IPC研究（1） -- signals

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IPC ---- Signals

Question:
Why is it unsafe to call printf() in a signal handler?
A: A signal handling fucntion could be interrupted in the middle and called again by something eles. So a signal handling function is not just recurisve, but also re-entrant. This means, any system calls that are not re-entrant but are called inside a signal handler are considered unsafe.
For system calls that are safe to call inside a signal handler, refer to page 524 in Book "Linux Programming".


Related system calls:
signal (deprecated)
kill
pause
sigaction (recommended)
sigemptyset
sigdelset
sigfillset
sigismember
sigpending
sigsuspend

Note:
When programming with signals, we should manipulater the signal set carefully.
Sigaction offers additional operations on sigset, eg. masking out a signal.
That's why sigaction is more robust then signal.

struct sigaction {
               void     (*sa_handler)(int);
               void     (*sa_sigaction)(int, siginfo_t *, void *);
               sigset_t   sa_mask;
               int        sa_flags;
               void     (*sa_restorer)(void);
           };

The field sa_mask could be used to set the signal mask.
The field sa_flags is used to modify the signal behavior. For example, many system calls are interruptalbe, if some interruptable system call receives a signal, it sets errno to EINTR and returns. If sa_flag is specified as SA_RESART, the interrruptable function will be restarted instead of being interrupted.

Analysis:
1. signal IPC机制是一种很基本的相对比较弱的IPC机制。它的“弱“主要表现在以下几个方面。
    第一，如果进程A和B要进行通信，他们之间可以传递的信息，其实就是个数字signum。所以，信息量是很少的。
    第二，signum有限，并且预定义了一些行为。如果你改变这些预定义的行为，会使得程序难以理解。这一点，我们可以通过NPTL的设计文档来了解更多。用LWP来实现线程，导致其间用signal通信很不方便，使得整个signal系统破破碎碎的，所以后来才有了NPTL支持。
    第三，A和B之间用signal进行通讯，会使得这两个进程耦合成都非常高。首先，A和B之间要现对每个SIGxxx有共同的行为定义；其次，如果A要发送signal给B，它必须要知道B的PID，如果AB不是父子进程的话，这一点有点麻烦。标准c库中，没有类似get_pid_by_name()这种函数。所以，你必须要自己实现这个函数。简单的方法是通过系统的/proc目录进行操作。
    第四，无论是获得pid还是本身signal通讯的维护，都需要额外的开销，给编程带来不利。
   


Example:
#include <unistd.h>
#include <signal.h>
#include <stdio.h>

void handler_usr1(int sig)
{
        printf("[%s] is running ... \n", __func__); /* unsafe, should not be called in a practical program */
        sleep(10);
        printf("[%s] exited \n", __func__);
}

void main()
{
        struct sigaction act;
        act.sa_handler = handler_usr1;
        sigfillset(&act.sa_mask); /* mask out all signals */
        act.sa_flags = 0;
        sigaction(SIGUSR1, &act, 0);

        while (1)
        {
                printf("[%s] is running ... \n", __func__);
                sleep(2);
        }
}


要通过进程名字来获得pid，可以通过/proc。如下：
/proc/23697 $ cat status
Name:    sigaction_test
State:    S (sleeping)
Tgid:    23697
Pid:    23697
PPid:    2309
TracerPid:    0
Uid:    1000    1000    1000    1000
Gid:    1000    1000    1000    1000
FDSize:    256
Groups:    4 20 24 46 105 119 122 1000
通过字符串，操作，可以获得PID。

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dirent.h>
#include <signal.h>

#define MAX_INFO_LEN 4096
/**
 * function: get_pid_by_name
 * @return: -1 if failed, pid of process[name] if succeeded
 **/
pid_t get_pid_by_name(const char* name)
{
    DIR *dir;
    struct dirent *ent;
    char *endptr;
    char buf[MAX_INFO_LEN];

    if ( !(dir=opendir("/proc")) )
    {
        perror("can not open /proc");
        exit(-1);
    }

    while( (ent = readdir(dir) ) != NULL)
    {
        long lpid = strtol(ent->d_name, &endptr, 10);
        if (*endptr != '\0') /* not a proc dir indicating a running process */
        {
            continue;
        }

        memset(buf, 0, sizeof(buf));
        snprintf(buf, sizeof(buf), "/proc/%ld/status", lpid);
        FILE *fp = fopen(buf, "r");

        if (fp)        /* not null */
        {
            if ( fgets(buf, sizeof(buf), fp) != NULL )
            {
                /* check the second token in the file, i.e. the program name */
                char *first = strtok(buf, "\t");
                char *second = strtok(NULL, "\n");
                printf("[%s]\n", second);
                if (!strcmp(second, name))
                {
                    fclose(fp);
                    closedir(dir);
                    return (pid_t)lpid;
                }
            }
            fclose(fp);
        }//examine "/proc/[lpid]/status"
        else
        {
            printf("open [%s] faild \n", buf);
        }
    }//while
    /* no corresponding process in /proc */
    return -1;
}

void main()
{
    char *name = "processB";
    pid_t pid = get_pid_by_name(name);
    if (pid == -1)
    {
        printf("no process found with name [%s] \n", name);
        exit(-1);
    }
    else
    {
        printf("sending [%s] SIGUSR1 \n", name);
        kill(pid, SIGUSR1);
    }
}