参考这篇文章：
http://www.cnblogs.com/mickole/p/3191832.html
《linux系统编程之信号（七）：被信号中断的系统调用和库函数处理方式》
一些IO系统调用执行时, 如 read 等待输入期间, 如果收到一个信号,系统将中断read, 转而执行信号处理函数. 当信号处理返回后, 系统遇到了一个问题: 是重新开始这个系统调用, 还是让系统调用失败?
早期UNIX系统的做法是, 中断系统调用, 并让系统调用失败, 比如read返回 -1, 同时设置 errno 为 EINTR中断了的系统调用是没有完成的调用, 它的失败是临时性的, 如果再次调用则可能成功, 这并不是真正的失败, 所以要对这种情况进行处理, 典型的方式为:
while (1) { n = read(fd, buf, BUFSIZ); if (n == -1 && errno != EINTR) { printf("read error\n"); break; } if (n == 0) { printf("read done\n"); break; } }
这样做逻辑比较繁琐, 事实上, 我们可以从信号的角度来解决这个问题, 安装信号的时候, 设置 SA_RESTART属性, 那么当信号处理函数返回后, 被该信号中断的系统调用将自动恢复.
#include#include #include #include #include <string.h> #include void sig_handler(int signum) { printf("in handler\n"); sleep(1); printf("handler return\n"); } int main(int argc, char **argv) { char buf[100]; int ret; struct sigaction action, old_action; action.sa_handler = sig_handler; sigemptyset(&action.sa_mask); action.sa_flags = 0; /* 版本1:不设置SA_RESTART属性 * 版本2:设置SA_RESTART属性 */ //action.sa_flags |= SA_RESTART; sigaction(SIGINT, NULL, &old_action); if (old_action.sa_handler != SIG_IGN) { sigaction(SIGINT, &action, NULL); } bzero(buf, 100); ret = read(0, buf, 100); if (ret == -1) { perror("read"); } printf("read %d bytes:\n", ret); printf("%s\n", buf); return 0; }
注意其中的：
/* 版本1:不设置SA_RESTART属性
* 版本2:设置SA_RESTART属性 */
//action.sa_flags |= SA_RESTART;
当sa_flags不设置：SA_RESTART时：
结果：
设置后：
当被中断后，重新执行
补充man手册关于这一块的说明：
Interruption of system calls and library functions by signal handlers
Interruption of system calls and library functions by signal handlers If a signal handler is invoked while a system call or library function call is blocked, then either: * the call is automatically restarted after the signal handler returns; or * the call fails with the error EINTR. Which of these two behaviors occurs depends on the interface and whether or not the signal handler was established using the SA_RESTART flag (see sigaction(2)). The details vary across UNIX systems; below, the details for Linux. If a blocked call to one of the following interfaces is interrupted by a signal handler, then the call will be automatically restarted after the signal handler returns if the SA_RESTART flag was used; otherwise the call will fail with the error EINTR: * read(2), readv(2), write(2), writev(2), and ioctl(2) calls on "slow" devices. A "slow" device is one where the I/O call may block for an indefinite time, for example, a terminal, pipe, or socket. (A disk is not a slow device according to this definition.) If an I/O call on a slow device has already transferred some data by the time it is interrupted by a signal handler, then the call will return a success status (normally, the number of bytes transferred). * open(2), if it can block (e.g., when opening a FIFO; see fifo(7)). * wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2). * Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2), recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a timeout has been set on the socket (see below). * File locking interfaces: flock(2) and fcntl(2) F_SETLKW. * POSIX message queue interfaces: mq_receive(3), mq_timedreceive(3), mq_send(3), and mq_timedsend(3). * futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always failed with EINTR). * POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3) (since Linux 2.6.22; beforehand, always failed with EINTR). The following interfaces are never restarted after being interrupted by a signal handler, regardless of the use of SA_RESTART; they always fail with the error EINTR when interrupted by a signal handler: * Socket interfaces, when a timeout has been set on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2), and recvmsg(2), if a receive timeout (SO_RCVTIMEO) has been set; connect(2), send(2), sendto(2), and sendmsg(2), if a send timeout (SO_SNDTIMEO) has been set. * Interfaces used to wait for signals: pause(2), sigsuspend(2), sigtimedwait(2), and sigwaitinfo(2). * File descriptor multiplexing interfaces: epoll_wait(2), epoll_pwait(2), poll(2), ppoll(2), select(2), and pselect(2). * System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and semtimedop(2). * Sleep interfaces: clock_nanosleep(2), nanosleep(2), and usleep(3). * read(2) from an inotify(7) file descriptor. * io_getevents(2). The sleep(3) function is also never restarted if interrupted by a handler, but gives a success return: the number of seconds remaining to sleep.
注意其中，不会进行重启的系统调用和库函数：
The following interfaces are never restarted after being interrupted
by a signal handler, regardless of the use of SA_RESTART; they always
fail with the error EINTR when interrupted by a signal handler:
* Socket interfaces, when a timeout has been set on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2), and recvmsg(2), if a receive timeout (SO_RCVTIMEO) has been set; connect(2), send(2), sendto(2), and sendmsg(2), if a send timeout (SO_SNDTIMEO) has been set. * Interfaces used to wait for signals: pause(2), sigsuspend(2), sigtimedwait(2), and sigwaitinfo(2). * File descriptor multiplexing interfaces: epoll_wait(2), epoll_pwait(2), poll(2), ppoll(2), select(2), and pselect(2). * System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and semtimedop(2). * Sleep interfaces: clock_nanosleep(2), nanosleep(2), and usleep(3). * read(2) from an inotify(7) file descriptor. * io_getevents(2). The sleep(3) function is also never restarted if interrupted by a handler, but gives a success return: the number of seconds remaining to sleep.