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#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
/* The printf text serve as further comments. Read it. */
int main(int argc, char* argv[])
{
pid_t pid;
char dummy[16];
if (argc < 2)
{
printf("You need to specify a program as argument 1.\n");
exit(1); /* Terminate the process with error (1). */
}
printf("P | Switch to a differen terminal.\n"
"P | Execute the command \"pstree $USER\".\n"
"P | It will show a list of processes you are running.\n"
"P | You should see the process '%s' once.\n"
"P | Press enter when you verified that.\n\n", argv[0]);
/* Wait until enter is pressed (gives you time...) */
fgets(dummy, 16, stdin);
printf("P | This process will now clone itself with fork.\n"
"P | One (almost) identical copy will be started.\n"
"P | The only difference is the result of fork()\n\n");
pid = fork();
if (pid == 0)
{
printf(" C | This is printed by the new process (the CHILD)\n"
" C | The child get a 0 return value from fork.\n\n");
printf(" C | Repeat the previous command in the other terminal.\n"
" C | You should see the process '%s' twice now.\n"
" C | (grep for it if you run many processes)\n"
" C | Press enter when you verified that.\n\n", argv[0]);
/* Wait until enter is pressed (gives you time...) */
fgets(dummy, 16, stdin);
printf(" C | The program specified as first argument (%s) on the \n"
" C | command line will now be started by the child process.\n"
" C | This will REPLACE the code of the child with the code\n"
" C | of the specified program.\n\n", argv[1]);
/* specify a terminal as argument 1 and check ptree again ...
* you will see that the child is replaced, it have the same
* process number, but a new command line
*/
execv(argv[1], argv + 1);
printf(" C | This will never be printed, since after starting the new\n"
" C | program by calling exec this old program is replaced by\n"
" C | the new. This is only printed if the new program could not\n"
" C | be started for some reason.\n\n");
exit(-1); /* Terminate the child with error (-1). */
}
else
{
int ret = 0; /* Used to save the result of the child. */
printf("P | This is printed by the origingal process (the PARENT)\n"
"P | It got the number (%d) from fork to identify the child.\n"
"P | This process will now work for 5 seconds.\n\n", (int)pid);
sleep(5); /* pretend */
printf("P | This is printed by the origingal process (the PARENT)\n"
"P | It got the number (%d) from fork to identify the child.\n"
"P | It is now used to wait for the result of that child\n\n",
(int)pid);
/* Wait for the child to finish and get it's status in ret. */
waitpid(pid, &ret, 0);
/* Print the exit status of the child. */
if (WIFEXITED(ret))
{
printf("P | Child finished with code %d\n\n", WEXITSTATUS(ret));
}
else if (WIFSIGNALED(ret))
{
printf("P | Child finished due to signal %d\n\n", WTERMSIG(ret));
}
}
return 0;
}
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