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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | SIGNAL SAFETY | NOTES | EXAMPLES | HISTORY | SEE ALSO | COLOPHON |
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SD_JOURNAL_STREAM_FD(3) sd_journal_stream_fd SD_JOURNAL_STREAM_FD(3)
sd_journal_stream_fd, sd_journal_stream_fd_with_namespace - Create
log stream file descriptor to the journal
#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);
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 executed.
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, LOG_ERR,
LOG_WARNING, LOG_NOTICE, LOG_INFO, LOG_DEBUG, as defined 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
information.
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
systemd-journald.service(8).
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.
The call returns a valid write-only file descriptor on success or
a negative errno-style error code.
sd_journal_stream_fd() and sd_journal_stream_fd_with_namespace()
are "async signal safe" in the meaning of signal-safety(7).
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.
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));
close(fd);
return 1;
}
fprintf(log, "Hello World!\n");
fprintf(log, SD_WARNING "This is a warning!\n");
fclose(log);
return 0;
}
sd_journal_stream_fd() was added in version 187.
sd_journal_stream_fd_with_namespace() was added in version 256.
systemd(1), sd-journal(3), sd-daemon(3), sd_journal_print(3),
syslog(3), fprintf(3), systemd.journal-fields(7)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 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 is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 SD_JOURNAL_STREAM_FD(3)
Pages that refer to this page: sd-journal(3), sd_journal_print(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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