current process into two processes  Or in other words  your nice linear easy to think of program suddenly becomes two separate programs running one piece of code    int pid   fork      if  pid    0          printf  I m the child        else         printf  I m the parent  my child is  i   pid           here we can kill the child  but that s not very parently of us      This can kind of blow your mind  Now you have one piece of code with pretty much identical state being executed by two processes  The child process inherits all the code and memory of the process that just created it  including starting from where the fork   call just left off  The only difference is the fork   return code to tell you if you are the parent or the child  If you are the parent  the return value is the id of the child   exec is a bit easier to grasp  you just tell exec to execute a process using the target executable and you don t have two processes running the same code or inheriting the same state  Like  Steve Hawkins says  exec can be used after you forkto execute in the current process the target executable

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fork     It creates a copy of running process  The running process is called parent process  amp  newly created process is called child process  The  way to differentiate the two is by looking at the returned value    fork   returns the process identifier  pid  of the child process in the parent fork   returns 0 in the child     exec      It initiates a new process within a process  It  loads a new program into the current process  replacing the existing one   fork     exec     When launching a new program is to firstly fork    creating a new process  and then exec    i e  load into memory and execute  the program binary it is supposed to run   int main  void          int pid   fork        if   pid    0                  execvp   find   argv                 Put the parent to sleep for 2 sec let the child finished executing      wait  2         return 0

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The main difference between fork   and exec   is that   The fork   system call creates a clone of the currently running program  The original program continues execution with the next line of code after the fork   function call  The clone also starts execution at the next line of code   Look at the following code that i got from http   timmurphy org 2014 04 26 using-fork-in-cc-a-minimum-working-example    include  lt stdio h gt   include  lt unistd h gt  int main int argc  char   argv        printf  --beginning of program n        int counter   0      pid t pid   fork        if  pid    0                   child process         int i   0          for    i  lt  5    i                        printf  child process  counter  d n     counter                       else if  pid  gt  0                   parent process         int j   0          for    j  lt  5    j                        printf  parent process  counter  d n     counter                       else                  fork failed         printf  fork   failed  n            return 1            printf  --end of program-- n        return 0      This program declares a counter variable  set to zero  before fork  ing  After the fork call  we have two processes running in parallel  both incrementing their own version of counter  Each process will run to completion and exit  Because the processes run in parallel  we have no way of knowing which will finish first  Running this program will print something similar to what is shown below  though results may vary from one run to the next   --beginning of program parent process  counter 1 parent process  counter 2 parent process  counter 3 child process  counter 1 parent process  counter 4 child process  counter 2 parent process  counter 5 child process  counter 3 --end of program-- child process  counter 4 child process  counter 5 --end of program--   The exec   family of system calls replaces the currently executing code of a process with another piece of code  The process retains its PID but it becomes a new program  For example  consider the following code     include  lt stdio h gt    include  lt unistd h gt   main      char program 80   args 3    int i   printf  Ready to exec      n     strcpy program  date     args 0   date    args 1   -u    args 2  NULL   i execvp program args    printf  i  d     did it work  n  i         This program calls the execvp   function to replace its code with the date program  If the code is stored in a file named exec1 c  then executing it produces the following output    Ready to exec       Tue Jul 15 20 17 53 UTC 2008    The program outputs the line -Ready to exec           and after calling the execvp   function  replaces its code with the date program  Note that the line -       did it work  is not displayed  because at that point the code has been replaced  Instead  we see the output of executing -date -u

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They are use together to create a new child process   First  calling fork creates a copy of the current process  the child process    Then  exec is called from within the child process to  replace  the copy of the parent process with the new process   The process goes something like this   child   fork       Fork returns a PID for the parent process  or 0 for the child  or -1 for Fail  if  child  lt  0        std  cout  lt  lt   Failed to fork GUI process   Exiting   lt  lt  std  endl      exit  -1     else if  child    0             This is the Child Process        Call one of the  exec  functions to create the child process     execvp  argv 0   const cast lt char   gt  argv      else                            This is the Parent Process       Continue executing parent process

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fork   creates a copy of the current process  with execution in the new child starting from just after the fork   call   After the fork    they re identical  except for the return value of the fork   function    RTFM for more details    The two processes can then diverge still further  with one unable to interfere with the other  except possibly through any shared file handles   exec   replaces the current process with a new one   It has nothing to do with fork    except that an exec   often follows fork   when what s wanted is to launch a different child process  rather than replace the current one

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The prime example to understand the fork   and exec   concept is the shell the command interpreter program that users typically executes after logging into the system The shell interprets the first word of command line as a command name  For many commands the shell forks and the child process execs the command associated with the name treating the remaining words on the command line as parameters to the command   The shell allows three types of commands  First  a command can be an executable file that contains object code produced by compilation of source code  a C program for example   Second  a command can be an executable file that contains a sequence of shell command lines  Finally  a command can be an internal shell command  instead of an executable file ex- cd ls etc

[c] Differences between fork and exec

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