sd_journal_stream_fd — 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 ); | 
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 STDOUT/STDERR 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 priority level prefixes (such as
                SD_WARNING) are interpreted, see
                sd-daemon(3)
                for more information.
It is recommended that applications log UTF-8 messages only with this API, but this is not enforced.
The call returns a valid write-only file descriptor on success or a negative errno-style error code.
The sd_journal_stream_fd()
                interface is available as shared library, which can
                be compiled and linked to with the
                libsystemd-journal
                pkg-config(1)
                file.
Creating a log stream suitable for fprintf(3):
#include <syslog.h>
#include <stdio.h>
#include <string.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: %m\n");
                close(fd);
                return 1;
        }
        fprintf(log, "Hello World!\n");
        fprintf(log, SD_WARNING "This is a warning!\n");
        fclose(log);
        return 0;
}