procps/doc/procps.3

.\" (C) Copyright 2020-2022 Jim Warner <james.warner@comcast.net>
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.TH PROCPS 3 "July 2022" "libproc-2"
.\" Please adjust this date whenever revising the manpage.
.\"
.nh
.SH NAME
procps \- API to access system level information in the /proc filesystem

.SH SYNOPSIS
Five distinct interfaces are represented in this synopsis and named after
the files they access in the /proc pseudo filesystem:
.BR diskstats ", " meminfo ", " slabinfo ", " stat " and " vmstat .

.nf
.RS +4
#include <procps/\fBnamed_interface\fR.h>

.RI "int\fB procps_new  \fR (struct info **" info );
.RI "int\fB procps_ref  \fR (struct info  *" info );
.RI "int\fB procps_unref\fR (struct info **" info );

.RB "struct result *" procps_get " ("
.RI "    struct info *" info ,
.RI "[   const char *" name ",      ]   \fBdiskstats\fR api only"
.RI "    enum item " item );

.RB "struct stack *" procps_select " ("
.RI "    struct info *" info ,
.RI "[   const char *" name ",      ]   \fBdiskstats\fR api only"
.RI "    enum item *" items ,
.RI "    int " numitems );

.RB "struct reaped *" procps_reap " ("
.RI "    struct info *" info ,
.RI "[   enum reap_type " what ",   ]   \fBstat\fR api only"
.RI "    enum item *" items ,
.RI "    int " numitems );

.RB "struct stack **" procps_sort " ("
.RI "    struct info *" info ,
.RI "    struct stack *" stacks [],
.RI "    int " numstacked ,
.RI "    enum item " sortitem ,
.RI "    enum sort_order " order );

.fi

The above functions and structures are generic but the specific
\fBnamed_interface\fR would also be part of any identifiers.
For example, `procps_new' would actually be `procps_\fBmeminfo\fR_new'
and `info' would really be `\fBdiskstats\fR_info', etc.

The same \fBnamed_interface\fR is used in each header file name with
an appended `.h' suffix.

Link with \fI\-lproc-2\fP.

.SH DESCRIPTION
.SS Overview
Central to these interfaces is a simple `result'
structure reflecting an `item' plus its value (in a union
with standard C language types as members).
All `result' structures are automatically allocated and
provided by the library.

By specifying an array of `items', these structures can be
organized as a `stack', potentially yielding many results
with a single function call.
Thus, a `stack' can be viewed as a variable length record
whose content and order is determined solely by the user.

As part of each interface there are two unique enumerators.
The `noop' and `extra' items exist to hold user values.
They are never set by the library, but the `extra'
result will be zeroed with each library interaction.

The \fBnamed_interface\fR header file will be an essential
document during user program development.
There you will find available items, their return type
(the `result' struct member name) and the source for such values.
Additional enumerators and structures are also documented there.

.SS Usage
The following would be a typical sequence of calls to
these interfaces.

.nf
.RB "1. " procps_new()
.RB "2. " procps_get() ", " procps_select() " or " procps_reap()
.RB "3. " procps_unref()
.fi

The \fBget\fR function is used to retrieve a `result' structure for
a single `item'.
Alternatively, a \fBGET\fR macro is available when only the return
value is of interest.

The \fBselect\fR function can retrieve multiple `result' structures
in a single `stack'.

For unpredictable variable outcomes, the \fBdiskstats\fR, \fBslabinfo\fR
and \fBstat\fR interfaces export a \fBreap\fR function.
It is used to retrieve multiple `stacks' each containing multiple
`result' structures.
Optionally, a user may choose to \fBsort\fR those results.

To exploit any `stack', and access individual `result' structures,
a \fIrelative_enum\fR is required as shown in the \fBVAL\fR macro
defined in the header file.
Such values could be hard coded as: 0 through numitems-1.
However, this need is typically satisfied by creating your own
enumerators corresponding to the order of the `items' array.

.SS Caveats
The \fBnew\fR, \fBref\fR, \fBunref\fR, \fBget\fR and \fBselect\fR
functions are available in all five interfaces.

For the \fBnew\fR and \fBunref\fR functions, the address of an \fIinfo\fR
struct pointer must be supplied.
With \fBnew\fR it must have been initialized to NULL.
With \fBunref\fR it will be reset to NULL if the reference count reaches zero.

In the case of the \fBdiskstats\fR interface, a \fIname\fR parameter
on the \fBget\fR and \fBselect\fR functions identifies a disk or
partition name

For the \fBstat\fR interface, a \fIwhat\fR parameter on the \fBreap\fR
function identifies whether data for just CPUs or both CPUs and NUMA
nodes is to be gathered.

When using the \fBsort\fR function, the parameters \fIstacks\fR and
\fInumstacked\fR would normally be those returned in the `reaped'
structure.

.SH RETURN VALUE
.SS Functions Returning an `int'
An error will be indicated by a negative number that
is always the inverse of some well known errno.h value.

Success is indicated by a zero return value.
However, the \fBref\fR and \fBunref\fR functions return
the current \fIinfo\fR structure reference count.

.SS Functions Returning an `address'
An error will be indicated by a NULL return pointer
with the reason found in the formal errno value.

Success is indicated by a pointer to the named structure.

.SH DEBUGGING
To aid in program development, there is a provision that can
help ensure `result' member references agree with library
expectations.
It assumes that a supplied macro in the header file is
used to access the `result' value.

This feature can be activated through either of the following
methods and any discrepancies will be written to \fBstderr\fR.

.IP 1) 3
Add CFLAGS='-DXTRA_PROCPS_DEBUG' to any other ./configure
options employed.

.IP 2) 3
Add #include <procps/xtra-procps-debug.h> to any program
\fIafter\fR the named interface includes.

.PP
This verification feature incurs substantial overhead.
Therefore, it is important that it \fInot\fR be activated
for a production/release build.

.SH SEE ALSO
.BR procps_misc (3),
.BR procps_pids (3),
.BR proc (5).