SOCKADDR_SNPRINTF(3) BSD Library Functions Manual SOCKADDR_SNPRINTF(3)
sockaddr_snprintf -- formatting function for socket address structures
System Utilities Library (libutil, -lutil)
sockaddr_snprintf(char *buf, size_t buflen, const char *fmt, const struct sockaddr *sa);
The sockaddr_snprintf() function formats a socket address into a form suitable for printing.
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 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 sup-
ported, then the string ``N/A'' is printed.
The resulting formatted string is placed into buf. Up to buflen characters are placed in
The following formatting characters are supported (immediately after a percent ('%') sign):
a The node address portion of the socket address is printed numerically. For AF_INET the
address is printed as: ``A.B.C.D'' and for AF_INET6 the address is printed as:
``A:B...[%if]'' using getnameinfo(3) internally with NI_NUMERICHOST. For AF_APPLETALK
the address is printed as: ``A.B'' For AF_LOCAL (AF_UNIX) the address is printed as:
``socket-path'' For AF_LINK the address is printed as: ``a.b.c.d.e.f'' using
link_ntoa(3), but the interface portion is skipped (see below). For AF_UNSPEC nothing
A The symbolic name of the address is printed. For AF_INET and AF_INET6 this is the host-
name associated with the address. For all other address families, it is the same as the
f The numeric value of the family of the address is printed.
l The length of the socket address is printed.
p For AF_INET, AF_INET6, and AF_APPLETALK the numeric value of the port portion of the
address is printed.
P For AF_INET and AF_INET6 this is the name of the service associated with the port num-
ber, if available. For all other address families, it is the same as the ``p'' format.
I For AF_LINK addresses, the interface name portion is printed.
F For AF_INET6 addresses, the flowinfo portion of the address is printed numerically.
R For AF_APPLETALK addresses, the netrange portion of the address is printed as:
S For AF_INET6 addresses, the scope portion of the address is printed numerically.
? If present between ``%'' and the format character, and the selected format does not
apply to the given address family, the ``N/A'' string is elided and no output results.
The sockaddr_snprintf() function returns the number of characters that are required to for-
mat the value val given the format string fmt excluding the terminating NUL. The returned
string in buf is always NUL-terminated. If the address family is not supported,
sockaddr_snprintf() returns -1 and sets errno to EAFNOSUPPORT. For AF_INET and AF_INET6
addresses sockaddr_snprintf() returns -1 if the getnameinfo(3) conversion failed, and errno
is set to the error value from getnameinfo(3).
If the buffer buf is too small to hold the formatted output, sockaddr_snprintf() will still
return the buffer, containing a truncated string.
getaddrinfo(3), getnameinfo(3), link_ntoa(3), snprintf(3)
The sockaddr_snprintf() first appeared in NetBSD 3.0.
The sockaddr_snprintf() interface is experimental and might change in the future.
There is no way to specify different formatting styles for particular addresses. For exam-
ple it would be useful to print AF_LINK addresses as ``%.2x:%.2x...'' instead of
This function is supposed to be quick, but getnameinfo(3) might use system calls to convert
the scope number to an interface name and the ``A'' and ``P'' format characters call
getaddrinfo(3) which may block for a noticeable period of time.
Not all formatting characters are supported by all address families and printing ``N/A'' is
not very convenient. The ``?'' character can suppress this, but other formatting (e.g.,
spacing or punctuation) will remain.
BSD April 9, 2005 BSD