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   mq_getattr    ( 3 )

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Имя (Name)

mq_getattr, mq_setattr - get/set message queue attributes

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Синопсис (Synopsis)

#include <mqueue.h>

int mq_getattr(mqd_t mqdes, struct mq_attr *attr); int mq_setattr(mqd_t mqdes, const struct mq_attr *restrict newattr, struct mq_attr *restrict oldattr);

Link with -lrt.

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Описание (Description)

mq_getattr() and mq_setattr() respectively retrieve and modify attributes of the message queue referred to by the message queue descriptor mqdes.

mq_getattr() returns an mq_attr structure in the buffer pointed by attr. This structure is defined as:

struct mq_attr { long mq_flags; /* Flags: 0 or O_NONBLOCK */ long mq_maxmsg; /* Max. # of messages on queue */ long mq_msgsize; /* Max. message size (bytes) */ long mq_curmsgs; /* # of messages currently in queue */ };

The mq_flags field contains flags associated with the open message queue description. This field is initialized when the queue is created by mq_open(3). The only flag that can appear in this field is O_NONBLOCK.

The mq_maxmsg and mq_msgsize fields are set when the message queue is created by mq_open(3). The mq_maxmsg field is an upper limit on the number of messages that may be placed on the queue using mq_send(3). The mq_msgsize field is an upper limit on the size of messages that may be placed on the queue. Both of these fields must have a value greater than zero. Two /proc files that place ceilings on the values for these fields are described in mq_overview(7).

The mq_curmsgs field returns the number of messages currently held in the queue.

mq_setattr() sets message queue attributes using information supplied in the mq_attr structure pointed to by newattr. The only attribute that can be modified is the setting of the O_NONBLOCK flag in mq_flags. The other fields in newattr are ignored. If the oldattr field is not NULL, then the buffer that it points to is used to return an mq_attr structure that contains the same information that is returned by mq_getattr().

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Возвращаемое значение (Return value)

On success mq_getattr() and mq_setattr() return 0; on error, -1 is returned, with errno set to indicate the error.

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Ошибки (Error)

EBADF The message queue descriptor specified in mqdes is invalid.

EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK.

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Атрибуты (Attributes)

For an explanation of the terms used in this section, see attributes(7).

┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface │ Attribute │ Value │ ├──────────────────────────────────────┼───────────────┼─────────┤ │mq_getattr(), mq_setattr() │ Thread safety │ MT-Safe │ └──────────────────────────────────────┴───────────────┴─────────┘

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Стандарты (Conforming to)

POSIX.1-2001, POSIX.1-2008.

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Примечание (Note)

On Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call.

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Примеры (Examples)

The program below can be used to show the default mq_maxmsg and
       mq_msgsize values that are assigned to a message queue that is
       created with a call to mq_open(3) in which the attr argument is
       NULL.  Here is an example run of the program:

           $ ./a.out /testq
           Maximum # of messages on queue:   10
           Maximum message size:             8192

       Since Linux 3.5, the following /proc files (described in
       mq_overview(7)) can be used to control the defaults:

           $ uname -sr
           Linux 3.8.0
           $ cat /proc/sys/fs/mqueue/msg_default
           10
           $ cat /proc/sys/fs/mqueue/msgsize_default
           8192

   Program source

       #include <mqueue.h>
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       int
       main(int argc, char *argv[])
       {
           mqd_t mqd;
           struct mq_attr attr;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s mq-name\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           mqd = mq_open(argv[1], O_CREAT | O_EXCL, S_IRUSR | S_IWUSR, NULL);
           if (mqd == (mqd_t) -1)
               errExit("mq_open");

           if (mq_getattr(mqd, &attr) == -1)
               errExit("mq_getattr");

           printf("Maximum # of messages on queue:   %ld\n", attr.mq_maxmsg);
           printf("Maximum message size:             %ld\n", attr.mq_msgsize);

           if (mq_unlink(argv[1]) == -1)
               errExit("mq_unlink");

           exit(EXIT_SUCCESS);
       }

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Смотри также (See also)

mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)

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