backend exit mystery

I have libpq client program that repeatedly connects to a DB, queries, and
then disconnects. After a seemingly random number of such successful
sessions (sometimes 30, sometimes hundreds), the backend mysteriously exits
after the client calls PQsetdbLogin(), and the client hangs. Any clues?
Details below...

Client: C program linked with 7.2.1 libpq on HP-UX B.11.00 E 9000/803.
Server: PostgreSQL 7.3.2 on i686-pc-linux-gnu, compiled by GCC 2.96.

Client code snippet:

1 if (text_db_conn == NULL || PQstatus(text_db_conn) != CONNECTION_OK) {
2 if (text_db_conn!=NULL) PQfinish(text_db_conn);
3 fprintf(stderr,"Connecting to DB...\n");
4 fflush(stderr);
5 text_db_conn = PQsetdbLogin(IP, PORT, NULL, NULL,
6 "mydb", "myuser", NULL);
7 if (PQstatus(text_db_conn) == CONNECTION_BAD) {
8 fprintf(stderr,"Connection attempt failed.\n");
9 } else {
10 fprintf(stderr,"Connected.\n");
11 }
12 }

Client hangs after line 5. Client backtrace when hanging:

(gdb) bt
#0 0x20da28 in _select_sys ()
#1 0x1ec788 in select ()
#2 0xb9818 in pqWait ()
#3 0x4000f0e0 in __d_trap_fptr ()
#4 0x1ec788 in select ()
Error accessing memory address 0xffffffbf: Bad address.

Server log (with server_min_messages = debug5) shows:

2003-10-10 17:04:01 [28501] DEBUG: BackendStartup: forked pid=20296
socket=8
2003-10-10 17:04:01 [20296] LOG: connection received: host=10.0.1.1
port=61438
2003-10-10 17:05:34 [28501] DEBUG: reaping dead processes
2003-10-10 17:05:34 [28501] DEBUG: child process (pid 20296) exited with
exit code 0

I attached to this backend before it exited, and got this backtrace:

(gdb) bt
#0 0x420e8182 in recv () from /lib/i686/libc.so.6
#1 0x081115c8 in secure_read (port=0x82be9f0, ptr=0x826f100, len=8192) at
be-secure.c:301
#2 0x08115322 in pq_recvbuf () at pqcomm.c:463
#3 0x08115439 in pq_getbytes (s=0xbfffdd70 "Ho*\b8?*\b8???d\210\024",
len=4) at pqcomm.c:538
#4 0x081472fa in ProcessStartupPacket (port=0x82be9f0, SSLdone=0 '\0') at
postmaster.c:1094
#5 0x08148914 in DoBackend (port=0x82be9f0) at postmaster.c:2178
#6 0x081483ff in BackendStartup (port=0x82be9f0) at postmaster.c:1924
#7 0x081471f1 in ServerLoop () at postmaster.c:1027
#8 0x08146be6 in PostmasterMain (argc=4, argv=0x82a5ae8) at
postmaster.c:788
#9 0x081160dc in main (argc=4, argv=0xbfffe8b4) at main.c:210
#10 0x42017499 in __libc_start_main () from /lib/i686/libc.so.6
(gdb) p debug_query_string
$1 = 0x0

TIA.