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sd_journal_stream_fd(3) — Linux manual page

NAME \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|SIGNAL SAFETY \|NOTES \|EXAMPLES \|HISTORY \|SEE ALSO \|COLOPHON

SD_JOURNAL_STREAM_FD(3)    sd_journal_stream_fdSD_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, intpriority,intlevel_prefix);int sd_journal_stream_fd_with_namespace(const char *name_space,const char *identifier,intpriority,intlevel_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 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 (seesystemd.journal-fields(7) for more information). The second argument shall be the default priority level for all messages. The priority level is one ofLOG_EMERG,LOG_ALERT,LOG_CRIT,LOG_ERR,LOG_WARNING,LOG_NOTICE,LOG_INFO,LOG_DEBUG, as defined in syslog.h, seesyslog(3) for details. The third argument is a boolean: if true kernel-style log level prefixes (such asSD_WARNING) are interpreted, seesd-daemon(3) for more information.sd_journal_stream_fd_with_namespace()is similar tosd_journal_stream_fd(), but takes an additionalname_space parameter that specifies which journal namespace to operate on. If specified asNULLthe call is identical tosd_journal_stream_fd(). For details about journal namespaces, seesystemd-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), andO_NONBLOCKis 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()andsd_journal_stream_fd_with_namespace()       are "async signal safe" in the meaning ofsignal-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 thelibsystemd pkg-config(1) file.

EXAMPLES top

       Creating a log stream suitable forfprintf(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;           }

HISTORY top

sd_journal_stream_fd()was added in version 187.sd_journal_stream_fd_with_namespace()was added in version 256.

COLOPHON top

       This page is part of thesystemd (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 isnot part of the original manual page), send a mail to       man-pages@man7.orgsystemd 258~rc2SD_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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