Fork function in C

Question

Below is an example of the Fork function in action  Below is also the output  My main question has to to do with the a fork is called how values are changed  So pid1 2 and 3 start off at 0 and get changed as the forks happen  Is this because each time a fork happens the values are copied to the child and the specific value gets changed in the parent  Basically how do values change with fork functions    include  lt stdio h gt   include  lt sys types h gt   include  lt unistd h gt   int main         pid t pid1  pid2  pid3      pid1 0  pid2 0  pid3 0      pid1  fork       A        if pid1  0           pid2 fork       B            pid3 fork       C          else           pid3 fork       D            if pid3  0                pid2 fork       E                      if  pid1    0  amp  amp  pid2    0               printf  Level 1 n            if pid1   0              printf  Level 2 n            if pid2   0             printf  Level 3 n            if pid3   0             printf  Level 4 n           return 0            Then this is the execution    ----A----D---------  pid1  0  pid2  0 as initialized   pid3  0  print  Level 2  and  Level 4                        ----E----  pid1  0  pid2  0  pid3  0  print  Level 2  and  Level 3                                  ----  pid1  0  pid2  0  pid3  0  print  Level 2              ----B----C----  pid1  0  pid2  0  pid3  0  print nothing                                 ----  pid1  0  pid2  0  pid3  0  print nothing                       ----C----  pid1  0  pid2  0  pid3  0  print nothing                                 ----  pid1  0  pid2  0  pid3  0  print nothing    Ideally below is how I would like to see it explained as this way makes sense to me  The   are where my main confusion lies  When the child forks for example pid1   fork    that creates a process with all the values of the parent  but does it then pass up a value like lets say 1 to the parents pid1  Meaning the child would have pid 1 0  pid2 0 and pid3 0 and the parent then as pid1 2 and pid2 and 3 equal to 0

User · Answer

System call fork   is used to create processes  It takes no arguments and returns a process ID  The purpose of fork   is to create a new process  which becomes the child process of the caller  After a new child process is created  both processes will execute the next instruction following the fork   system call  Therefore  we have to distinguish the parent from the child  This can be done by testing the returned value of fork     Fork is a system call and you shouldnt think of it as a normal C function  When a fork   occurs you effectively create two new processes with their own address space Variable that are initialized before the fork   call store the same values in both the address space  However values modified within the address space of either of the process remain unaffected in other process one of which is parent and the other is child  So if   pid fork      If in the subsequent blocks of code you check the value of pid Both processes run for the entire length of your code  So how do we distinguish them  Again Fork is a system call and here is difference Inside the newly created child process pid will store 0 while in the parent process it would store a positive value A negative value inside pid indicates a fork error   When we test the value of pid to find whether it is equal to zero or greater than it we are effectively finding out whether we are in the child process or the parent process   Read more about Fork

User · Answer

First a link to some documentation of fork    http   pubs opengroup org onlinepubs 009695399 functions fork html  The pid is provided by the kernel  Every time the kernel create a new process it will increase the internal pid counter and assign the new process this new unique pid and also make sure there are no duplicates  Once the pid reaches some high number it will wrap and start over again   So you never know what pid you will get from fork    only that the parent will keep it s unique pid and that fork will make sure that the child process will have a new unique pid  This is stated in the documentation provided above   If you continue reading the documentation you will see that fork   return 0 for the child process and the new unique pid of the child will be returned to the parent  If the child want to know it s own new pid you will have to query for it using getpid     pid t pid   fork   if pid    0        printf  this is a child  my new unique pid is  d n   getpid       else       printf  this is the parent  my pid is  d and I have a child with pid  d  n   getpid    pid       and below is some inline comments on your code   include  lt stdio h gt   include  lt sys types h gt   include  lt unistd h gt   int main         pid t pid1  pid2  pid3      pid1 0  pid2 0  pid3 0      pid1  fork       A        if pid1    0              This is child A            pid2 fork       B            pid3 fork       C          else              This is parent A               Child B and C will never reach this code            pid3 fork       D            if pid3  0                   This is child D fork ed from parent A                pid2 fork       E                      if  pid1    0  amp  amp  pid2    0                    pid1 will never be 0 here so this is dead code                printf  Level 1 n                      if pid1   0                   This is always true for both parent and child E                printf  Level 2 n                      if pid2   0                  This is parent E  same as parent A                printf  Level 3 n                      if pid3   0                  This is parent D  same as parent A                printf  Level 4 n                        return 0

User · Answer

int a   fork       Creates a duplicate process  clone    which shares the execution stack   The difference between the parent and the child is the return value of the function   The child getting 0 returned  and the parent getting the new pid   Each time the addresses and the values of the stack variables are copied   The execution continues at the point it already got to in the code   At each fork  only one value is modified - the return value from fork

User · Answer

I think every process you make start executing the line you create so something like this     pid fork   at line 6  fork function returns 2 values  you have 2 pids  first pid 0 for child and pid gt 0 for parent  so you can use if to separate             sleep int time  to see clearly      lt 0 fail       0 child      gt 0 parent    int main int argc  char   argv        pid t childpid1  childpid2      printf  pid   process identification n        printf  ppid   parent process identification n        childpid1   fork        if  childpid1    -1            printf  Fork error   n              if  childpid1    0            sleep 1           printf  child 1  -- gt  pid    d and  ppid    d n                   getpid    getppid           else           childpid2   fork            if  childpid2    0                sleep 2               printf  child 2  -- gt  pid    d and ppid    d n                       getpid    getppid               else               sleep 3               printf  parent -- gt  pid    d n   getpid                         return 0       pid   process identification   ppid   parent process identification   child 1  -- gt  pid   2399 and  ppid   2398   child 2  -- gt  pid   2400 and ppid   2398   parent -- gt  pid   2398   linux die net  some uni stuff

[c] Fork() function in C

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