Re: Libpq: copy file to bytea column - Mailing list pgsql-general

seiliki wrote:
>>> The data types of tableout.c1 and tablein.c1 are both bytea. 

[...]

>>> However, I get the following errors from log when calling 
>>> libpq functions PQputCopyData() and PQputCopyEnd().
>>> 
>>> 2010-03-06 20:47:42 CST ERROR:  invalid byte sequence for encoding "UTF8": 0x00

[...]

> > When you use COPY FROM STDIN, you send data from the client to the
> > server, which is something quite different. Data sent from the client
> > are expected to be in the client encoding, not in the server encoding.

[...]

> Changing client encoding can not make PQputCopyData() work, 
> either. The problem seems to be caused by the fact that 
> PQputCopyData() does not accept zero value  ('\0' in C), 
> which I think can not be encoded to any character regardless 
> of which client encoding is used.
> 
> Yes, the data I am trying to import is binary data. That is 
> why I use bytea to store it.
> 
> What I can't understand is why PQputCopyData() encodes 
> incoming character string, which was exported by "COPY ... TO 
> '/tmp/t'" SQL command under psql prompt, for bytea columns 
> and refuse to accept or misinterpret zero value.

Sorry, that was my mistake. I did not see that you said that the
columns were bytea. So you can safely ignore everything I said before.

> Below is the more or less self contained code that also 
> yields the same problem.
> ----------------------------
> #define BYTEA "abcd\\011\\000ABCDEFGHIJKLMNO"

I looked it again, and the problem is the number of backslashes.

You are using text mode COPY, not binary COPY, so all the
non-printable characters are escaped with backslash sequences.

As described in
http://www.postgresql.org/docs/8.4/static/datatype-binary.html
certain octets must be escaped.
The problem is that there are two levels of escaping:
First there is some escaping for text literals, and then there
is the bytea escape syntax.

So for the zero octet, \000 would be the bytea escape, and since
backslash is a special character for text literals, it must be
escaped itself.

Ok, so double backslash.

Now since you represent the string as a literal in C, you have a third
level of escaping. So each backslash must again be represented by a
double backslash.

This leads to C strings like "\\\\000".

Your "\\000" would send '\000' to PostgreSQL, which the string
parser interprets as character zero, see
http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-CONSTANTS

This is rejected as string constant right away.

[...]

> Another issue I can not understand is that both 
> PQputCopyData() and PQputCopyEnd() always return 1 even 
> though the copy operation actually fails. That being said, 
> the above code always shows "Done" but actually error occurs.

As described in
http://www.postgresql.org/docs/current/static/libpq-copy.html
you'll have to call PQgetResult after PQputCopyEnd to get the
result of the COPY statement.

Here's my modified version of your sample that works for me:

#include <stdio.h>
#include <string.h>
#include <libpq-fe.h>

#define BYTEA "abcd\\\\011\\\\000ABCDEFGHIJKLMNO"

int main()
{
    PGconn *PgConnect;
    PGresult *result;
    PgConnect = PQconnectdb("");
    if (!PgConnect)
        puts("Fail to connect.");
    else {
        result = PQexec(PgConnect,"COPY tablein FROM STDIN");
        if (PGRES_COPY_IN == PQresultStatus(result)) {
            if (PQputCopyData(PgConnect,BYTEA,strlen(BYTEA)) == 1) {
                if (PQputCopyEnd(PgConnect,NULL) == 1) {
                    PQclear(result);
                    result = PQgetResult(PgConnect);
                    if (PGRES_COMMAND_OK == PQresultStatus(result))
                        puts("Done");
                    else
                        puts(PQerrorMessage(PgConnect));
                }
                else
                    puts(PQerrorMessage(PgConnect));
            }
            else
                puts(PQerrorMessage(PgConnect));
        }
        else
            puts(PQerrorMessage(PgConnect));
        PQfinish(PgConnect);
    }
}

The problems I spotted in your version:
- there must be four backslashes in the #definition
- you forgot to call PQgetResult
- the second argument of PQputCopyData must be the length of the buffer,
  not the length of the resulting bytea field (which is 21, by the way)
- you did not free the memory of "result"

Note that my sample also cannot serve as a textbook example, since
for example PQexec could return a NULL result if it runs out of memory.

Yours,
Laurenz Albe