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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |EXAMPLES |SEE ALSO |COLOPHON | |
mq_getattr(3) Library Functions Manualmq_getattr(3)mq_getattr, mq_setattr - get/set message queue attributes
Real-time library (librt,-lrt)
#include <mqueue.h>int mq_getattr(mqd_tmqdes, struct mq_attr *attr);int mq_setattr(mqd_tmqdes, const struct mq_attr *restrictnewattr,struct mq_attr *restrictoldattr);
mq_getattr() andmq_setattr() respectively retrieve and modify attributes of the message queue referred to by the message queue descriptormqdes.mq_getattr() returns anmq_attr structure in the buffer pointed byattr. 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 */ }; Themq_flags field contains flags associated with the open message queue description. This field is initialized when the queue is created bymq_open(3). The only flag that can appear in this field isO_NONBLOCK. Themq_maxmsg andmq_msgsize fields are set when the message queue is created bymq_open(3). Themq_maxmsg field is an upper limit on the number of messages that may be placed on the queue usingmq_send(3). Themq_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 inmq_overview(7). Themq_curmsgs field returns the number of messages currently held in the queue.mq_setattr() sets message queue attributes using information supplied in themq_attr structure pointed to bynewattr. The only attribute that can be modified is the setting of theO_NONBLOCK flag inmq_flags. The other fields innewattr are ignored. If theoldattr field is not NULL, then the buffer that it points to is used to return anmq_attr structure that contains the same information that is returned bymq_getattr().On successmq_getattr() andmq_setattr() return 0; on error, -1 is returned, witherrno set to indicate the error.
EBADFThe message queue descriptor specified inmqdes is invalid.EINVALnewattr->mq_flags contained set bits other thanO_NONBLOCK.
For an explanation of the terms used in this section, seeattributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface│Attribute│Value│ ├──────────────────────────────────────┼───────────────┼─────────┤ │mq_getattr(),mq_setattr() │ Thread safety │ MT-Safe │ └──────────────────────────────────────┴───────────────┴─────────┘
On Linux,mq_getattr() andmq_setattr() are library functions layered on top of themq_getsetattr(2) system call.
POSIX.1-2008.
POSIX.1-2001.
The program below can be used to show the defaultmq_maxmsg andmq_msgsize values that are assigned to a message queue that is created with a call tomq_open(3) in which theattr 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 inmq_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; 8192Program source #include <fcntl.h> #include <mqueue.h> #include <stdio.h> #include <stdlib.h> #include <sys/stat.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, 0600, 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); }mq_close(3),mq_notify(3),mq_open(3),mq_receive(3),mq_send(3),mq_unlink(3),mq_overview(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17mq_getattr(3)Pages that refer to this page:mq_getsetattr(2), mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), systemd.socket(5), mq_overview(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |