由于您有无法控制的线程,我建议您编写一个预加载的库来调用system()
调用(也可能是popen()
等)与您自己的实现。我也会在库中包含你的SIGCHLD
处理程序。
如果你不想通过env LD_PRELOAD=libwhatever.so yourprogram
运行您的程序,你可以像
const char *libs;
libs = getenv("LD_PRELOAD");
if (!libs || !*libs) {
setenv("LD_PRELOAD", "libwhatever.so", 1);
execv(argv[0], argv);
_exit(127);
}
在程序的开始添加的东西,有一个与LD_PRELOAD正确设置它重新执行本身。 (请注意,有怪癖考虑,如果你的程序是setuid或setgid;参见man ld.so
对细节特别是,如果libwhatever.so
没有安装在系统库目录,你必须指定一个完整路径。)
一个可能的方法可能会使用待定子项的无锁数组(由C编译器提供的原子内置函数)。而不是waitpid()
,您的system()
实施分配其中一个条目,将子PID保留在那里,然后等待子信号退出,而不是调用waitpid()
。
下面是一个示例实现:
#define _GNU_SOURCE
#define _POSIX_C_SOURCE 200809L
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <signal.h>
#include <semaphore.h>
#include <dlfcn.h>
#include <errno.h>
/* Maximum number of concurrent children waited for.
*/
#define MAX_CHILDS 256
/* Lockless array of child processes waited for.
*/
static pid_t child_pid[MAX_CHILDS] = { 0 }; /* 0 is not a valid PID */
static sem_t child_sem[MAX_CHILDS];
static int child_status[MAX_CHILDS];
/* Helper function: allocate a child process.
* Returns the index, or -1 if all in use.
*/
static inline int child_get(const pid_t pid)
{
int i = MAX_CHILDS;
while (i-->0)
if (__sync_bool_compare_and_swap(&child_pid[i], (pid_t)0, pid)) {
sem_init(&child_sem[i], 0, 0);
return i;
}
return -1;
}
/* Helper function: release a child descriptor.
*/
static inline void child_put(const int i)
{
sem_destroy(&child_sem[i]);
__sync_fetch_and_and(&child_pid[i], (pid_t)0);
}
/* SIGCHLD signal handler.
* Note: Both waitpid() and sem_post() are async-signal safe.
*/
static void sigchld_handler(int signum __attribute__((unused)),
siginfo_t *info __attribute__((unused)),
void *context __attribute__((unused)))
{
pid_t p;
int status, i;
while (1) {
p = waitpid((pid_t)-1, &status, WNOHANG);
if (p == (pid_t)0 || p == (pid_t)-1)
break;
i = MAX_CHILDS;
while (i-->0)
if (p == __sync_fetch_and_or(&child_pid[i], (pid_t)0)) {
child_status[i] = status;
sem_post(&child_sem[i]);
break;
}
/* Log p and status? */
}
}
/* Helper function: close descriptor, without affecting errno.
*/
static inline int closefd(const int fd)
{
int result, saved_errno;
if (fd == -1)
return EINVAL;
saved_errno = errno;
do {
result = close(fd);
} while (result == -1 && errno == EINTR);
if (result == -1)
result = errno;
else
result = 0;
errno = saved_errno;
return result;
}
/* Helper function: Create a close-on-exec socket pair.
*/
static int commsocket(int fd[2])
{
int result;
if (socketpair(AF_UNIX, SOCK_STREAM, 0, fd)) {
fd[0] = -1;
fd[1] = -1;
return errno;
}
do {
result = fcntl(fd[0], F_SETFD, FD_CLOEXEC);
} while (result == -1 && errno == EINTR);
if (result == -1) {
closefd(fd[0]);
closefd(fd[1]);
return errno;
}
do {
result = fcntl(fd[1], F_SETFD, FD_CLOEXEC);
} while (result == -1 && errno == EINTR);
if (result == -1) {
closefd(fd[0]);
closefd(fd[1]);
return errno;
}
return 0;
}
/* New system() implementation.
*/
int system(const char *command)
{
pid_t child;
int i, status, commfd[2];
ssize_t n;
/* Allocate the child process. */
i = child_get((pid_t)-1);
if (i < 0) {
/* "fork failed" */
errno = EAGAIN;
return -1;
}
/* Create a close-on-exec socket pair. */
if (commsocket(commfd)) {
child_put(i);
/* "fork failed" */
errno = EAGAIN;
return -1;
}
/* Create the child process. */
child = fork();
if (child == (pid_t)-1)
return -1;
/* Child process? */
if (!child) {
char *args[4] = { "sh", "-c", (char *)command, NULL };
/* If command is NULL, return 7 if sh is available. */
if (!command)
args[2] = "exit 7";
/* Close parent end of comms socket. */
closefd(commfd[0]);
/* Receive one char before continuing. */
do {
n = read(commfd[1], &status, 1);
} while (n == (ssize_t)-1 && errno == EINTR);
if (n != 1) {
closefd(commfd[1]);
_exit(127);
}
/* We won't receive anything else. */
shutdown(commfd[1], SHUT_RD);
/* Execute the command. If successful, this closes the comms socket. */
execv("/bin/sh", args);
/* Failed. Return the errno to the parent. */
status = errno;
{
const char *p = (const char *)&status;
const char *const q = (const char *)&status + sizeof status;
while (p < q) {
n = write(commfd[1], p, (size_t)(q - p));
if (n > (ssize_t)0)
p += n;
else
if (n != (ssize_t)-1)
break;
else
if (errno != EINTR)
break;
}
}
/* Explicitly close the socket pair. */
shutdown(commfd[1], SHUT_RDWR);
closefd(commfd[1]);
_exit(127);
}
/* Parent process. Close the child end of the comms socket. */
closefd(commfd[1]);
/* Update the child PID in the array. */
__sync_bool_compare_and_swap(&child_pid[i], (pid_t)-1, child);
/* Let the child proceed, by sending a char via the socket. */
status = 0;
do {
n = write(commfd[0], &status, 1);
} while (n == (ssize_t)-1 && errno == EINTR);
if (n != 1) {
/* Release the child entry. */
child_put(i);
closefd(commfd[0]);
/* Kill the child. */
kill(child, SIGKILL);
/* "fork failed". */
errno = EAGAIN;
return -1;
}
/* Won't send anything else over the comms socket. */
shutdown(commfd[0], SHUT_WR);
/* Try reading an int from the comms socket. */
{
char *p = (char *)&status;
char *const q = (char *)&status + sizeof status;
while (p < q) {
n = read(commfd[0], p, (size_t)(q - p));
if (n > (ssize_t)0)
p += n;
else
if (n != (ssize_t)-1)
break;
else
if (errno != EINTR)
break;
}
/* Socket closed with nothing read? */
if (n == (ssize_t)0 && p == (char *)&status)
status = 0;
else
if (p != q)
status = EAGAIN; /* Incomplete error code, use EAGAIN. */
/* Close the comms socket. */
shutdown(commfd[0], SHUT_RDWR);
closefd(commfd[0]);
}
/* Wait for the command to complete. */
sem_wait(&child_sem[i]);
/* Did the command execution fail? */
if (status) {
child_put(i);
errno = status;
return -1;
}
/* Command was executed. Return the exit status. */
status = child_status[i];
child_put(i);
/* If command is NULL, then the return value is nonzero
* iff the exit status was 7. */
if (!command) {
if (WIFEXITED(status) && WEXITSTATUS(status) == 7)
status = 1;
else
status = 0;
}
return status;
}
/* Library initialization.
* Sets the sigchld handler,
* makes sure pthread library is loaded, and
* unsets the LD_PRELOAD environment variable.
*/
static void init(void) __attribute__((constructor));
static void init(void)
{
struct sigaction act;
int saved_errno;
saved_errno = errno;
sigemptyset(&act.sa_mask);
act.sa_sigaction = sigchld_handler;
act.sa_flags = SA_NOCLDSTOP | SA_RESTART | SA_SIGINFO;
sigaction(SIGCHLD, &act, NULL);
(void)dlopen("libpthread.so.0", RTLD_NOW | RTLD_GLOBAL);
unsetenv("LD_PRELOAD");
errno = saved_errno;
}
如果保存上面说child.c
,你可以把它编译成libchild.so
使用
gcc -W -Wall -O3 -fpic -fPIC -c child.c -lpthread
gcc -W -Wall -O3 -shared -Wl,-soname,libchild.so child.o -ldl -lpthread -o libchild.so
如果你有一个测试程序,它system()
电话在各种线程中,您可以使用system()
插入(和孩子自动收获)使用
env LD_PRELOAD=/path/to/libchild.so test-program
请注意,根据正是那些不是你的控制之下线程做,你可能需要干预更多的功能,包括signal()
,sigaction()
,sigprocmask()
,pthread_sigmask()
,等等,以确保这些线程不会改变您的SIGCHLD
处理程序的处置(由libchild.so
库安装后)。
如果这些外的控制线程使用popen()
,您可以设置是(和pclose()
)非常相似的代码上面system()
,只是分成两个部分。 (如果您想知道为什么我的system()
代码不能向父进程报告exec()
失败,那是因为我通常使用此代码的变体,它将该命令作为字符串数组使用;这样,它就能正确地报告if该命令没有找到,或者由于权限不足等原因无法执行。在这种特殊情况下,该命令始终为/bin/sh
。但是,由于无论如何都需要使用通信套接字来避免在子出口和最新的PID在* child_pid [] *数组中,我决定留下“额外”代码。)
不应该system()阻止SIGCHLD吗?而你的意思是“SIGCHLD在不同的线程中调用”? – LostBoy
也许你可以使用一个库(Glib,Pocolib,Boost,QtCore等),它比'system(3)'具有更好的进程控制能力。 –
@LostBoy'system()'确实阻塞了SIGCHLD,但仅限于调用线程。它使用'sigprocmask()',它被记录为“一个进程中的每个线程都有自己的信号掩码。” – shoosh