sd_journal_stream_fd(3) — Linux manual page


SD_JOURNAL_STREAM_FD(3)   sd_journal_stream_fd   SD_JOURNAL_STREAM_FD(3)

NAME         top

       sd_journal_stream_fd, sd_journal_stream_fd_with_namespace -
       Create log stream file descriptor to the journal

SYNOPSIS         top

       #include <systemd/sd-journal.h>

       int sd_journal_stream_fd(const char *identifier, int priority,
                                int level_prefix);

       int sd_journal_stream_fd_with_namespace(const char *name_space,
                                               const char *identifier,
                                               int priority,
                                               int level_prefix);

DESCRIPTION         top

       sd_journal_stream_fd() may be used to create a log stream file
       descriptor. Log messages written to this file descriptor as
       simple newline-separated text strings are written to the journal.
       This file descriptor can be used internally by applications or be
       made standard output or standard error of other processes

       sd_journal_stream_fd() takes a short program identifier string as
       first argument, which will be written to the journal as
       SYSLOG_IDENTIFIER= field for each log entry (see
       systemd.journal-fields(7) for more information). The second
       argument shall be the default priority level for all messages.
       The priority level is one of LOG_EMERG, LOG_ALERT, LOG_CRIT,
       in syslog.h, see syslog(3) for details. The third argument is a
       boolean: if true kernel-style log level prefixes (such as
       SD_WARNING) are interpreted, see sd-daemon(3) for more

       sd_journal_stream_fd_with_namespace() is similar to
       sd_journal_stream_fd(), but takes an additional name_space
       parameter that specifies which journal namespace to operate on.
       If specified as NULL the call is identical to
       sd_journal_stream_fd(). For details about journal namespaces, see

       It is recommended that applications log UTF-8 messages only with
       this API, but this is not enforced.

       Each invocation of these functions allocates a new log stream
       file descriptor, that is not shared with prior or later
       invocations. The file descriptor is write-only (its reading
       direction is shut down), and O_NONBLOCK is turned off initially.

RETURN VALUE         top

       The call returns a valid write-only file descriptor on success or
       a negative errno-style error code.

SIGNAL SAFETY         top

       sd_journal_stream_fd() and sd_journal_stream_fd_with_namespace()
       are "async signal safe" in the meaning of signal-safety(7).

NOTES         top

       All functions listed here are thread-safe and may be called in
       parallel from multiple threads.

       Functions described here are available as a shared library, which
       can be compiled against and linked to with the
       libsystemd pkg-config(1) file.

EXAMPLES         top

       Creating a log stream suitable for fprintf(3):

           /* SPDX-License-Identifier: MIT-0 */

           #define _GNU_SOURCE 1
           #include <errno.h>
           #include <syslog.h>
           #include <stdio.h>
           #include <unistd.h>
           #include <systemd/sd-journal.h>
           #include <systemd/sd-daemon.h>

           int main(int argc, char *argv[]) {
             int fd;
             FILE *log;

             fd = sd_journal_stream_fd("test", LOG_INFO, 1);
             if (fd < 0) {
               fprintf(stderr, "Failed to create stream fd: %s\n", strerror(-fd));
               return 1;

             log = fdopen(fd, "w");
             if (!log) {
               fprintf(stderr, "Failed to create file object: %s\n", strerror(errno));
               return 1;
             fprintf(log, "Hello World!\n");
             fprintf(log, SD_WARNING "This is a warning!\n");
             return 0;

HISTORY         top

       sd_journal_stream_fd() was added in version 187.

       sd_journal_stream_fd_with_namespace() was added in version 256.

SEE ALSO         top

       systemd(1), sd-journal(3), sd-daemon(3), sd_journal_print(3),
       syslog(3), fprintf(3), systemd.journal-fields(7)

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2024-06-13.)  If you discover any rendering
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       (which is not part of the original manual page), send a mail to

systemd 257~devel                                SD_JOURNAL_STREAM_FD(3)

Pages that refer to this page: sd-journal(3)sd_journal_print(3)systemd.directives(7)systemd.index(7)