[英]sem_wait is ignored in my program
I write a simple project on shared memory in linux. 我在linux上的共享内存上编写了一个简单的项目。 Two programs share memory, one is writing letters to it and second is reading them from it.
两个程序共享内存,一个程序向其中写入字母,第二个程序从中读取它们。 I decided to use semaphores in order to ensure that no new letter is produced until it is read.
我决定使用信号量,以确保在读取之前不会产生新的字母。
The problem is that my writer process is ignoring sem_wait( reading ) when its value is 0 and it should wait. 问题是我的编写器进程在其值为0且应等待时忽略了sem_wait(reading)。 It finishes its job before the reader even starts.
它甚至在读者开始之前就完成了工作。 I run it through
./writer & ./reader
. 我通过
./writer & ./reader
运行它。
I enclose the code. 我附上代码。 There are a few unused elements here because it is not the final version yet.
这里有一些未使用的元素,因为它不是最终版本。 However problem already arised.
但是问题已经出现。
/* writer.c */
#include <stdio.h>
#include <stdlib.h>
#include <sys/shm.h>
#include <unistd.h>
#include <stdlib.h>
#include <semaphore.h>
int main( int argc, char *argv[] )
{
key_t shmkey = 0xF00;
int bytes = sizeof(char)*3 + sizeof(sem_t) * 3;
int shmid;
char* sharedMemory;
sem_t *writing, *reading, *working;
if ( (shmid = shmget( shmkey, bytes, IPC_CREAT | IPC_EXCL | 0666 )) < 0 )
{
shmdt( (void*) sharedMemory );
shmctl( shmid, IPC_RMID, NULL );
return 1;
}
if ( (sharedMemory = (char*) shmat( shmid, NULL, 0 )) == (char*) -1 )
{
shmdt( (void*) sharedMemory );
shmctl( shmid, IPC_RMID, NULL );
return 1;
}
writing = (sem_t*)(sharedMemory + 3);
reading = writing + 1;
working = reading + 1;
sem_init( writing, 0, 0 );
sem_init( reading, 0, 0 );
sharedMemory[2] = 'w'; // writer is running
char c;
for( c = 'a'; c <= 'z'; ++c )
{
*sharedMemory = c;
sem_post( writing );
sem_wait( reading );
}
sharedMemory[2] = 'q';
while ( sharedMemory[2] != 'w' );
sharedMemory[2] = 'q';
shmdt( (void*) sharedMemory );
shmctl( shmid, IPC_RMID, NULL );
return 0;
}
And the reader, 还有读者
/* reader.c */
#include <stdio.h>
#include <stdlib.h>
#include <sys/shm.h>
#include <unistd.h>
#include <stdlib.h>
#include <semaphore.h>
int main( int argc, char *argv[] )
{
key_t shmkey = 0xF00;
int bytes = sizeof(char)*3 + sizeof(sem_t) * 3;
int shmid;
char* sharedMemory;
sem_t *writing, *reading, *working;
sleep(1); // wait until writer allocates fresh memory
if ( (shmid = shmget( shmkey, bytes, 0666 )) < 0 )
{
shmdt( (void*) sharedMemory );
return 1;
}
if ( (sharedMemory = (char*) shmat( shmid, NULL, 0 )) == (char*) -1 )
{
shmdt( (void*) sharedMemory );
return 1;
}
if ( sharedMemory[2] != 'w' ) // is writer running?
{
shmdt( (void*) sharedMemory );
return 1;
}
writing = (sem_t*)(sharedMemory + 3);
reading = writing + 1;
working = reading + 1;
//sleep(5); //@REMOVE
char c;
do
{
sem_wait( writing );
c = *sharedMemory;
sem_post( reading );
printf( "%c\n", c );
} while ( sharedMemory[2] == 'w' );
sharedMemory[2] = 'w';
shmdt( (void*) sharedMemory );
return 0;
}
sharedMemory + 3
is not properly aligned for type sem_t
. sharedMemory + 3
对于sem_t
类型未正确对齐。 Since you don't know the alignment requirement for sem_t
, you need to make sure that your sem_t
objects start at an offset in the shared memory segment that is a multiple of sizeof(sem_t)
(this works because the alignment requirement of any object evenly divides its size). 由于您不知道
sem_t
的对齐要求,因此需要确保sem_t
对象从共享内存段中的偏移量开始,该偏移量是sizeof(sem_t)
的倍数(这很有效,因为任何对象的对齐需求均等划分大小)。
Note that you should be checking the return values of sem_wait
and sem_post
. 请注意,您应该检查
sem_wait
和sem_post
的返回值。 Then you could inspect errno
if they fail and that would give you information on why they're failing (however in your case I suspect the errno
value may have been less than helpful). 然后,您可以检查
errno
是否失败,这将为您提供有关它们失败原因的信息(但是,在您的情况下,我怀疑errno
值可能没有帮助)。
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