I have a pretty basic question about the use of the 'socketpair' call. I'll refer to this code example:


int main(void)
{
int sv[2]; /* the pair of socket descriptors */
char buf; /* for data exchange between processes */

if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == -1) {
perror("socketpair");
exit(1);
}

if (!fork()) { /* child */
read(sv[1], &buf, 1);
printf("child: read '%c'\n", buf);
buf = toupper(buf); /* make it uppercase */
write(sv[1], &buf, 1);
printf("child: sent '%c'\n", buf);

} else { /* parent */
write(sv[0], "b", 1);
printf("parent: sent 'b'\n");
read(sv[0], &buf, 1);
printf("parent: read '%c'\n", buf);
wait(NULL); /* wait for child to die */
}

return 0;
}

The child process is reading and writing to sv[1] and the parent to sv[0]. In general, is that the way one should write the code, or code the parent, for instance, read and/or write to sv[1] as well?

That’s a perfectly reasonable way to write the code.
It is possible, but wouldn’t make much sense in most situations (and perhaps could become deadlocked if you don’t be careful) for the parent to read and write from both sockets. But the choice of which socket to use is arbitrary (as the the standard says, “The two sockets shall be identical.”).