Advanced Programming in the UNIX® Environment

ptg10805159

Section 17.4 Passing File Descriptors 643

process table entry

...

fd 0:

fd 1:

fd 2:

fd 3:

flagsfd pointerfile

process table entry

...

fd 0:

fd 1:

fd 2:

fd 3:

fd 4:

flagsfd pointerfile

file status flags

current file offset

v-node pointer

file table

v-node information

v_data

i-node information

current file size

i_vnode

v-node table

Figure 17.11 Passing an open file from the top process to the bottom process

Aprocess (normally a server) that wants to pass a descriptor to another process

calls eithersend_fdorsend_err.The process waiting to receive the descriptor (the

client) callsrecv_fd.

The send_fdfunction sends the descriptor fd_to_send across using the UNIX

domain socket represented byfd.Thesend_errfunction sends theerrmsgusingfd,

followed by thestatusbyte. The value ofstatusmust be in the range−1through−255.

Clients call recv_fd to receive a descriptor.Ifall is OK (the sender called

send_fd), the non-negative descriptor is returned as the value of the function.

Otherwise, the value returned is thestatusthat was sent bysend_err(anegative value

in the range−1through− 255 ).Additionally, if an error message was sent by the server,

the client’suserfuncis called to process the message. The first argument touserfuncis

the constant STDERR_FILENO,followed by a pointer to the error message and its

length. Thereturn value fromuserfuncis the number of bytes written or a negative

number on error.Often, the client specifies the normalwritefunction as theuserfunc.

We implement our own protocol that is used by these three functions. To send a

descriptor,send_fdsends two bytes of 0, followed by the actual descriptor.Tosend an

error,send_errsends theerrmsg,followed by a byte of 0, followed by the absolute

value of thestatusbyte(1through 255).Therecv_fdfunction reads everything on the