.\"	$OpenBSD: profil.2,v 1.17 2025/07/13 23:38:55 jsg Exp $
.\"	$NetBSD: profil.2,v 1.3 1995/11/22 23:07:23 cgd Exp $
.\"
.\" Copyright (c) 1993
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.\" This code is derived from software contributed to Berkeley by
.\" Donn Seeley of BSDI.
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.\"	@(#)profil.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: July 13 2025 $
.Dt PROFIL 2
.Os
.Sh NAME
.Nm profil
.Nd control process profiling
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn profil "void *buf" "size_t buflen" "size_t samplesize" "u_long offset" "u_int scale" "int dirfd"
.Sh DESCRIPTION
The
.Fn profil
function is only available inside programs compiled with the
.Fl pg
compiler / linker option.
.Fl pg
selects the profiling version of the C-run-time code
.Pa ( gcrt0.o ) ,
which places an ELF note into the binary which enables the system call.
The profiling subsystem only works with static binaries, so the
.Fl static
link option is also required.
.Pp
The first call to
.Fn profil
happens at startup inside
.Pa gcrt0.o
and sets up a region of memory
.Fa buf
that is
.Fa buflen
bytes long to contain a
.Va struct gmonhdr ,
a samples buffer of size
.Fa samplesize ,
and a sufficiently-sized arc-table.
.Pp
.Fa dirfd
can indicate the path for placing the output file
(environment variable
.Ev PROFDIR
is the usual choice).
Otherwise, \-1 indicates the current (starting) directory location.
.Pp
Program execution then continues with profiling operational.
During execution, profiling can be selectively stopped and restarted using
.Xr moncontrol 3 .
.Pp
While profiling is enabled, at every clock tick, the kernel updates an
appropriate count in the samples buffer.
.Pp
The samples buffer contains
.Fa samplesize
bytes and is divided into a series of 16-bit bins.
Each bin counts the number of times the program counter was in a particular
address range in the process when a clock tick occurred while profiling
was enabled.
For a given program counter address, the number of the corresponding bin
is given by the relation:
.Bd -literal -offset indent
[(pc - offset) / 2] * scale / 65536
.Ed
.Pp
The
.Fa offset
parameter is the lowest address at which the kernel takes program
counter samples.
The
.Fa scale
parameter ranges from 1 to 65536 and can be used to change the
span of the bins.
A scale of 65536 maps each bin to 2 bytes of address range;
a scale of 32768 gives 4 bytes, 16384 gives 8 bytes and so on.
Intermediate values provide approximate intermediate ranges.
A
.Fa scale
value of 0 disables profiling.
.Pp
At normal program termination, the C-run-time completes the data in the
buffer to final format, and proceeds into
.Xr _exit 2 .
The kernel then constructs a pathname
.Pa gmon.progname.pid.out
and stores the data to the filesystem (either at the starting directory,
or the optional directory indicated by the environment variable
.Ev PROFDIR ) .
.Pp
Further processing is then done using
.Xr gprof 1 .
.Sh RETURN VALUES
.Rv -std
.Sh FILES
.Bl -tag -width gmon.progname.pid.out -compact
.It Pa /usr/lib/gcrt0.o
profiling C run-time startup file
.It Pa gmon.progname.pid.out
conventional name for profiling output file
.El
.Sh ERRORS
The following error may be reported:
.Bl -tag -width Er
.It Bq Er EPERM
The program was not linked with
.Fl pg .
.It Bq Er EALREADY
An attempt was made to change the profile buffer.
.It Bq Er EBADF
The
.Fa dirfd
argument is not a valid file descriptor.
.It Bq Er ENOTDIR
The
.Fa dirfd
argument does not refer to a directory.
.It Bq Er EINVAL
The
.Fa scale
value is too large.
.El
.Sh SEE ALSO
.Xr gprof 1
.Sh HISTORY
The
.Fn profil
system call first appeared in
.At v5 .
.Pp
Historically, profile information was written to the file by the C-run-time
exit processing code using
.Xr open 2 ,
.Xr write 2 ,
and such \(em which is incompatible with modern privilege separation practices
like
.Xr chroot 2 ,
.Xr pledge 2 ,
.Xr setresuid 2 ,
and
.Xr unveil 2 .
This replacement
.Fn profil
interface was redesigned so the kernel writes out the profiling information
on behalf of the terminating process.
.Sh BUGS
The
.Fa samples
argument should really be a vector of type
.Fa "unsigned short" .