如何解决管道似乎无法在exec覆盖的fork进程之间进行通信
我一直在尝试学习如何在IPC中使用管道,尽管我可以使用一些简单的示例,但是除了那些非常简单的程序之外,我什么也做不到。
一方面,我认为我可能存在一些比赛条件问题-我计划在管道正常运行后使信号灯正常工作-因此,如果这是问题,我很高兴得知它。
另一方面,我只是不知道我的代码在哪里掉落...
我有3个拼图:
- 外部流程-分叉,执行并设置管道
- 内部流程-执行到分叉上并将消息通过管道传递到外部
- DISPL流程-执行到fork上,而printf则是从OUTER管道发送的消息
这3个进程是派生和执行正常的,我只是从中什么也没读懂INNER管道,这导致消息缓冲区包含一个空字符串。因此,DISPL从不显示任何内容。
我希望DISPL显示9个字符的每个块以及所包含的内容。没有读取缓冲区的组合,无法打印出漂亮的照片,因为那时我还是一无所获。
我的问题是,为什么这些管道不传输任何数据?
一如既往,我们很乐意接受所有帮助。
外部:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#define READ 0
#define WRITE 1
#define READ_BLOCK_SIZE 9
#define PROCESS_COUNT 2
#define SLEEP_TIME 2
int pfdInnerPipe[2];
int pfdDisplPipe[2];
int main()
{
pid_t processID;
char readBuffer[READ_BLOCK_SIZE+1];
ssize_t bytesRead;
char pipeFdStr_inner[10];
char pipeFdStr_displ[10];
if (pipe(pfdInnerPipe) < 0 || pipe(pfdDisplPipe) < 0) exit(1);
sprintf(pipeFdStr_inner,"%d",pfdInnerPipe[WRITE]);
sprintf(pipeFdStr_displ,pfdDisplPipe[READ]);
for (int count = 0; count < PROCESS_COUNT; count++)
{
processID = fork();
switch (processID)
{
case 0:
if (count == 0) // spawn inner
{
// Inner will only write to pipe 1
close(pfdInnerPipe[READ]);
close(pfdDisplPipe[WRITE]);
close(pfdDisplPipe[READ]);
execl("./pipe_inner.exe","pipe_inner.exe",pipeFdStr_inner,(char *)NULL);
exit(2);
} else if (count == 1) // spawn display
{
// Display will only read from display pipe
close(pfdDisplPipe[WRITE]);
close(pfdInnerPipe[WRITE]);
close(pfdInnerPipe[READ]);
execl("./pipe_displ.exe","pipe_displ.exe",pipeFdStr_displ,(char *)NULL);
exit(2);
}
break;
case -1:
perror("fork failed");
exit(3);
break;
default :
continue;
}
}
// parent process
// parent will only read from INNER pipe and write to DISPL pipe
close(pfdDisplPipe[READ]);
close(pfdInnerPipe[WRITE]);
sleep(SLEEP_TIME); // allow time for something to be on the pipe
char messBuffer[] = "";
bytesRead = read(pipeFdStr_inner[READ],readBuffer,READ_BLOCK_SIZE);
while (bytesRead > 0)
{
readBuffer[bytesRead] = '\0';
strcat(readBuffer,messBuffer);
printf("Outer: Read %li bytes\n",bytesRead);
printf("Outer: Message Buffer: %s\n",readBuffer);
bytesRead = read(pipeFdStr_inner[READ],READ_BLOCK_SIZE);
}
close(pipeFdStr_inner[READ]);
write(pipeFdStr_displ[WRITE],messBuffer,strlen(messBuffer));
sleep(SLEEP_TIME); // keep the pipe open to read from
close(pipeFdStr_displ[WRITE]);
}
内部:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define READ_BLOCK_SIZE 9
#define SLEEP_TIME 10
int main(int argc,char *argv[])
{
int writeFd;
char *strFromChild = "Message from INNER to OUTER";
if(argc != 2) {
exit(1);
}
writeFd = atoi(argv[1]);
write(writeFd,strFromChild,strlen(strFromChild));
sleep(SLEEP_TIME); // keep pipe open for a while
close(writeFd);
}
DISPL:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define READ_BLOCK_SIZE 9
#define SLEEP_TIME 5
int main(int argc,char *argv[])
{
int readFd;
char readBuffer[READ_BLOCK_SIZE+1];
ssize_t bytesRead;
if(argc != 2) {
exit(1);
}
readFd = atoi(argv[1]);
sleep(SLEEP_TIME); // allow time for everything else to happen
bytesRead = read(readFd,READ_BLOCK_SIZE);
while (bytesRead > 0) {
readBuffer[bytesRead] = '\0';
printf("Display: Read %li bytes - '%s'\n",bytesRead,readBuffer);
bytesRead = read(readFd,READ_BLOCK_SIZE);
}
printf("Display: Finished reading from pipe 2\n");
close(readFd);
}
解决方法
踢自己的时间...您拥有:
bytesRead = read(pipeFdStr_inner[READ],readBuffer,READ_BLOCK_SIZE);
while (bytesRead > 0)
{
readBuffer[bytesRead] = '\0';
strcat(readBuffer,messBuffer);
printf("Outer: Read %li bytes\n",bytesRead);
printf("Outer: Message Buffer: %s\n",readBuffer);
bytesRead = read(pipeFdStr_inner[READ],READ_BLOCK_SIZE);
}
close(pipeFdStr_inner[READ]);
write(pipeFdStr_displ[WRITE],messBuffer,strlen(messBuffer));
您正在读取和写入的“文件描述符”是字符串的第一个字符,它们会自动转换为int
。您需要使用:
bytesRead = read(pfdInnerPipe[READ],readBuffer);
bytesRead = read(pfdInnerPipe[READ],READ_BLOCK_SIZE);
}
close(pfdInnerPipe[READ]);
write(pfdDisplPipe[WRITE],strlen(messBuffer));
通过对系统调用进行错误检查发现了问题。选中的read()
报告:
outer: 2020-09-30 23:15:48.086 - pid=36674: failed to read from pipe
error (9) Bad file descriptor
不难看出文件描述符,并发现它不是文件描述符。
我使用了一些代码,它们在GitHub上的https://player.vimeo.com/video/111111111(堆栈溢出问题)存储库中以SOQ子目录中的文件stderr.c
和stderr.h
的形式提供。 outer.c
的经过充分检查的版本是这样的,我也同样错误地检查了inner.c
和displ.c
。请注意,我更改了程序名称,删除了pipe_
前缀和.exe
后缀。此外,outer.c
等待其两个子都死掉,然后退出自身。
#include "stderr.h"
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
#define READ 0
#define WRITE 1
#define READ_BLOCK_SIZE 9
#define PROCESS_COUNT 2
#define SLEEP_TIME 2
int pfdInnerPipe[2];
int pfdDisplPipe[2];
int main(int argc,char **argv)
{
if (argc > 0)
err_setarg0(argv[0]);
err_setlogopts(ERR_PID | ERR_MILLI);
pid_t processID;
char readBuffer[READ_BLOCK_SIZE + 1];
ssize_t bytesRead;
char pipeFdStr_inner[10];
char pipeFdStr_displ[10];
if (pipe(pfdInnerPipe) < 0 || pipe(pfdDisplPipe) < 0)
exit(1);
err_remark("inner (%d,%d),displ (%d,%d)\n",pfdInnerPipe[READ],pfdInnerPipe[WRITE],pfdDisplPipe[READ],pfdDisplPipe[WRITE]);
sprintf(pipeFdStr_inner,"%d",pfdInnerPipe[WRITE]);
sprintf(pipeFdStr_displ,pfdDisplPipe[READ]);
for (int count = 0; count < PROCESS_COUNT; count++)
{
processID = fork();
switch (processID)
{
case 0:
if (count == 0) // spawn inner
{
// Inner will only write to pipe 1
close(pfdInnerPipe[READ]);
close(pfdDisplPipe[WRITE]);
close(pfdDisplPipe[READ]);
// execl("./pipe_inner.exe","pipe_inner.exe",pipeFdStr_inner,(char *)NULL);
execl("./inner","inner",(char *)NULL);
err_syserr("failed to execute ./inner");
exit(2);
}
else if (count == 1) // spawn display
{
// Display will only read from display pipe
close(pfdDisplPipe[WRITE]);
close(pfdInnerPipe[WRITE]);
close(pfdInnerPipe[READ]);
// execl("./pipe_displ.exe","pipe_displ.exe",pipeFdStr_displ,(char *)NULL);
execl("./displ","displ",(char *)NULL);
err_syserr("failed to execute ./displ");
exit(2);
}
break;
case -1:
perror("fork failed");
exit(3);
break;
default:
err_remark("forked %d\n",processID);
continue;
}
}
// parent process
// parent will only read from INNER pipe and write to DISPL pipe
close(pfdDisplPipe[READ]);
close(pfdInnerPipe[WRITE]);
sleep(SLEEP_TIME); // allow time for something to be on the pipe
char messBuffer[] = "";
bytesRead = read(pfdInnerPipe[READ],READ_BLOCK_SIZE);
if (bytesRead < 0)
err_syserr("failed to read from pipe\n");
err_remark("read %zd bytes [[%.*s]]\n",bytesRead,(int)bytesRead,readBuffer);
while (bytesRead > 0)
{
readBuffer[bytesRead] = '\0';
strcat(readBuffer,messBuffer);
printf("Outer: Read %li bytes\n",bytesRead);
printf("Outer: Message Buffer: %s\n",readBuffer);
bytesRead = read(pfdInnerPipe[READ],READ_BLOCK_SIZE);
if (bytesRead < 0)
err_syserr("failed to read from pipe\n");
}
close(pfdInnerPipe[READ]);
write(pfdDisplPipe[WRITE],strlen(messBuffer));
sleep(SLEEP_TIME); // keep the pipe open to read from
close(pfdDisplPipe[WRITE]);
int status;
int corpse;
while ((corpse = wait(&status)) > 0)
err_remark("child %d exited with status 0x%.4X\n",corpse,status);
err_remark("exits\n");
return 0;
}
样本输出:
outer: 2020-09-30 23:26:24.222 - pid=36852: inner (3,4),displ (5,6)
outer: 2020-09-30 23:26:24.224 - pid=36852: forked 36853
outer: 2020-09-30 23:26:24.224 - pid=36852: forked 36854
inner: 2020-09-30 23:26:24.437 - pid=36853: fd = 4
displ: 2020-09-30 23:26:24.590 - pid=36854: fd = 5
outer: 2020-09-30 23:26:26.226 - pid=36852: read 9 bytes [[Message f]]
Outer: Read 9 bytes
Outer: Message Buffer: Message f
Outer: Read 9 bytes
Outer: Message Buffer: rom INNER
Outer: Read 9 bytes
Outer: Message Buffer: to OUTER
inner: 2020-09-30 23:26:34.441 - pid=36853: exiting
outer: 2020-09-30 23:26:36.441 - pid=36852: child 36853 exited with status 0x0000
Display: Finished reading from pipe 2
displ: 2020-09-30 23:26:36.441 - pid=36854: exiting
outer: 2020-09-30 23:26:36.442 - pid=36852: child 36854 exited with status 0x0000
outer: 2020-09-30 23:26:36.442 - pid=36852: exits
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