管道似乎无法在由 exec() 覆盖的 fork() 进程之间进行通信
Pipes appear not to communicate between fork() processes overlaid by exec()
我一直在努力学习如何使用 IPC 管道，虽然 我可以使用简单的示例，但除了那些非常简单的程序，我什么都学不会正确行事。
一方面，我想我可能有一些竞争条件问题 - 我计划在管道工作后让信号量工作 - 所以如果这是问题所在，我'我很高兴听到这个消息。
另一方面，我只是不知道我的代码哪里出错了...
这个拼图我有 3 块：
- 外部进程 - 分叉、执行和设置管道
- INNER 进程 - 执行到 fork 上并将消息通过管道传递给 OUTER
- DISPL 进程 - 执行到叉子上并打印从 OUTER
传输的消息
这 3 个进程分叉并执行得很好，我只是从来没有从 INNER 管道中读取任何内容，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 能够显示每个 9 个字符的块及其包含的内容。没有读取缓冲区的组合以获得漂亮的打印，因为无论如何我在那个时候什么也得不到。
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, "%d", 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] = '[=10=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pipeFdStr_inner[READ], readBuffer, 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);
}
显示：
#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, readBuffer, READ_BLOCK_SIZE);
while (bytesRead > 0) {
readBuffer[bytesRead] = '[=12=]';
printf("Display: Read %li bytes - '%s'\n", bytesRead, readBuffer);
bytesRead = read(readFd, readBuffer, READ_BLOCK_SIZE);
}
printf("Display: Finished reading from pipe 2\n");
close(readFd);
}
是时候踢自己了……你有：
bytesRead = read(pipeFdStr_inner[READ], readBuffer, READ_BLOCK_SIZE);
while (bytesRead > 0)
{
readBuffer[bytesRead] = '[=10=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pipeFdStr_inner[READ], readBuffer, READ_BLOCK_SIZE);
}
close(pipeFdStr_inner[READ]);
write(pipeFdStr_displ[WRITE], messBuffer, strlen(messBuffer));
您正在读取和写入的 'file descriptors' 是字符串的第一个字符，它会自动转换为 int。您需要使用：
bytesRead = read(pfdInnerPipe[READ], readBuffer, READ_BLOCK_SIZE);
while (bytesRead > 0)
{
readBuffer[bytesRead] = '[=11=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pfdInnerPipe[READ], readBuffer, READ_BLOCK_SIZE);
}
close(pfdInnerPipe[READ]);
write(pfdDisplPipe[WRITE], messBuffer, strlen(messBuffer));
问题是通过检查系统调用的错误发现的。检查的 read() 报告：
outer: 2020-09-30 23:15:48.086 - pid=36674: failed to read from pipe
error (9) Bad file descriptor
不难仔细查看文件描述符，发现它不是文件描述符。
我使用了我的 SOQ (Stack Overflow Questions) repository on GitHub as files stderr.c and stderr.h in the src/libsoq sub-directory 中可用的一些代码。 outer.c的合理全检版本是这样的，我也同样错误检查了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 在退出之前等待它的 children 死亡。
#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, "%d", 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", pipeFdStr_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", pipeFdStr_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], readBuffer, 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] = '[=13=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pfdInnerPipe[READ], readBuffer, READ_BLOCK_SIZE);
if (bytesRead < 0)
err_syserr("failed to read from pipe\n");
}
close(pfdInnerPipe[READ]);
write(pfdDisplPipe[WRITE], messBuffer, 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;
}
示例输出：
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
我一直在努力学习如何使用 IPC 管道，虽然 我可以使用简单的示例，但除了那些非常简单的程序，我什么都学不会正确行事。
一方面，我想我可能有一些竞争条件问题 - 我计划在管道工作后让信号量工作 - 所以如果这是问题所在，我'我很高兴听到这个消息。
另一方面，我只是不知道我的代码哪里出错了...
这个拼图我有 3 块：
- 外部进程 - 分叉、执行和设置管道
- INNER 进程 - 执行到 fork 上并将消息通过管道传递给 OUTER
- DISPL 进程 - 执行到叉子上并打印从 OUTER 传输的消息
这 3 个进程分叉并执行得很好，我只是从来没有从 INNER 管道中读取任何内容，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 能够显示每个 9 个字符的块及其包含的内容。没有读取缓冲区的组合以获得漂亮的打印，因为无论如何我在那个时候什么也得不到。
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, "%d", 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] = '[=10=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pipeFdStr_inner[READ], readBuffer, 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);
}
显示：
#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, readBuffer, READ_BLOCK_SIZE);
while (bytesRead > 0) {
readBuffer[bytesRead] = '[=12=]';
printf("Display: Read %li bytes - '%s'\n", bytesRead, readBuffer);
bytesRead = read(readFd, readBuffer, READ_BLOCK_SIZE);
}
printf("Display: Finished reading from pipe 2\n");
close(readFd);
}
是时候踢自己了……你有：
bytesRead = read(pipeFdStr_inner[READ], readBuffer, READ_BLOCK_SIZE);
while (bytesRead > 0)
{
readBuffer[bytesRead] = '[=10=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pipeFdStr_inner[READ], readBuffer, READ_BLOCK_SIZE);
}
close(pipeFdStr_inner[READ]);
write(pipeFdStr_displ[WRITE], messBuffer, strlen(messBuffer));
您正在读取和写入的 'file descriptors' 是字符串的第一个字符，它会自动转换为 int。您需要使用：
bytesRead = read(pfdInnerPipe[READ], readBuffer, READ_BLOCK_SIZE);
while (bytesRead > 0)
{
readBuffer[bytesRead] = '[=11=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pfdInnerPipe[READ], readBuffer, READ_BLOCK_SIZE);
}
close(pfdInnerPipe[READ]);
write(pfdDisplPipe[WRITE], messBuffer, strlen(messBuffer));
问题是通过检查系统调用的错误发现的。检查的 read() 报告：
outer: 2020-09-30 23:15:48.086 - pid=36674: failed to read from pipe
error (9) Bad file descriptor
不难仔细查看文件描述符，发现它不是文件描述符。
我使用了我的 SOQ (Stack Overflow Questions) repository on GitHub as files stderr.c and stderr.h in the src/libsoq sub-directory 中可用的一些代码。 outer.c的合理全检版本是这样的，我也同样错误检查了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 在退出之前等待它的 children 死亡。
#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, "%d", 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", pipeFdStr_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", pipeFdStr_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], readBuffer, 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] = '[=13=]';
strcat(readBuffer, messBuffer);
printf("Outer: Read %li bytes\n", bytesRead);
printf("Outer: Message Buffer: %s\n", readBuffer);
bytesRead = read(pfdInnerPipe[READ], readBuffer, READ_BLOCK_SIZE);
if (bytesRead < 0)
err_syserr("failed to read from pipe\n");
}
close(pfdInnerPipe[READ]);
write(pfdDisplPipe[WRITE], messBuffer, 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;
}
示例输出：
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