socket.h) - glibc

Split from the chapter glibc: Sockets into the per-function layout; see the method page and the glibc ledger for coverage status.

Field	Value
Header	`sys/socket.h` (multiarch path `/usr/include/x86_64-linux-gnu/sys/socket.h`)
Standards	POSIX.1-2001/2008, SVr4, 4.4BSD; `getsockname` first appeared in 4.2BSD (`getsockname(2)`, `getpeername(2)` CONFORMING TO)
Exported	`getsockname@@GLIBC_2.2.5`, `getpeername@@GLIBC_2.2.5`, both weak syscall aliases (nm, host glibc 2.41)
Thread-safety	MT-Safe (manual safety annotations, `socket.texi` nodes Reading Address and Who is Connected; the man2 pages predate the ATTRIBUTES convention)
Ledger ID	GLIBC-getsockname, GLIBC-getpeername

Declaration and gates

#include <sys/socket.h>

int getsockname(int fd, struct sockaddr *restrict addr,
                socklen_t *restrict len);
int getpeername(int fd, struct sockaddr *restrict addr,
                socklen_t *restrict len);

Both unconditional: getsockname at /usr/include/x86_64-linux-gnu/ sys/socket.h lines 116-117, getpeername at lines 130-131, with the opaque __SOCKADDR_ARG union elided to its struct sockaddr * reading here.

Semantics

getsockname reports the address the socket fd is currently bound to; after bind with port 0 or an autobind it is the way to learn the port the kernel picked. getpeername reports the address of the connected peer – on a forked server the standard way to re-derive the client identity from the bare descriptor. Both take a value-result len: initialize it to the buffer size, and on return it holds the real size; if the buffer was too small the address is truncated and len comes back larger than what was passed (getsockname(2), getpeername(2)). The manual recommends allocating the proper namespace-specific type (sockaddr_in etc.) and casting to struct sockaddr * (manual, Reading Address).

Gotchas and portability

Truncation is signalled only by *len growing past the supplied value; callers that ignore the returned length can silently use clipped addresses – for AF_UNIX that clips the sun_path tail.
“You can’t read the address of a socket in the file namespace” (manual, Reading Address): glibc’s historic file-namespace sockets answer neither function.
getpeername on an unconnected TCP socket fails with ENOTCONN; on UDP it reports the default peer set by connect(2).
Raw syscall pages: getsockname; sibling library pages: socket/bind/connect, sockaddr variants.
musl 1.2.5: thin wrappers present (src/network/getsockname.c, getpeername.c).

Errors

EBADF, EFAULT, EINVAL (invalid len), ENOBUFS, ENOTSOCK; getpeername adds ENOTCONN (getsockname(2), getpeername(2)).

References

glibc manual: socket.texi, nodes Reading Address, Who is Connected
man2: getsockname.2, getpeername.2
header: /usr/include/x86_64-linux-gnu/sys/socket.h
symbols: nm -D --with-symbol-versions libc.so.6

Keyboard shortcuts