《UNP》描述符传递的例子

#include "unp.h"
int my_open(const char *,int);
int
main(int argc, char **argv)
{
        int fd,n;
        char buff[BUFFSIZE];
        if(argc!=2)
                err_quit("usage:mycat ");
        if((fd=open(argv[1],O_RDONLY))<0)
                err_sys("cannot open %s",argv[1]);
        while((n=Read(fd,buff,BUFFSIZE))>0){
                printf("%d\n",n);
                Write(STDOUT_FILENO,buff,n);
                }
        exit(0);
}

int
my_open(const char *pathname, int mode)
{
        int fd,sockfd[2],status;
        pid_t childpid;
        char c,argsockfd[10],argmode[10];
        Socketpair(AF_LOCAL,SOCK_STREAM,0,sockfd);
        if((childpid=Fork())==0){
                Close(sockfd[0]);
                snprintf(argsockfd,sizeof(argsockfd),"%d",sockfd[1]);
                snprintf(argmode,sizeof(argmode),"%d",mode);
                execl("./openfile","openfile",argsockfd,pathname,argmode,(char*)NULL);
                err_sys("execl error");
        }
        Close(sockfd[1]);
        Waitpid(childpid,&status,0);//这个status是openfile主函数的返回值。
        if(WIFEXITED(status)==0)
                err_quit("child did not terminate");
        if((status=WEXITSTATUS(status))==0)
                Read_fd(sockfd[0],&c,1,&fd);
        else{
                errno=status;
                fd=-1;
        }
        Close(sockfd[0]);
        return(fd);
}

#include "unp.h"
int
main(int argc,char **argv)
{
        int fd;
        if(argc!=4)
                err_quit("openfile   ");
        if((fd=open(argv[2],atoi(argv[3])))<0)
                exit((errno>0)?errno:255);
        if(write_fd(atoi(argv[1]),"",1,fd)<0)
                exit((errno>0)?errno:255);
        exit(0);
}

//read_fd()
#include        "unp.h"

ssize_t
read_fd(int fd, void *ptr, size_t nbytes, int *recvfd)
{
        struct msghdr   msg;
        struct iovec    iov[1];
        ssize_t                 n;

#ifdef  HAVE_MSGHDR_MSG_CONTROL
        union {
          struct cmsghdr        cm;
          char                          control[CMSG_SPACE(sizeof(int))];
        } control_un;
        struct cmsghdr  *cmptr;

        msg.msg_control = control_un.control;
        msg.msg_controllen = sizeof(control_un.control);
#else
        int                             newfd;

        msg.msg_accrights = (caddr_t) &newfd;
        msg.msg_accrightslen = sizeof(int);
#endif

        msg.msg_name = NULL;
        msg.msg_namelen = 0;

        iov[0].iov_base = ptr;
        iov[0].iov_len = nbytes;
        msg.msg_iov = iov;
        msg.msg_iovlen = 1;

        if ( (n = recvmsg(fd, &msg, 0)) <= 0)
                return(n);

#ifdef  HAVE_MSGHDR_MSG_CONTROL
        if ( (cmptr = CMSG_FIRSTHDR(&msg)) != NULL &&
            cmptr->cmsg_len == CMSG_LEN(sizeof(int))) {
                if (cmptr->cmsg_level != SOL_SOCKET)
                        err_quit("control level != SOL_SOCKET");
//cmsg_level和cmsg_type见unp图14-11
                if (cmptr->cmsg_type != SCM_RIGHTS)
                        err_quit("control type != SCM_RIGHTS");
                *recvfd = *((int *) CMSG_DATA(cmptr));
        } else
                *recvfd = -1;           /* descriptor was not passed */
#else
/* *INDENT-OFF* */
        if (msg.msg_accrightslen == sizeof(int))
                *recvfd = newfd;
        else
                *recvfd = -1;           /* descriptor was not passed */
/* *INDENT-ON* */
#endif

        return(n);
}
/* end read_fd */

ssize_t
Read_fd(int fd, void *ptr, size_t nbytes, int *recvfd)
{
        ssize_t         n;

        if ( (n = read_fd(fd, ptr, nbytes, recvfd)) < 0)
                err_sys("read_fd error");

        return(n);
}

一、snprintf()

int snprintf ( char * s, size_t n, const char * format, ... );

/* snprintf example */
#include 

int main ()
{
  char buffer [100];
  int cx;

  cx = snprintf ( buffer, 100, "The half of %d is %d", 60, 60/2 );

  if (cx>=0 && cx<100)      // check returned value

    snprintf ( buffer+cx, 100-cx, ", and the half of that is %d.", 60/2/2 );

  puts (buffer);

  return 0;
}

Output:
The half of 60 is 30, and the half of that is 15.

二、几个宏

子进程的结束状态返回后存于status，底下有几个宏可判别结束情况
WIFEXITED(status)如果子进程正常结束则为非0值。
WEXITSTATUS(status)取得子进程exit()返回的结束代码，一般会先用WIFEXITED 来判断是否正常结束才能使用此宏。
WIFSIGNALED(status)如果子进程是因为信号而结束则此宏值为真

WTERMSIG(status)取得子进程因信号而中止的信号代码，一般会先用WIFSIGNALED 来判断后才使用此宏。
WIFSTOPPED(status)如果子进程处于暂停执行情况则此宏值为真。一般只有使用WUNTRACED 时才会有此情况。
WSTOPSIG(status)取得引发子进程暂停的信号代码，一般会先用WIFSTOPPED 来判断后才使用此宏。

三、socketpair函数

#include 
int socketpair(int family, int type, int protocol, int sockfd[2]);
//成功返回非0，出错为-1；

本函数会返回两个彼此连接的描述符。