libcap-2.19/doc/cap_from_text.3

.\"
.\" written by Andrew Main <zefram@dcs.warwick.ac.uk>
.\"
.TH CAP_FROM_TEXT 3 "2008-05-10" "" "Linux Programmer's Manual"
.SH NAME
cap_from_text, cap_to_text, cap_to_name, cap_from_name \- capability
state textual representation translation
.SH SYNOPSIS
.B #include <sys/capability.h>
.sp
.BI "cap_t cap_from_text(const char *" buf_p );
.sp
.BI "char *cap_to_text(cap_t " caps ", ssize_t *" length_p );
.sp
.BI "int cap_from_name(const char *" name ", cap_value_t *" cap_p );
.sp
.BI "char *cap_to_name(cap_value_t " cap );
.sp
Link with \fI-lcap\fP.
.SH DESCRIPTION
These functions translate a capability state between
an internal representation and a textual one.
The internal representation is managed by the capability
functions in working storage. The textual representation is a structured,
human-readable string suitable for display.
.PP
.BR cap_from_text ()
allocates and initializes a capability state in working storage. It
then sets the contents of this newly created capability state to the
state represented by a human-readable, nul-terminated character
string pointed to by
.IR buf_p .
It returns a pointer to the newly created capability state.
When the capability state in working storage is no longer required,
the caller should free any releasable memory
by calling
.BR cap_free ()
with
.I cap_t
as an argument.  The function returns an error if it cannot parse the
contents of the string pointed to by
.I buf_p
or does not recognize any
.I capability_name
or flag character as valid.  The function also returns an error if any flag
is both set and cleared within a single clause.
.PP
.BR cap_to_text ()
converts the capability state in working storage identified by
.I cap_p
into a nul-terminated human-readable string.  This function allocates
any memory necessary to contain the string, and returns a pointer to
the string.  If the pointer
.I len_p
is not NULL,
the function shall also return the full length of the string (not including
the nul terminator) in the location pointed to by
.IR len_p .
The capability state in working storage, identified by
.IR cap_p ,
is completely represented in the character string.
When the capability state in working storage is no longer required,
the caller should free any releasable memory by calling
.BR cap_free ()
with the returned string pointer as an argument.
.PP
.BR cap_from_name ()
converts a text representation of a capability, such as "cap_chown",
to its numerical representation
.RB ( CAP_CHOWN=0 ),
writing the decoded value into
.IR *cap_p .
If
.I cap_p
is NULL
no result is written, but the return code of the function indicates
whether or not the specified capability can be represented by the
library.
.PP
.BR cap_to_name ()
converts a capability index value,
.IR cap ,
to a libcap-allocated textual string. This string should be
deallocated with
.BR cap_free ().
.SH "TEXTUAL REPRESENTATION"
A textual representation of capability sets consists of one or more
whitespace-separated
.IR clauses .
Each clause specifies some operations on a capability set; the set
starts out with all capabilities lowered, and the meaning of the
string is the state of the capability set after all the clauses have
been applied in order.
.PP
Each clause consists of a list of comma-separated capability names
(or the word
.RB ` all '),
followed by an
.IR action-list .
An action-list consists of a sequence of
.I operator flag
pairs.  Legal operators are:
.RB ` = "', '" + "', and `" - "'."
Legal flags are:
.RB ` e "', `" i "', and `" p "'."
These flags are case-sensitive and specify the Effective, Inheritable
and Permitted sets respectively.
.PP
In the capability name lists, all names are case-insensitive.  The
special name
.RB ` all '
specifies all capabilities; it is equivalent to a list naming every
capability individually.
.PP
Unnamed capabilities can also be specified by number. This feature
ensures that libcap can support capabilities that were not allocated
at the time libcap was compiled. However, generally upgrading libcap
will add names for recently allocated capabilities.
.PP
The
.RB ` = '
operator indicates that the listed capabilities are first reset in
all three capability sets.  The subsequent flags (which are optional
when associated with this operator) indicate that the listed
capabilities for the corresponding set are to be raised.  For example:
"all=p" means lower every capability in the Effective and Inheritable
sets but raise all of the Permitted capabilities;
or, "cap_fowner=ep" means raise the Effective and Permitted
override-file-ownership capability, while lowering this Inheritable
capability.
.PP
In the case that the leading operator is
.RB ` = ',
and no list of capabilities is provided, the action-list is assumed to
refer to `all' capabilities.  For example, the following three
clauses are equivalent to each other (and indicate a completely empty
capability set): "all="; "="; "cap_chown,<every-other-capability>=".
.PP
The operators, `+' and `-' both require an explicit preceding
capability list and one or more explicit trailing flags.  The `+'
operator will raise all of the listed capabilities in the flagged
capability sets.  The `-' operator will lower all of the listed
capabilities in the flagged capability sets.  For example:
"all+p" will raise all of the Permitted capabilities; "cap_fowner+p-i"
will raise the override-file-ownership capability in the Permitted
capability set and lower this Inheritable capability;
"cap_fowner+pe-i" and "cap_fowner=+pe" are equivalent.
.SH "RETURN VALUE"
.BR cap_from_text (),
.BR cap_to_text ()
and
.BR cap_to_text ()
return a non-NULL value on success, and NULL on failure.
.BR cap_from_text ()
returns 0 for success, and -1 on failure (unknown capability).
.PP
On failure,
.I errno
is set to 
.BR EINVAL ,
or 
.BR ENOMEM .
.SH "CONFORMING TO"
.BR cap_from_text ()
and
.BR cap_to_text ()
are specified by the withdrawn POSIX.1e draft specification.
.BR cap_from_name ()
and
.BR cap_to_name ()
are Linux extensions.
.SH EXAMPLE
The example program below demonstrates the use of
.BR cap_from_text ()
and
.BR cap_to_text ().
The following shell session shows a some example runs:
.in +4n
.nf

$ ./a.out "cap_chown=p cap_chown+e"
caps_to_text() returned "= cap_chown+ep"
$ ./a.out "all=pe cap_chown-e cap_kill-pe"
caps_to_text() returned "=ep cap_chown-e cap_kill-ep"

.fi
.in
The source code of the program is as follows:
.nf

#include <stdlib.h>
#include <stdio.h>
#include <sys/capability.h>

#define handle_error(msg) \\
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
    cap_t caps;
    char *txt_caps;

    if (argc != 2) {
        fprintf(stderr, "%s <textual\-cap\-set>\\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    caps = cap_from_text(argv[1]);
    if (caps == NULL)
        handle_error("cap_from_text");

    txt_caps = cap_to_text(caps, NULL);
    if (txt_caps == NULL)
        handle_error("cap_to_text");

    printf("caps_to_text() returned \\"%s\\"\\n", txt_caps);

    if (cap_free(txt_caps) != 0 || cap_free(caps) != 0)
        handle_error("cap_free");

    exit(EXIT_SUCCESS);
}
.fi
.SH "SEE ALSO"
.BR libcap (3),
.BR cap_clear (3),
.BR cap_compare (3),
.BR cap_copy_ext (3),
.BR cap_get_file (3),
.BR cap_get_proc (3),
.BR cap_init (3),
.BR capabilities (7)