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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | DEBUGGING | ENVIRONMENT VARIABLE(S) | SEE ALSO | COLOPHON |
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PROCPS_PIDS(3) Library Functions Manual PROCPS_PIDS(3)
procps_pids - API to access process information in the /proc
filesystem
#include <libproc2/pids.h>
int procps_pids_new (struct pids_info **info, enum pids_item *items, int numitems);
int procps_pids_ref (struct pids_info *info);
int procps_pids_unref (struct pids_info **info);
struct pids_stack *procps_pids_get (
struct pids_info *info,
enum pids_fetch_type which);
struct pids_fetch *procps_pids_reap (
struct pids_info *info,
enum pids_fetch_type which);
struct pids_fetch *procps_pids_select (
struct pids_info *info,
unsigned *these,
int numthese,
enum pids_select_type which);
struct pids_stack **procps_pids_sort (
struct pids_info *info,
struct pids_stack *stacks[],
int numstacked,
enum pids_item sortitem,
enum pids_sort_order order);
int procps_pids_reset (
struct pids_info *info,
enum pids_item *newitems,
int newnumitems);
struct pids_stack *fatal_proc_unmounted (
struct pids_info *info,
int return_self);
Link with -lproc2.
Overview
Central to this interface 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 this 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 pids.h 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.
Usage
The following would be a typical sequence of calls to this
interface.
1. fatal_proc_unmounted()
2. procps_pids_new()
3. procps_pids_get(), procps_pids_reap() or procps_pids_select()
4. procps_pids_unref()
The get function is an iterator for successive PIDs/TIDs,
returning those ‘items’ previously identified via new or reset.
Two functions support unpredictable variable outcomes. The reap
function gathers data for all processes while the select function
deals with specific PIDs or UIDs. Both can return multiple
‘stacks’ each containing multiple ‘result’ structures.
Optionally, a user may choose to sort such results
To exploit any ‘stack’, and access individual ‘result’ structures,
a relative_enum is required as shown in the VAL 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.
Caveats
The <pids> API differs from others in that those items of interest
must be provided at new or reset time, the latter being unique to
this API. If either the items or numitems parameter is zero at
new time, then reset becomes mandatory before issuing any other
call. Otherwise, procps_pids_reset() should only be called when
the order or number of items in your stacks is changed.
For the new and unref functions, the address of an info struct
pointer must be supplied. With new it must have been initialized
to NULL. With unref it will be reset to NULL if the reference
count reaches zero.
The get and reap functions use the which parameter to specify
whether just tasks or both tasks and threads are to be fetched.
The select function requires an array of PIDs or UIDs as these
along with numthese to identify which processes are to be fetched.
This function then operates as a subset of reap.
When using the sort function, the parameters stacks and numstacked
would normally be those returned in the ‘pids_fetch’ structure.
Lastly, a fatal_proc_unmounted function may be called before any
other function to ensure that the /proc/ directory is mounted. As
such, the info parameter would be NULL and the return_self
parameter zero. If, however, some items are desired for the
issuing program (a return_self other than zero) then the new call
must precede it to identify the items and obtain the required info
pointer.
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 ref and
unref functions return the current info structure reference count.
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.
However, if one survives the fatal_proc_unmounted call, NULL is
always returned when return_self is zero.
To aid in program development, there are two procps-ng provisions
that can be exploited.
The first is a supplied file named ‘libproc.supp’ which may be
useful when developing a multi-threaded application. When used
with the valgrind ‘--suppressions=’ option, warnings associated
with the procps library itself are avoided.
Such warnings arise because the library handles heap based
allocations in a thread-safe manner. A single-threaded
application will not receive those warnings.
The second provision 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 stderr.
1) Add CFLAGS='-DXTRA_PROCPS_DEBUG' to any other ./configure
options your project may employ.
2) Add #include <libproc2/xtra-procps-debug.h> to any program
after the #include <libproc2/pids.h>.
This verification feature incurs substantial overhead. Therefore,
it is important that it not be activated for a production/release
build.
The value set for the following is unimportant, just its presence.
LIBPROC_HIDE_KERNEL
This will hide kernel threads which would otherwise be
returned with a procps_pids_get, procps_pids_select or
procps_pids_reap call.
procps(3), procps_misc(3), proc(5).
This page is part of the procps-ng (/proc filesystem utilities)
project. Information about the project can be found at
⟨https://gitlab.com/procps-ng/procps⟩. If you have a bug report
for this manual page, see
⟨https://gitlab.com/procps-ng/procps/blob/master/Documentation/bugs.md⟩.
This page was obtained from the project's upstream Git repository
⟨https://gitlab.com/procps-ng/procps.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-07-30.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
procps-ng 2004-09-03 PROCPS_PIDS(3)
Pages that refer to this page: procps(3), procps_misc(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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