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UTRACE(2) System Calls Manual UTRACE(2)

utrace —
insert user record in ktrace log

#include <sys/types.h>
#include <sys/param.h>
#include <sys/uio.h>
#include <sys/ktrace.h>

int
utrace(const char *label, void *addr, size_t len);

Adds a record to the process trace with information supplied by the user. The record is identified by label and contains len bytes from memory pointed to by addr. This call only has an effect if the calling process is being traced.

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.

[]
The currently running kernel was compiled without ktrace(2) support (option KTRACE).
[]
The length of the label string was longer than KTR_USER_MAXIDLEN-1.
[]
The specified data length len was bigger than KTR_USER_MAXLEN.

kdump(1), ktrace(1), ktrace(2), options(4)

The utrace() system call first appeared in FreeBSD 2.2. It was added to OpenBSD in OpenBSD 5.4. The label argument is an extension.
June 1, 2013 OpenBSD-current