i'm prefacing stating homework assignment , i'm stuck while writing sigchld handler. need access variables within child process.
#include <stdlib.h> #include <stdio.h> #include <signal.h> #include <unistd.h> #include <sys/types.h> #include <sys/wait.h> #define text_len 10 #define num_rescue_plows 4 #define num_victims_to_rescue 40 /* purpose: keep trap of number of victims have been rescued. */ int numrescuedvictims = 0; /* purpose: keep track of process id's of rescuing snow plows. */ pid_t plowpid[num_rescue_plows]; /* purpose: note @ least 1 child plow has finished. reports *process id , number of victims rescued each child plow. *'signum' tells signal number. no return value */ // may want define sigchld listener here // should have loop wait()s children , // , prints how many victims each has rescued. void listensigchld(int sig) { int status; pid_t pidwait; while((pidwait = wait(&status)) > 0) printf("plow %d rescued %d victim(s)", plowid, numvictimsrescued); } /* purpose: handle being informed of rescued victim. 'signum' tells *signal number. no return value. */ // may want define sigusr1 listener here // increments 'numrescuedvictims' , prints total number rescued victims void listensigusr1(int sig) { numrescuedvictims += &numvictimsrescued; printf("we've rescued %d victims!" numrescuedvictims); } /* purpose: make 'num_rescue_plows' processes run 'rescuingplow' *rescue stuck victims, , tell them quit after *'num_victims_to_rescue' victims have been rescued. ignores parameters. *returns 'exit_success' os. */ int main () { // i. applicability validity check: // ii. rescue victims: // ii.a. install 'sigusr1' handler: signal(sigusr1, listensigusr1); // install 'sigusr1' handler // install sigchld handler here signal(sigchld, listensigchld); // ii.b. tell num_rescue_plows plows start rescuing victims: int i; int mypid= getpid(); (i = 0; < num_rescue_plows; i++) { // fork() , save in plowpid[i] plowpid[i] = fork(); // if plowpid[i] less 0 do: if(plowpid[i] < 0) { fprintf(stderr,"dude, system way busy play rescuer!\n"); return(exit_failure); } // if plowpid[i] equal 0 do: else if(plowpid[i] == 0) { char pidtext[text_len]; char indextext[text_len]; } snprintf(pidtext,text_len,"%d",mypid); snprintf(indextext,text_len,"%d",i); execl("./rescuingplow","rescuingplow",pidtext,indextext,null); fprintf(stderr,"dude, stole plow!!\n"); return(exit_failure); } // ii.c. wait until victims have been rescued: while (numrescuedvictims < num_victims_to_rescue) { sleep(1); printf("searching victims . . .\n"); } // iii. finished: // loop send sigterm num_rescue_plows plow processes (int = 0; < num_rescue_plows; i++) { kill(plowpid[i], sigterm); } int tosleep= num_rescue_plows; // sleep() can interrupted sigchld. whenever interrupted // returns number of seconds still remain on alarm // clock. let's wait until has slept full amount incase // prematured interrupted sigchld. { tosleep= sleep(tosleep); } while (tosleep > 0); printf("ready next snow storm!\n"); return(exit_success); } `
and here's what's going end being spawned processes. although haven't worked through yet.
/* * rescuingplow.c * * compile $ gcc rescuingplow.c -o rescuingplow */ #include <stdlib.h> #include <stdio.h> #include <signal.h> #include <unistd.h> #include <sys/types.h> #include <sys/wait.h> /* purpose: keep track of number of victims process * rescued. */ int numvictimsrescued = 0; /* purpose: return number of victims rescued os. 'signum' * tells signal number. no return value. */ // may want write sigterm handling function // returns os 'numvictimsrescued'. /* purpose: rescue victims @ random intervals , inform parent process * sending sigusr1 until receiving sigterm. first parameter (after * program name) tells parent's process id. second parameter tells * plow's index. */ int main (int argc, char* argv[]) { // i. applicability validity check: pid_t parentpid; int plowid; if (argc < 3) { fprintf(stderr,"usage: rescuingplow <parentpid> <plowid>\n"); return(exit_failure); } parentpid = atoi(argv[1]); plowid = atoi(argv[2]); // ii. rescuing victims until told stop: // ii.a. install signal handler: // install sigterm handler here srand(plowid); // uniquely initialize random number generator act independently of each other // ii.b. rescue victims: // write endless loop that: // (1) 'sleep((rand() % 6) + 1);' // (2) increments 'numvictimsrescued' // (3) 'printf("plow %d rescued %d victim(s)!\n",plowid,numvictimsrescued);' // (4) send 'sigusr1' 'parentpid' // iii. finished: return(exit_success); } i'm not sure i'm going or how deal that. i'm pretty confident can deal of other problems exist.
without using ipc mechanics child can pass value of 8 bits parent process.
those 8 bits send child parameter call exit() , receive parent applying macro wexitstatus() value of status returned successful call wait() or waitpid(). please see man 2 exit , man 2 wait details.
if remember correctly 8 bits minimum defined standard. implementations might allow more bits.
update:
example on how use wait():
int child_exit_code = -1; int status = -1; pid_t pid = wait(&status); if (-1 != pid) child_exit_code = wexitstatus(status); 
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