libpq: lo_open() returns 0

Question

I am trying to create large objects using libpq and encountered the problem. In the following code:

Oid                 oid;
int                 fd;

oid = lo_creat( pgConn, INV_READ | INV_WRITE );
fd = lo_open( pgConn, oid , INV_WRITE );

lo_open() returns 0, which is not an error according to the docs, but causes subsequent lo_write() to fail with "Invalid descriptor" error.

I read about functions "taking place within an SQL transaction block" and just tried to envelop my code in

PQexec( pgConn, "BEGIN" );
...
PQexec( pgConn, "COMMIT" );

The things changed. lo_open() still returned 0, but if I immediately call it once more, it worked!

The same is true for reading.

I feel I missed something, but cannot guess what.

UPD: After running the code from the answer below it occurred to me that 0 was a valid descriptor, and I got the initial error because I had not explicitly started a transaction. Surprisingly, I have to call "BEGIN" and "COMMIT" when reading large objects, too.

Answer 1

It's hard to tell you anything when you don't check or quote your error messages.

Please try this SSCCE and go from there:

#include <stdio.h>
#include <stdlib.h>

#include <libpq-fe.h>
#include <libpq/libpq-fs.h>

void check_PGconn(PGconn *conn, ConnStatusType status_ok, char* errmsg)
{
    if (PQstatus(conn) != status_ok) {
        if (errmsg!=NULL) {
            fprintf(stderr, "%s: %s", errmsg, PQerrorMessage(conn));
        } else {
            fprintf(stderr, "%s", PQerrorMessage(conn));
        }
        PQfinish(conn);
        exit(1);
    }
}

int check_PGresult(PGconn *conn, PGresult *res, ExecStatusType status_ok, char* errmsg)
{
    if (PQresultStatus(res) != status_ok) {
        if (errmsg!=NULL) {
            fprintf(stderr, "%s: %s", errmsg, PQerrorMessage(conn));
        } else {
            fprintf(stderr, "%s", PQerrorMessage(conn));
        }
        return 0;
    }
    return 1;
}

int main(int argc, char** argv)
{
    char* conninfo;
    PGconn* conn;
    PGresult* res;
    int res_ok;
    Oid loid;
    int lofd;
    int lo_res;
    static char lo_content[] = "Lorem ipsum dolor sit amet, fabulas conclusionemque ius ad.";

    if (argc > 1) conninfo = argv[1]; else conninfo = "";

    conn = PQconnectdb(conninfo);
    check_PGconn(conn, CONNECTION_OK, "Connection to database failed");

    /* Create large object */

    res = PQexec(conn, "BEGIN");
    res_ok = check_PGresult(conn, res, PGRES_COMMAND_OK, "BEGIN failed");
    if ( !res_ok ) exit(1);
    PQclear(res);

    loid = lo_creat(conn, INV_READ|INV_WRITE);
    if (loid == 0) {
        fprintf(stderr, "lo_creat failed: %s", PQerrorMessage(conn));
        exit(1);
    }

    lofd = lo_open(conn, loid, INV_WRITE);
    if ( lofd == -1 ) {
        fprintf(stderr, "lo_open failed: %s", PQerrorMessage(conn));
        exit(1);
    }

    lo_res = lo_write(conn, lofd, lo_content, sizeof(lo_content));
    if ( lo_res == -1 ) {
        fprintf(stderr, "lo_write failed: %s", PQerrorMessage(conn));
        exit(1);
    }

    lo_res = lo_close(conn, lofd);
    if ( lo_res == -1 ) {
        fprintf(stderr, "lo_close failed: %s", PQerrorMessage(conn));
        exit(1);
    }

    res = PQexec(conn, "COMMIT");
    res_ok = check_PGresult(conn, res, PGRES_COMMAND_OK, "COMMIT failed");
    if ( !res_ok ) exit(1);
    PQclear(res);

    /* Delete large object */

    res = PQexec(conn, "BEGIN");
    res_ok = check_PGresult(conn, res, PGRES_COMMAND_OK, "BEGIN failed");
    if ( !res_ok ) exit(1);
    PQclear(res);

    lo_res = lo_unlink(conn, loid);
    if ( lo_res == -1 ) {
        fprintf(stderr, "lo_unlink failed: %s", PQerrorMessage(conn));
        exit(1);
    }

    res = PQexec(conn, "COMMIT");
    res_ok = check_PGresult(conn, res, PGRES_COMMAND_OK, "COMMIT failed");
    if ( !res_ok ) exit(1);
    PQclear(res);

    PQfinish(conn);
}