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16.3.3 Reading the Address of a Socket

Use the function getsockname to examine the address of an Internet socket. The prototype for this function is in the header file sys/socket.h.

Function: int getsockname (int socket, struct sockaddr *addr, socklen_t *length-ptr)

Preliminary: | MT-Safe | AS-Safe | AC-Safe mem/hurd | See POSIX Safety Concepts.

The getsockname function returns information about the address of the socket socket in the locations specified by the addr and length-ptr arguments. Note that the length-ptr is a pointer; you should initialize it to be the allocation size of addr, and on return it contains the actual size of the address data.

The format of the address data depends on the socket namespace. The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much. The usual practice is to allocate a place for the value using the proper data type for the socket’s namespace, then cast its address to struct sockaddr * to pass it to getsockname.

The return value is 0 on success and -1 on error. The following errno error conditions are defined for this function:


The socket argument is not a valid file descriptor.


The descriptor socket is not a socket.


There are not enough internal buffers available for the operation.

You can’t read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there’s no way to find a file’s name from a descriptor for that file.

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