lib/libutil/sockaddr_snprintf.3

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.\" by Christos Zoulas.
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.Dd January 2, 2021
.Dt SOCKADDR_SNPRINTF 3
.Os
.Sh NAME
.Nm sockaddr_snprintf
.Nd formatting function for socket address structures
.Sh LIBRARY
.Lb libutil
.Sh SYNOPSIS
.In util.h
.Ft int
.Fn sockaddr_snprintf "char *buf" "size_t buflen" "const char *fmt" "const struct sockaddr *sa"
.Sh DESCRIPTION
The
.Fn sockaddr_snprintf
function formats a socket address into a form suitable for printing.
.Pp
This function is convenient because it is protocol independent, i.e. one does
not need to know the address family of the sockaddr in order to print it.
The
.Xr printf 3
like format string specifies how the address is going to be printed.
Some formatting characters are only supported by some address families.
If a certain formatting character is not supported, then the string
.Dq N/A
is printed.
.Pp
The resulting formatted string is placed into
.Fa buf .
Up to
.Fa buflen
characters are placed in
.Fa buf .
.Pp
The following formatting characters are supported (immediately
after a percent
.Pq Sq %
sign):
.Bl -tag -width %a
.It a
The node address portion of the socket address is printed numerically.
For
.Dv AF_INET
the address is printed as
.Dq A.B.C.D ,
and
for
.Dv AF_INET6
the address is printed as
.Dq A:B...[%if]
using
.Xr getnameinfo 3
internally with
.Dv NI_NUMERICHOST .
For
.Dv AF_LOCAL
.Pq Dv AF_UNIX
the address is printed as
.Dq socket-path .
For
.Dv AF_LINK
the address is printed as
.Dq a.b.c.d.e.f
using
.Xr link_ntoa 3 ,
but the interface portion is skipped (see below).
For
.Dv AF_UNSPEC
nothing is printed.
.It A
The symbolic name of the address is printed.
For
.Dv AF_INET
and
.Dv AF_INET6
this is the hostname associated with the address.
For all other address families, it is the same as the
.Dq a
format.
.It D
Debugging output:
For all addresses, print all their fields as
.Dq field_name=value .
.It f
The numeric value of the family of the address is printed.
.It l
The length of the socket address is printed.
.It p
For
.Dv AF_INET
and
.Dv AF_INET6
the numeric value of the port portion of the address is printed.
.It P
For
.Dv AF_INET
and
.Dv AF_INET6
this is the name of the service associated with the port number, if
available.
For all other address families, it is the same as the
.Dq p
format.
.It I
For
.Dv AF_LINK
addresses, the interface name portion is printed.
.It F
For
.Dv AF_INET6
addresses, the flowinfo portion of the address is printed numerically.
.It R
For
.Dv AF_APPLETALK
addresses,
always prints
.Dq N/A ,
because not supported on
.Dx .
.It S
For
.Dv AF_INET6
addresses, the scope portion of the address is printed numerically.
.It ?
If present between
.Dq %
and the format character, and the selected format does not apply to
the given address family, the
.Dq N/A
string is elided and no output results.
.El
.Sh RETURN VALUES
The
.Fn sockaddr_snprintf
function returns the number of characters that are required to format the
value
.Fa val
given the format string
.Fa fmt
excluding the terminating NUL.
The returned string in
.Fa buf
is always NUL-terminated.
If the address family is not supported,
.Fn sockaddr_snprintf
returns \-1 and sets
.Va errno
to
.Er EAFNOSUPPORT .
For
.Dv AF_INET
and
.Dv AF_INET6
addresses
.Fn sockaddr_snprintf
returns \-1 if the
.Xr getnameinfo 3
conversion failed, and
.Fa errno
is set to the error value from
.Xr getnameinfo 3 .
.Sh ERRORS
If the buffer
.Fa buf
is too small to hold the formatted output,
.Fn sockaddr_snprintf
will still return the buffer, containing a truncated string.
.Sh SEE ALSO
.Xr getaddrinfo 3 ,
.Xr getnameinfo 3 ,
.Xr link_ntoa 3 ,
.Xr snprintf 3
.Sh HISTORY
The
.Fn sockaddr_snprintf
first appeared in
.Nx 3.0 .
.Sh BUGS
The
.Fn sockaddr_snprintf
interface is experimental and might change in the future.
.Pp
There is no way to specify different formatting styles for particular
addresses.
For example it would be useful to print
.Dv AF_LINK
addresses as
.Dq %.2x:%.2x...
instead of
.Dq %x.%x...
.Pp
This function is supposed to be quick, but
.Xr getnameinfo 3
might use system calls to convert the scope number to an interface
name and the
.Dq A
and
.Dq P
format characters call
.Xr getaddrinfo 3
which may block for a noticeable period of time.
.Pp
Not all formatting characters are supported by all address families and
printing
.Dq N/A
is not very convenient.
The
.Dq \&?
character can suppress this, but other formatting (e.g., spacing or
punctuation) will remain.