叉子过程不从管道读取
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我正在尝试使用fork（）创建一个单独的“记录器”进程，它会在收到消息时将消息记录到文件中。但是我有一个问题，子进程似乎在一个字符串后“放弃”。
我甚至不确定这是否是一个很好的方法来进行记录器处理 - 但这是输出和代码
任何想法或帮助将不胜感激 - 谢谢
int main()
{
int pipefd[2], nbytes;
pid_t pid;
char szLogtest[] = "Log Event: Blah blah blah
";
char szLogtest2[] = "Log Event: Blah blah 2
";
char readbuffer[512];
pipe(pipefd);
if ((pid = fork()) == -1)
{
fprintf(stderr, "Fork error!
");
exit(1);
}
else if (pid == 0)
{
printf("I am child!
");
close(pipefd[1]); // close the write end of pipe
while(1)
{
nbytes = read(pipefd[0], readbuffer, sizeof(readbuffer));
printf("Nbytes is %d
", nbytes);
if (nbytes)
{
printf("Received string: %s", readbuffer);
}
nbytes = 0;
}
}
else
{
printf("I am parent!
");
close(pipefd[0]); //close read end of pipe
write(pipefd[1], szLogtest, (strlen(szLogtest)+1));
write(pipefd[1], szLogtest2, (strlen(szLogtest2)+1));
}
return 0;
}
答案
Correct diagnosis in comments
正如Ian Abbot在comment中正确诊断的那样，当前的问题是你在消息中间收到空字节，但是试图将消息视为字符串。这切碎了信息。
你也在sizzzzlerz得到comment的明智建议。
Do not send null bytes
最简单的解决方法是不向子进程发送空字节。然后，除非要将信息添加到日志打印（例如时间信息），否则子进程如何拆分读取并不重要。这是一个例子：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
static void wait_for(pid_t pid)
{
int corpse;
int status;
while ((corpse = wait(&status)) > 0)
{
printf("%d: PID %d exit status 0x%.4X
", (int)getpid(), corpse, status);
if (corpse == pid)
break;
}
}
static void write_log(int fd, const char *msg)
{
ssize_t len = strlen(msg);
if (write(fd, msg, len) != len)
{
fprintf(stderr, "%d: failed to write message to logger
", (int)getpid());
exit(EXIT_FAILURE);
}
printf("%d: Wrote: %zd [%s]
", (int)getpid(), len, msg);
}
int main(void)
{
int pipefd[2];
pid_t pid;
char szLogtest1[] = "Log Event: Blah blah blah
";
char szLogtest2[] = "Log Event: Blah blah 2
";
pipe(pipefd);
if ((pid = fork()) == -1)
{
fprintf(stderr, "Fork error!
");
exit(1);
}
else if (pid == 0)
{
printf("%d: I am child!
", (int)getpid());
close(pipefd[1]); // close the write end of pipe
while (1)
{
char readbuffer[512];
int nbytes = read(pipefd[0], readbuffer, sizeof(readbuffer));
if (nbytes <= 0)
{
close(pipefd[0]);
printf("EOF on pipe -exiting
");
exit(EXIT_SUCCESS);
}
printf("%d: Logging string: %d [%s]
", (int)getpid(), nbytes, readbuffer);
}
}
else
{
printf("%d: I am parent!
", (int)getpid());
close(pipefd[0]); // close read end of pipe
write_log(pipefd[1], szLogtest1);
write_log(pipefd[1], szLogtest2);
close(pipefd[1]);
wait_for(pid);
}
return 0;
}
请注意，大多数消息都以生成它的进程的PID为前缀。它可以更容易地查看多进程调试中发生的情况。
该程序的示例输出（log61）：
$ ./log61
48941: I am parent!
48941: Wrote: 26 [Log Event: Blah blah blah
]
48941: Wrote: 23 [Log Event: Blah blah 2
]
48942: I am child!
48942: Logging string: 49 [Log Event: Blah blah blah
Log Event: Blah blah 2
]
EOF on pipe -exiting
48941: PID 48942 exit status 0x0000
$
Prefix messages with length
另一个简单（但不是那么简单）修复是将消息分成不超过255个字节的块，并写入1个字节的长度，然后写入那么多字节的数据。然后，孩子读取1字节长度和多个字节的数据。此代码创建一个与read_log()函数并行的write_log()函数，并且write_log()函数也被修改。
#include <assert.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
static void wait_for(pid_t pid)
{
int corpse;
int status;
while ((corpse = wait(&status)) > 0)
{
printf("%d: PID %d exit status 0x%.4X
", (int)getpid(), corpse, status);
if (corpse == pid)
break;
}
}
static void write_log(int fd, const char *msg)
{
ssize_t len = strlen(msg);
unsigned char byte = len;
assert(len > 0 && len <= UCHAR_MAX);
if (write(fd, &byte, sizeof(byte)) != sizeof(byte) ||
write(fd, msg, len) != len)
{
fprintf(stderr, "%d: failed to write message to logger
", (int)getpid());
exit(EXIT_FAILURE);
}
printf("%d: Wrote: %zd [%s]
", (int)getpid(), len, msg);
}
static int read_log(int fd, size_t buflen, char buffer[buflen])
{
unsigned char byte;
if (read(fd, &byte, sizeof(byte)) != sizeof(byte))
{
fprintf(stderr, "%d: EOF on file descriptor %d
", (int)getpid(), fd);
return 0;
}
if (buflen < (size_t)(byte + 1)) // avoid signed/unsigned comparison
{
fprintf(stderr, "%d: buffer length %zu cannot hold %d bytes
",
(int)getpid(), buflen, byte + 1);
exit(EXIT_FAILURE);
}
if (read(fd, buffer, byte) != byte)
{
fprintf(stderr, "%d: failed to read %d bytes from file descriptor %d
",
(int)getpid(), byte, fd);
exit(EXIT_FAILURE);
}
buffer[byte] = ' ';
return byte;
}
int main(void)
{
int pipefd[2];
pid_t pid;
char szLogtest1[] = "Log Event: Blah blah blah
";
char szLogtest2[] = "Log Event: Blah blah 2
";
pipe(pipefd);
if ((pid = fork()) == -1)
{
fprintf(stderr, "Fork error!
");
exit(1);
}
else if (pid == 0)
{
printf("%d: I am child!
", (int)getpid());
close(pipefd[1]); // close the write end of pipe
char readbuffer[512];
int nbytes;
while ((nbytes = read_log(pipefd[0], sizeof(readbuffer), readbuffer)) > 0)
{
printf("%d: Logging string: %d [%s]
", (int)getpid(), nbytes, readbuffer);
}
close(pipefd[0]);
printf("EOF on pipe -exiting
");
exit(EXIT_SUCCESS);
}
else
{
printf("%d: I am parent!
", (int)getpid());
close(pipefd[0]); // close read end of pipe
write_log(pipefd[1], szLogtest1);
write_log(pipefd[1], szLogtest2);
close(pipefd[1]);
wait_for(pid);
}
return 0;
}
该程序的示例输出（log59）：
$ ./log59
48993: I am parent!
48993: Wrote: 26 [Log Event: Blah blah blah
]
48993: Wrote: 23 [Log Event: Blah blah 2
]
48994: I am child!
48994: Logging string: 26 [Log Event: Blah blah blah
]
48994: Logging string: 23 [Log Event: Blah blah 2
]
48994: EOF on file descriptor 3
EOF on pipe -exiting
48993: PID 48994 exit status 0x0000
$
如您所见，单独的消息是单独记录的，与第一个示例中的联合消息不同。
Encoding very long messages
如果将长消息作为一个单元处理至关重要，但它们相当罕见，则可以发送1..254作为单个单元消息的长度，并发送255以指示'254字节块跟随但其不完整' 。或者，实际上，您可以发送1..255来指示最多255个字节的完整消息，并且0表示“这是一个长度超过255个字节的消息的下一个255字节分期”。 （请注意，您必须只发送255个字节，以便有至少1个字节的后续消息。）这会修改write_log()和read_log()函数，但主代码不必更改 - 除非必须测试多块功能。
此代码比以前的版本更复杂。显示一些调试打印已注释掉。
/* SO 4481-0272 */
/* Variant 3 - handle long messages */
#include <assert.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
static void wait_for(pid_t pid)
{
int corpse;
int status;
while ((corpse = wait(&status)) > 0)
{
printf("%d: PID %d exit status 0x%.4X
", (int)getpid(), corpse, status);
if (corpse == pid)
break;
}
}
static void write_log_segment(int fd, size_t lencode, size_t length, const char *msg)
{
unsigned char byte = lencode;
assert(lencode == length || (lencode == 0 && length == UCHAR_MAX));
if (write(fd, &byte, sizeof(byte)) != sizeof(byte) ||
write(fd, msg, length) != (ssize_t)length) // avoid signed/unsigned comparison
{
fprintf(stderr, "%d: failed to write message to logger
", (int)getpid());
exit(EXIT_FAILURE);
}
//printf("%d: Wrote segment: %zu (code %zu) [%s]
", (int)getpid(), length, lencode, msg);
}
static void write_log(int fd, const char *msg)
{
size_t len = strlen(msg);
printf("%d: Write buffer: %zu [%s]
", (int)getpid(), len, msg);
while (len > 0)
{
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