Advanced Programming in the UNIX® Environment

ptg10805159

332 Signals Chapter 10

#include "apue.h"

#include <pwd.h>

static void

my_alarm(int signo)

{

struct passwd *rootptr;

printf("in signal handler\n");

if ((rootptr = getpwnam("root")) == NULL)

err_sys("getpwnam(root) error");

alarm(1);

}

int

main(void)

{

struct passwd *ptr;

signal(SIGALRM, my_alarm);

alarm(1);

for ( ; ; ) {

if ((ptr = getpwnam("sar")) == NULL)

err_sys("getpwnam error");

if (strcmp(ptr->pw_name, "sar") != 0)

printf("return value corrupted!, pw_name = %s\n",

ptr->pw_name);

}

}

Figure 10.5Call a nonreentrant function from a signal handler

When this program was run, the results wererandom. Usually,the program would

be terminated by aSIGSEGVsignal when the signal handler returned after several

iterations. An examination of thecorefile showed that themainfunction had called

getpwnam,but that whengetpwnamcalledfree,the signal handler interrupted it and

calledgetpwnam,which in turn calledfree.The data structures maintained by

mallocandfreehad been corrupted when the signal handler (indirectly) calledfree

while themainfunction was also callingfree.Occasionally,the program would run

for several seconds beforecrashing with aSIGSEGVerror.When themainfunction did

runcorrectly after the signal had been caught, the return value was sometimes

corrupted and sometimes fine.

As shown by this example, if we call a nonreentrant function from a signal handler,

the results areunpredictable.

10.7 SIGCLDSemantics

Twosignals that continually generate confusion areSIGCLDandSIGCHLD.The name

SIGCLD(without theH) is fromSystem V,and this signal has different semantics from

the BSD signal, namedSIGCHLD.The POSIX.1 signal is also namedSIGCHLD.