Index · Directives systemd 250


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 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.

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

Each invocation of sd_journal_stream_fd() 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

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

Signal safety

sd_journal_stream_fd() is "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.

These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd 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");
    return 1;
  fprintf(log, "Hello World!\n");
  fprintf(log, SD_WARNING "This is a warning!\n");
  return 0;

See Also

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