Ubuntu Manpage: cap_from_text, cap_to_text, cap_to_name, cap_from_name

NAME

       cap_from_text,   cap_to_text,   cap_to_name,  cap_from_name  -  capability  state  textual
       representation translation

SYNOPSIS

       #include <sys/capability.h>

       cap_t cap_from_text(const char *buf_p);

       char *cap_to_text(cap_t caps, ssize_t *length_p);

       int cap_from_name(const char *name, cap_value_t *cap_p);

       char *cap_to_name(cap_value_t cap);

       Link with -lcap.

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.

       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 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 cap_free() with
       cap_t as an argument.  The function returns an error if it cannot parse  the  contents  of
       the string pointed to by buf_p or does not recognize any 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.

       cap_to_text()  converts the capability state in working storage identified by 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 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 len_p.  The capability state in working storage,
       identified by 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 cap_free() with the returned string pointer as an argument.

       cap_from_name() converts a text representation of a capability, such  as  "cap_chown",  to
       its  numerical  representation  (CAP_CHOWN=0),  writing the decoded value into *cap_p.  If
       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.

       cap_to_name()  converts  a  capability  index  value,  cap,  to a libcap-allocated textual
       string. This string should be deallocated with cap_free().

TEXTUAL REPRESENTATION

A textual representation of capability sets consists of one or more whitespace-separated
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.

Each clause consists of a list of comma-separated capability names (or the word `all'),
followed by an action-list. An action-list consists of a sequence of operator flag pairs.
Legal operators are: `=', '+', and `-'. Legal flags are: `e', `i', and `p'. These flags
are case-sensitive and specify the Effective, Inheritable and Permitted sets respectively.

In the capability name lists, all names are case-insensitive. The special name `all'
specifies all capabilities; it is equivalent to a list naming every capability
individually.

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.

The `=' 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.

In the case that the leading operator is `=', 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>=".

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.

RETURN VALUE

       cap_from_text(),  cap_to_text()  and cap_to_name() return a non-NULL value on success, and
       NULL on failure.  cap_from_name() returns 0  for  success,  and  -1  on  failure  (unknown
       capability).

       On failure, errno is set to EINVAL, or ENOMEM.

CONFORMING TO

       cap_from_text()   and   cap_to_text()  are  specified  by  the  withdrawn  POSIX.1e  draft
       specification.  cap_from_name() and cap_to_name() are Linux extensions.

EXAMPLE

       The example program below demonstrates the use of cap_from_text() and cap_to_text().   The
       following shell session shows a some example runs:

           $ ./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"

       The source code of the program is as follows:

       #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);
       }