Please explain the exec function and its family

Question

What is the exec   function and its family   Why is this function used and how does its work   Please anyone explain these functions

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Functions in the exec   family have different behaviours    l   arguments are passed as a list of strings to the main   v   arguments are passed as an array of strings to the main   p   path s to search for the new running program e   the environment can be specified by the caller   You can mix them  therefore you have    int execl const char  path  const char  arg        int execlp const char  file  const char  arg        int execle const char  path  const char  arg       char   const envp     int execv const char  path  char  const argv     int execvp const char  file  char  const argv     int execvpe const char  file  char  const argv    char  const envp       For all of them the initial argument is the name of a file that is to be executed   For more information read exec 3  man page   man 3 exec    if you are running a UNIX system

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The exec 3 3p  functions replace the current process with another  That is  the current process stops  and another runs instead  taking over some of the resources the original program had

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Simplistically  in UNIX  you have the concept of processes and programs  A process is an environment in which a program executes  The simple idea behind the UNIX  quot execution model quot  is that there are two operations you can do  The first is to fork    which creates a brand new process containing a duplicate  mostly  of the current program  including its state  There are a few differences between the two processes which allow them to figure out which is the parent and which is the child  The second is to exec    which replaces the program in the current process with a brand new program  From those two simple operations  the entire UNIX execution model can be constructed   To add some more detail to the above  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 makes a near 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   Only one process calls fork   but two processes return from that call - sounds bizarre but it s really quite elegant The new process  called the child  gets a different process ID  PID  and has the PID of the old process  the parent  as its parent PID  PPID   Because the two processes are now running exactly the same code  they need to be able to tell which is which - the return code of fork   provides this information - the child gets 0  the parent gets the PID of the child  if the fork   fails  no child is created and the parent gets an error code   That way  the parent knows the PID of the child and can communicate with it  kill it  wait for it and so on  the child can always find its parent process with a call to getppid     The exec   call replaces the entire current contents of the 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 call fork   without a following exec   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  For this situation  the program contains both the parent and the child code  Similarly  programs that know they re finished and just want to run another program don t need to fork    exec   and then wait   waitpid   for the child  They can just load the child directly into their current process space with exec    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  Under Linux  fork   only makes a copy of the page tables and a new task structure  exec   will do the grunt work of  quot separating quot  the memory of the two processes  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  before modifications are allowed  Linux also has a vfork    even more optimised  which shares just about everything between the two processes  Because of that  there are certain restrictions in what the child can do  and the parent halts until the child calls exec   or  exit    The parent has to be stopped  and the child is not permitted to return from the current function  since the two processes even share the same stack  This is slightly more efficient for the classic use case of fork   followed immediately by exec    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

User · Answer

The exec family of functions make your process execute a different program  replacing the old program it was running  I e   if you call  execl   bin ls    ls   NULL     then the ls program is executed with the process id  current working dir and user group  access rights  of the process that called execl  Afterwards  the original program is not running anymore   To start a new process  the fork system call is used  To execute a program without replacing the original  you need to fork  then exec

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what is the exec function and its family    The exec function  family is all functions used to execute a file  such as execl  execlp  execle  execv  and execvp They are all frontends for execve and provide different methods of calling it      why is this function used   Exec functions are used when you want to execute  launch  a file  program       and how does it work    They work by overwriting the current process image with the one that you launched  They replace  by ending  the currently running process  the one that called the exec command  with the new process that has launched   For more details  see this link

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When a process uses fork    it creates a duplicate copy of itself and this duplicates becomes the child of the process  The fork   is implemented using clone   system call in linux which returns twice from kernel    A non-zero value Process ID of child  is returned to the parent  A value of zero is returned to the child  In case the child is not created successfully due to any issues like low memory  -1 is returned to the fork      Let   s understand this with an example   pid   fork        Both child and parent will now start execution from here  if pid  lt  0          child was not created successfully     return 1    else if pid    0           This is the child process        Child process code goes here   else          Parent process code goes here   printf  This is code common to parent and child      In the example  we have assumed that exec   is not used inside the child process   But a parent and child differs in some of the PCB process control block  attributes  These are    PID - Both child and parent have a different Process ID  Pending Signals - The child doesn   t inherit Parent   s pending signals  It will be empty for the child process when created  Memory Locks - The child doesn   t inherit its parent   s memory locks  Memory locks are locks which can be used to lock a memory area and then this memory area cannot be swapped to disk  Record Locks - The child doesn   t inherit its parent   s record locks  Record locks are associated with a file block or an entire file  Process resource utilisation and CPU time consumed is set to zero for the child  The child also doesn   t inherit timers from the parent    But what about the child memory  Is a new address space created for a child   The answers in no  After the fork    both parent and child share the memory address space of parent  In linux  these address space are divided into multiple pages  Only when the child writes to one of the parent memory pages  a duplicate of that page is created for the child  This is also known as copy on write Copy parent pages only when the child writes to it    Let   s understand copy on write with an example   int x   2  pid   fork    if pid    0        x   10         child is changing the value of x or writing to a page        One of the parent stack page will contain this local               variable  That page will be duplicated for child and it will store the value 10 in x in duplicated page      else       x   4      But why is copy on write necessary   A typical process creation takes place through fork  -exec   combination  Let   s first understand what exec   does   Exec   group of functions replaces the child   s address space with a new program  Once exec   is called within a child  a separate address space will be created for the child which is totally different from the parent   s one   If there was no copy on write mechanism associated with fork    duplicate pages would have created for the child and all the data would have been copied to child   s pages  Allocating new memory and copying data is a very expensive process takes processor   s time and other system resources   We also know that in most cases  the child is going to call exec   and that would replace the child   s memory with a new program  So the first copy which we did would have been a waste if copy on write was not there   pid   fork    if pid    0        execlp   bin ls   ls  NULL       printf  will this line be printed       Think about it        A new memory space will be created for the child and that   memory will contain the   bin ls  program text section   it s stack  data section and heap section else       wait NULL          parent is waiting for the child  Once child terminates  parent will get its exit status and can then continue   return 1     Both child and parent will exit with status code 1    Why does parent waits for a child process    The parent can assign a task to it   s child and wait till it completes it   s task  Then it can carry some other work  Once the child terminates  all the resources associated with child are freed except for the process control block  Now  the child is in zombie state  Using wait    parent can inquire about the status of child and then ask the kernel to free the PCB  In case parent doesn   t uses wait  the child will remain in the zombie state    Why is exec   system call necessary   It   s not necessary to use exec   with fork    If the code that the child will execute is within the program associated with parent  exec   is not needed   But think of cases when the child has to run multiple programs  Let   s take the example of shell program  It supports multiple commands like find  mv  cp  date etc  Will be it right to include program code associated with these commands in one program or have child load these programs into the memory when required   It all depends on your use case  You have a web server which given an input x that returns the 2 x to the clients  For each request  the web server creates a new child and asks it to compute  Will you write a separate program to calculate this and use exec    Or you will just write computation code inside the parent program   Usually  a process creation involves a combination of fork    exec    wait   and exit   calls

User · Answer

exec is often used in conjunction with fork  which I saw that you also asked about  so I will discuss this with that in mind   exec turns the current process into another program   If you ever watched Doctor Who  then this is like when he regenerates -- his old body is replaced with a new body   The way that this happens with your program and exec is that a lot of the resources that the OS kernel checks to see if the file you are passing to exec as the program argument  first argument  is executable by the current user  user id of the process making the exec call  and if so it replaces the virtual memory mapping of the current process with a virtual memory the new process and copies the argv and envp data that were passed in the exec call into an area of this new virtual memory map   Several other things may also happen here  but the files that were open for the program that called exec will still be open for the new program and they will share the same process ID  but the program that called exec will cease  unless exec failed    The reason that this is done this way is that by separating running  a  new  program into two steps like this you can do some things between the two steps   The most common thing to do is to make sure that the new program has certain files opened as certain file descriptors    remember here that file descriptors are not the same as FILE    but are int values that the kernel knows about    Doing this you can   int X   open    output file txt   O WRONLY    pid t fk   fork    if   fk       in child        dup2 X  1      fd 1 is standard output                     so this makes standard out refer to the same file as X         close X           I m using execl here rather than exec because        it s easier to type the arguments         execl   bin echo     bin echo    hello world         exit 127      should not get here      else if  fk    -1           An error happened and you should do something about it         perror  fork       print an error message      close X      The parent doesn t need this anymore      This accomplishes running    bin echo  hello world   gt    output file txt   from the command shell

[c] Please explain the exec() function and its family

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