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SENDSYSLOG(2) System Calls Manual SENDSYSLOG(2)

sendsyslog —
send a message to syslogd

#include <sys/syslog.h>
#include <sys/types.h>

int
sendsyslog(const char *msg, size_t len, int flags);

The sendsyslog() function is used to transmit a syslog(3) formatted message direct to syslogd(8) without requiring the allocation of a socket. The msg is not NUL terminated and its len is limited to 8192 bytes. If LOG_CONS is specified in the flags argument, and syslogd(8) is not accepting messages, the message will be sent to the console. This is used internally by syslog_r(3), so that messages can be sent during difficult situations. If sending to syslogd(8) fails, dropped messages are counted. When syslogd(8) works again, a warning with the counter and error number is logged.

To receive messages from the kernel, syslogd(8) has to create a datagram socket pair and register one end. This registration is done by opening the /dev/klog device and passing one file descriptor of the socket pair as argument to ioctl(2) LIOCSFD invoked on the klog file descriptor. After that the messages can be be read from the other end of the socket pair. By utilizing /dev/klog the access to log messages is limited to processes that may open this device.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

sendsyslog() can fail if:
[]
An invalid user space address was specified for a parameter.
[]
The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.
[]
The system was unable to allocate an internal buffer. The operation may succeed when buffers become available.
[]
The message cannot be sent, likely because syslogd(8) is not running.

syslog_r(3), syslogd(8)

The sendsyslog() function call appeared in OpenBSD 5.6. The flags argument was added in OpenBSD 6.0.
September 27, 2017 OpenBSD-current