procps/doc/procps.3

.\" (C) Copyright 2020 Jim Warner <warnerjc@comcast.net>
.\"
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.TH PROCPS_LINUX_VERSION 3 "June 2020" "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>

.BI "int procps_new   (struct info **" info );
.BI "int procps_ref   (struct info  *" info );
.BI "int procps_unref (struct info **" info );

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

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

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

.BI "struct stack **procps_sort ("
.BI "    struct info *" info ,
.BI "    struct stack *" stacks[] ,
.BI "    int " numstacked ,
.BI "    enum item " sortitem ,
.BI "    enum sort_order " order );
.RE
.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\-lprocps\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 items.
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

Optionally, a user may choose to sort results returned from
a \fBreap\fR function call.

.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

The \fBdiskstats\fR, \fBslabinfo\fR and \fBstat\fR interfaces support
unpredictable variable outcomes.
As such, they export a \fBreap\fR function to retrieve multiple `stacks'
with a single invocation.

Those same interfaces also offer a \fBsort\fR function whose
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 wrtten 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).