问题：项目开发中，碰到一个很奇怪的问题：当tcsh启动的子程序退出之后，tcsh本身无法退出，并占用大量CPU资源。
背景：应用程序在fork之后，用tcsh启动另一个子进程，执行特定任务。进程之间使用sockepair(pipe)进行进程间通讯；为简化编程，将子进程的socket fd重定向为stdin和stdout。
具体症状：
Strace tcsh程序：
fstat(250, 0x7fbfffc240)= -1 EBADF (Bad file descriptor)fstat(251, 0x7fbfffc240)= -1 EBADF (Bad file descriptor)fstat(252, 0x7fbfffc240)= -1 EBADF (Bad file descriptor)fstat(253, 0x7fbfffc240)= -1 EBADF (Bad file descriptor)fstat(254, 0x7fbfffc240)= -1 EBADF (Bad file descriptor)fstat(255, 0x7fbfffc240)= -1 EBADF (Bad file descriptor)write(17, "[1] 30274\n", 10) = -1 EPIPE (Broken pipe)--- SIGPIPE (Broken pipe) @ 0 (0) ---lseek(16, 0, SEEK_END) = -1 ESPIPE (Illegal seek)rt_sigprocmask(SIG_BLOCK, [], [], 8) = 0rt_sigprocmask(SIG_SETMASK, [], [], 8) = 0fstat(0, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(1, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(2, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(3, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(4, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(5, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(6, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(7, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(8, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(9, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(10, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(11, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(12, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(13, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(14, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(20, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)fstat(21, 0x7fbfffc240) = -1 EBADF (Bad file descriptor)
tcsh不断打印上述系统调用，可判断tcsh陷入了死循环，无法退出。其中
write(17, "[1] 30274\n", 10) = -1 EPIPE (Broken pipe)
--- SIGPIPE (Broken pipe) @ 0 (0) ---
lseek(16, 0, SEEK_END) = -1 ESPIPE (Illegal seek)
初步这两个系统调用是引发死循环的关键。为什么write到17文件描述符。(如果有经验，估计在这一步就可知道问题的根源。)
Lsof tcsh进程：
COMMAND PID USER FD TYPE DEVICESIZENODE NAMEcsh30246 tachyon cwd DIR 253,74096 108511249 /h/work/tcpproxycsh30246 tachyon rtd DIR 253,0 20480 2 /csh30246 tachyon txt REG 253,0 349696 163890 /bin/tcshcsh30246 tachyon mem REG 253,3 48524688 1951970 /usr/lib/locale/locale-archivecsh30246 tachyon mem REG 253,3 44473 1757371 /usr/share/locale/en/LC_MESSAGES/tcshcsh30246 tachyon mem REG 253,3 21546 1425608 /usr/lib64/gconv/gconv-modules.cachecsh30246 tachyon mem REG 253,3 10128 1425550 /usr/lib64/gconv/ISO8859-1.socsh30246 tachyon mem REG 253,0 105080 196842 /lib64/ld-2.3.4.socsh30246 tachyon mem REG 253,0 1493409 196843 /lib64/tls/libc-2.3.4.socsh30246 tachyon mem REG 253,0 15728 196865 /lib64/libtermcap.so.2.0.8csh30246 tachyon mem REG 253,0 30070 196861 /lib64/libcrypt-2.3.4.socsh30246 tachyon 16u unix 0x0000010293993640 31854667 socketcsh30246 tachyon 17u unix 0x0000010293993640 31854667 socketcsh30246 tachyon 18u REG 253,7 25770181 108511260 /****.logcsh30246 tachyon 19u unix 0x0000010293993640 31854667 socket
tcsh没有0，1，2的文件描述符，只有3个指向同一个unix地址的socket地址，基本断定： 16为stdin，17为stdout，18为stderr，19未知。
问题就变成：tcsh不断尝试往stdout输入特定信息！[1] 30274这个信息，应该是tcsh所起子进程的PID。
这样就基本清楚了：tcsh收到子进程的状态信息([1] PID)，并将收到的信息输出。由于stdout已被重定向成unix socket，在输出时，发生SIGPIPE (Broken pipe)事件。 SIGPIPE的默认行为是终止程序，显然tcsh没终止，并不断尝试输出。为什么tcsh不终止？难道tcsh内部实现屏蔽了该信号？为了解决这个问题，有两个办法：看tcsh的源码；看是否自己的程序改变了tcsh的信号处理。
最终，从自己的程序入手，发现果然是自己的程序改变了tcsh的信号配置。
A child created via fork(2) inherits a copy of its parent's signal dispositions. During an execve(2), the dispositions of handled signals are reset to the default; the dispositions of ignored signals are left unchanged.
问题的结论：
1) 自己的程序忽略了SIGPIPE；
2) fork之后，重定向了stdin和stdout;
3) tcsh启动子程序，捕获到子进程的状态(后台启动返回PID信息)，并尝试输出到stdout(socket)，因socket已被关闭，写失败并导致SIGPIPE。因SIGPIPE已被忽略，导致tcsh不断重试，无法退出！
重现程序：
#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <signal.h>void Usage(char* program){fprintf(stderr, "Usage: %s binary\n", program);exit(-1);}void sighandler(int sig){/* fprintf(stderr, "recieve SIGPIPE\n"); */}int main(int argc, char** argv){if(argc < 2){Usage(argv[0]);}char* binary = argv[1];/* signal(SIGPIPE, sighandler); */signal(SIGPIPE, SIG_IGN);/* Get a socketpair we'll use for stdin and stdout. */int sp[2];if (socketpair(AF_UNIX, SOCK_STREAM, PF_UNSPEC, sp) == -1) {perror("socketpair error");return -1;}pid_t pid = fork();if (pid == 0) {signal(SIGTERM, SIG_DFL); signal(SIGHUP, SIG_DFL);close(sp[0]);if (dup2(sp[1], 0) == -1 || dup2(0, 1) == -1) {char msg[128];snprintf(msg, sizeof msg, "dup2 failed in child process %d", getpid());}sleep(1);fprintf(stderr, "start child process\n");setsid();execvp(binary, &argv[1]);char msg[512];snprintf(msg, sizeof msg, "exec %s failed", binary);exit(-1);}else if(pid < 0){perror("fork");exit(-1);}printf("parent process\n");close(sp[1]);close(sp[0]);while(1){char* msg = "hello test\n";/* write(sp[0], msg, strlen(msg)); */sleep(1);printf("write successful\n");}return 0;}
gcc -o pipe pipe_broke.c
./pipe /usr/bin/strace -e trace=write,lseek -o strace.log csh -c "sleep 10 & "
Note: bash has no this problem! csh -> tcsh
