74 lines
2.5 KiB
C
74 lines
2.5 KiB
C
|
// In-class exercise: protect non-reentrant function using signal
|
||
|
|
// blocking. This prevents stalls but NOT may yield incorrect results
|
||
|
|
// if the signals are unblocked before the CRITICAL section can be
|
||
|
|
// completed.
|
||
|
|
//
|
||
|
|
// Adapted from Stevens and Rago Fig 10.5.
|
||
|
|
#include <stdio.h>
|
||
|
|
#include <signal.h>
|
||
|
|
#include <unistd.h>
|
||
|
|
#include <stdlib.h>
|
||
|
|
#include <sys/types.h>
|
||
|
|
#include <sys/stat.h>
|
||
|
|
#include <fcntl.h>
|
||
|
|
#include <errno.h>
|
||
|
|
#include <pwd.h>
|
||
|
|
#include <string.h>
|
||
|
|
|
||
|
|
#define USERNAME "kauffman"
|
||
|
|
|
||
|
|
static void alarm_handler(int signo) {
|
||
|
|
printf("in signal handler\n");
|
||
|
|
sigset_t block_all, defaults; // two sets of signals
|
||
|
|
sigfillset( &block_all ); // contains all signals
|
||
|
|
sigprocmask(SIG_SETMASK, &block_all, &defaults); // block all signals
|
||
|
|
struct passwd *rootptr = NULL;
|
||
|
|
|
||
|
|
// NON-REENTRANT FUNCTION CALL IN SIGNAL HANDLER
|
||
|
|
rootptr = getpwnam("root"); // fetch record associated with root password
|
||
|
|
|
||
|
|
if (rootptr == NULL){ // check for presence of root
|
||
|
|
printf("no root, that's weird...\n");
|
||
|
|
}
|
||
|
|
else{
|
||
|
|
printf("root looks okay\n");
|
||
|
|
}
|
||
|
|
alarm(1); // reset the alarm
|
||
|
|
sigprocmask(SIG_SETMASK, &defaults, NULL); // un-block signals
|
||
|
|
printf("leaving signal handler\n");
|
||
|
|
}
|
||
|
|
|
||
|
|
int main(void) {
|
||
|
|
signal(SIGALRM, alarm_handler); // handle alarms in with the above function
|
||
|
|
alarm(1);
|
||
|
|
|
||
|
|
int successes = 0;
|
||
|
|
printf("Repeatedly checking on user '%s'\n",USERNAME);
|
||
|
|
while(1){
|
||
|
|
// Block signals while calling non-reentrant function
|
||
|
|
sigset_t block_all, defaults; // two sets of signals
|
||
|
|
sigfillset( &block_all ); // contains all signals
|
||
|
|
sigprocmask(SIG_SETMASK, &block_all, &defaults); // block all signals
|
||
|
|
struct passwd *ptr = getpwnam(USERNAME); // non-reentrant call
|
||
|
|
// sigprocmask(SIG_SETMASK, &defaults, NULL); // un-block signals, incorrect position
|
||
|
|
|
||
|
|
if(ptr == NULL){
|
||
|
|
perror("couldn't find user");
|
||
|
|
exit(1);
|
||
|
|
}
|
||
|
|
if(strcmp(ptr->pw_name, USERNAME) != 0){ // should always be equal to USERNAME
|
||
|
|
printf("return value corrupted! pw_name = %s\n",
|
||
|
|
ptr->pw_name);
|
||
|
|
exit(1);
|
||
|
|
}
|
||
|
|
|
||
|
|
sigprocmask(SIG_SETMASK, &defaults, NULL); // un-block signals, correct position
|
||
|
|
|
||
|
|
successes++; // Show progress of loop
|
||
|
|
if(successes % 100000 == 0){
|
||
|
|
printf("%d successes so far\n",successes);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
exit(0);
|
||
|
|
}
|