void openlog(const char *ident, int logopt, int facility);
int setlogmask(int maskpri);
void syslog(int priority, const char *message, ... /* arguments */);
IEEE Std 1003.1-2017
syslog() function shall send a message to an implementation-defined logging facility, which may log it in an
implementation-defined system log, write it to the system console, forward it to a list of users, or forward it to the logging
facility on another host over the network. The logged message shall include a message header and a message body. The message header
contains at least a timestamp and a tag string.
The message body is generated from the message and following arguments in the same manner as if these were arguments to
printf(), except that the additional conversion specification
%m shall be
recognized; it shall convert no arguments, shall cause the output of the error message string associated with the value of
errno on entry to
syslog(), and may be mixed with argument specifications of the
If a complete conversion specification with the m conversion specifier character is not just
%m, the behavior is
undefined. A trailing
<newline> may be added if needed.
Values of the priority argument are formed by OR'ing together a severity-level value and an optional facility value. If no facility value is specified, the current default facility value is used.
Possible values of severity level include:
LOG_EMERG- A panic condition.
LOG_ALERTA condition that should be corrected immediately, such as a corrupted system database.
LOG_CRIT- Critical conditions, such as hard device errors.
LOG_WARNING- Warning messages.
LOG_NOTICE- Conditions that are not error conditions, but that may require special handling.
LOG_INFO- Informational messages.
LOG_DEBUG- Messages that contain information normally of use only when debugging a program.
The facility indicates the application or system component generating the message. Possible facility values include:
LOG_USER- Messages generated by arbitrary processes. This is the default facility identifier if none is specified.
LOG_LOCAL0- Reserved for local use.
LOG_LOCAL1- Reserved for local use.
LOG_LOCAL2- Reserved for local use.
LOG_LOCAL3- Reserved for local use.
LOG_LOCAL4- Reserved for local use.
LOG_LOCAL5- Reserved for local use.
LOG_LOCAL6- Reserved for local use.
LOG_LOCAL7- Reserved for local use.
openlog() function shall set process attributes that affect subsequent calls to
syslog(). The ident
argument is a string that is prepended to every message. The logopt argument indicates logging options. Values for
logopt are constructed by a bitwise-inclusive OR of zero or more of the following:
LOG_PID- Log the process ID with each message. This is useful for identifying specific processes.
LOG_CONS- Write messages to the system console if they cannot be sent to the logging facility. The
syslog()function ensures that the process does not acquire the console as a controlling terminal in the process of writing the message.
LOG_NDELAY- Open the connection to the logging facility immediately. Normally the open is delayed until the first message is logged. This is useful for programs that need to manage the order in which file descriptors are allocated.
LOG_ODELAY- Delay open until
LOG_NOWAIT- Do not wait for child processes that may have been created during the course of logging the message. This option should be used by processes that enable notification of child termination using
syslog()may otherwise block waiting for a child whose exit status has already been collected.
The facility argument encodes a default facility to be assigned to all messages that do not have an explicit facility
already encoded. The initial default facility is
syslog() functions may allocate a file descriptor. It is not necessary to call
prior to calling
closelog() function shall close any open file descriptors allocated by previous calls to
setlogmask() function shall set the log priority mask for the current process to maskpri and return the
previous mask. If the maskpri argument is 0, the current log mask is not modified. Calls by the current process to
syslog() with a priority not set in maskpri shall be rejected. The default log mask allows all priorities to be
logged. A call to
openlog() is not required prior to calling
Symbolic constants for use as values of the logopt, facility, priority, and maskpri arguments are
defined in the
setlogmask() function shall return the previous log priority mask. The
syslog() functions shall not return a value.
No errors are defined.