Linux / Perl - O que acontece quando um processo é bifurcado?

Question

Eu li sobre o fork e pelo que entendi, o processo é clonado, mas qual processo? O script em si ou o processo que iniciou o script?

Por exemplo:

Estou executando o rTorrent na minha máquina e quando um torrent é concluído, tenho um script executado contra ele. Esse script busca dados da web, portanto, leva alguns segundos para ser concluído. Durante esse tempo, meu processo rtorrent está congelado. Então eu fiz o fork do script usando o seguinte

my $pid = fork();
if ($pid) == 0) { blah blah blah; exit 0; }

Se eu executar este script da CLI, ele retorna ao shell em um segundo enquanto ele é executado em segundo plano, exatamente como pretendi. No entanto, quando eu corro do rTorrent, parece ser ainda mais lento do que antes. Então o que exatamente foi bifurcado? O processo do rtorrent se clonou e meu script foi executado, ou meu script foi clonado? Espero que isso faça sentido.

    

Answer 1

De Programação Avançada no Ambiente UNIX por W. Richard Stevens (pg. 188):

8.3 fork function

The only way a new process is created by the Unix kernel is when an existing process calls the fork function. (This doesn't apply to the special processes that we mentioned in the previous section—the swapper, init, and pagedaemon. These processes are created specially by the kernel as part of bootstrapping.)

#include <sys/types.h>
#include <unistd.h>

pid_t fork(void);
/* Returns: 0 in child, process ID of child in parent, -1 on error */

The new process created by fork is called the child process. The function is called once but returns twice. The only difference in the returns is that the return value in the child is 0 while the return value in the parent is the process ID of the new child. The reason the child's process ID is returned to the parent is because a process can have more than one child, so there is no function that allows a process to obtain the process IDs of its children. The reason fork returns 0 to the child is because a process can have only a single parent, so the child can always call getppid to obtain the process ID of its parent. (Process ID 0 is always in use by the swapper, so it's not possible for 0 to be the process ID of a child.)

Both the child and parent continue executing with the instruction that follows the call to fork. The child is a copy of the parent. For example, the child gets a copy of the parent's data space, heap, and stack. Note that this is a copy for the child—the parent and child do not share these portions of memory. Often the parent and child share the text segment (Section 7.6), if it is read-only.

No Linux, o operador fork do Perl chama o sistema fork e retorna undef na falha em vez de -1.

Stevens fornece listas (p. 192) de propriedades herdadas e diferenças entre os processos pais e seus filhos bifurcados:

Besides open files, there are numerous other properties of the parent that are inherited by the child:

• real user ID, real group ID, effective user ID, effective group ID
• supplementary group IDs
• process group ID
• session ID
• controlling terminal
• set-user-ID flag and set-group-ID flag
• current working directory
• root directory
• file mode creation mask
• signal mask and dispositions
• the close-on-exec flag for any open file descriptors
• environment
• attached shared memory segments
• resource limits

The differences between the parent and child are

• the return value from fork
• the process IDs are different
• the two processes have different parent process IDs—the parent process ID of the child is the parent; the parent process ID of the parent doesn't change
• the child's values for tms_utime, tms_stime, tms_cutime, and tms_ustime are set to 0
• file locks set by the parent are not inherited by the child
• pending alarms are cleared for the child
• the set of pending signals for the child is set to the empty set