| Server IP : 103.119.228.120 / Your IP : 3.144.116.195 Web Server : Apache System : Linux v8.techscape8.com 3.10.0-1160.119.1.el7.tuxcare.els2.x86_64 #1 SMP Mon Jul 15 12:09:18 UTC 2024 x86_64 User : nobody ( 99) PHP Version : 5.6.40 Disable Function : shell_exec,symlink,system,exec,proc_get_status,proc_nice,proc_terminate,define_syslog_variables,syslog,openlog,closelog,escapeshellcmd,passthru,ocinum cols,ini_alter,leak,listen,chgrp,apache_note,apache_setenv,debugger_on,debugger_off,ftp_exec,dl,dll,myshellexec,proc_open,socket_bind,proc_close,escapeshellarg,parse_ini_filepopen,fpassthru,exec,passthru,escapeshellarg,escapeshellcmd,proc_close,proc_open,ini_alter,popen,show_source,proc_nice,proc_terminate,proc_get_status,proc_close,pfsockopen,leak,apache_child_terminate,posix_kill,posix_mkfifo,posix_setpgid,posix_setsid,posix_setuid,dl,symlink,shell_exec,system,dl,passthru,escapeshellarg,escapeshellcmd,myshellexec,c99_buff_prepare,c99_sess_put,fpassthru,getdisfunc,fx29exec,fx29exec2,is_windows,disp_freespace,fx29sh_getupdate,fx29_buff_prepare,fx29_sess_put,fx29shexit,fx29fsearch,fx29ftpbrutecheck,fx29sh_tools,fx29sh_about,milw0rm,imagez,sh_name,myshellexec,checkproxyhost,dosyayicek,c99_buff_prepare,c99_sess_put,c99getsource,c99sh_getupdate,c99fsearch,c99shexit,view_perms,posix_getpwuid,posix_getgrgid,posix_kill,parse_perms,parsesort,view_perms_color,set_encoder_input,ls_setcheckboxall,ls_reverse_all,rsg_read,rsg_glob,selfURL,dispsecinfo,unix2DosTime,addFile,system,get_users,view_size,DirFiles,DirFilesWide,DirPrintHTMLHeaders,GetFilesTotal,GetTitles,GetTimeTotal,GetMatchesCount,GetFileMatchesCount,GetResultFiles,fs_copy_dir,fs_copy_obj,fs_move_dir,fs_move_obj,fs_rmdir,SearchText,getmicrotime MySQL : ON | cURL : ON | WGET : ON | Perl : ON | Python : ON | Sudo : ON | Pkexec : ON Directory : /usr/share/doc/postgresql-9.2.24/html/ |
Upload File : |
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>C++ Applications</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 9.2.24 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="ECPG - Embedded SQL in C"
HREF="ecpg.html"><LINK
REL="PREVIOUS"
TITLE="Large Objects"
HREF="ecpg-lo.html"><LINK
REL="NEXT"
TITLE="Embedded SQL Commands"
HREF="ecpg-sql-commands.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2017-11-06T22:43:11"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.2.24 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Large Objects"
HREF="ecpg-lo.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="ecpg.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 33. <SPAN
CLASS="APPLICATION"
>ECPG</SPAN
> - Embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> in C</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Embedded SQL Commands"
HREF="ecpg-sql-commands.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ECPG-CPP"
>33.13. <ACRONYM
CLASS="ACRONYM"
>C++</ACRONYM
> Applications</A
></H1
><P
> ECPG has some limited support for C++ applications. This section
describes some caveats.
</P
><P
> The <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor takes an input file
written in C (or something like C) and embedded SQL commands,
converts the embedded SQL commands into C language chunks, and
finally generates a <TT
CLASS="FILENAME"
>.c</TT
> file. The header file
declarations of the library functions used by the C language chunks
that <TT
CLASS="COMMAND"
>ecpg</TT
> generates are wrapped
in <TT
CLASS="LITERAL"
>extern "C" { ... }</TT
> blocks when used under
C++, so they should work seamlessly in C++.
</P
><P
> In general, however, the <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor only
understands C; it does not handle the special syntax and reserved
words of the C++ language. So, some embedded SQL code written in
C++ application code that uses complicated features specific to C++
might fail to be preprocessed correctly or might not work as
expected.
</P
><P
> A safe way to use the embedded SQL code in a C++ application is
hiding the ECPG calls in a C module, which the C++ application code
calls into to access the database, and linking that together with
the rest of the C++ code. See <A
HREF="ecpg-cpp.html#ECPG-CPP-AND-C"
>Section 33.13.2</A
>
about that.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-CPP-SCOPE"
>33.13.1. Scope for Host Variables</A
></H2
><P
> The <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor understands the scope of
variables in C. In the C language, this is rather simple because
the scopes of variables is based on their code blocks. In C++,
however, the class member variables are referenced in a different
code block from the declared position, so
the <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor will not understand the
scope of the class member variables.
</P
><P
> For example, in the following case, the <TT
CLASS="COMMAND"
>ecpg</TT
>
preprocessor cannot find any declaration for the
variable <TT
CLASS="LITERAL"
>dbname</TT
> in the <TT
CLASS="LITERAL"
>test</TT
>
method, so an error will occur.
</P><PRE
CLASS="PROGRAMLISTING"
>class TestCpp
{
EXEC SQL BEGIN DECLARE SECTION;
char dbname[1024];
EXEC SQL END DECLARE SECTION;
public:
TestCpp();
void test();
~TestCpp();
};
TestCpp::TestCpp()
{
EXEC SQL CONNECT TO testdb1;
}
void Test::test()
{
EXEC SQL SELECT current_database() INTO :dbname;
printf("current_database = %s\n", dbname);
}
TestCpp::~TestCpp()
{
EXEC SQL DISCONNECT ALL;
}</PRE
><P>
This code will result in an error like this:
</P><PRE
CLASS="SCREEN"
><KBD
CLASS="USERINPUT"
>ecpg test_cpp.pgc</KBD
>
test_cpp.pgc:28: ERROR: variable "dbname" is not declared</PRE
><P>
</P
><P
> To avoid this scope issue, the <TT
CLASS="LITERAL"
>test</TT
> method
could be modified to use a local variable as intermediate storage.
But this approach is only a poor workaround, because it uglifies
the code and reduces performance.
</P><PRE
CLASS="PROGRAMLISTING"
>void TestCpp::test()
{
EXEC SQL BEGIN DECLARE SECTION;
char tmp[1024];
EXEC SQL END DECLARE SECTION;
EXEC SQL SELECT current_database() INTO :tmp;
strlcpy(dbname, tmp, sizeof(tmp));
printf("current_database = %s\n", dbname);
}</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-CPP-AND-C"
>33.13.2. C++ Application Development with External C Module</A
></H2
><P
> If you understand these technical limitations of
the <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor in C++, you might come to
the conclusion that linking C objects and C++ objects at the link
stage to enable C++ applications to use ECPG features could be
better than writing some embedded SQL commands in C++ code
directly. This section describes a way to separate some embedded
SQL commands from C++ application code with a simple example. In
this example, the application is implemented in C++, while C and
ECPG is used to connect to the PostgreSQL server.
</P
><P
> Three kinds of files have to be created: a C file
(<TT
CLASS="FILENAME"
>*.pgc</TT
>), a header file, and a C++ file:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="FILENAME"
>test_mod.pgc</TT
></DT
><DD
><P
> A sub-routine module to execute SQL commands embedded in C.
It is going to be converted
into <TT
CLASS="FILENAME"
>test_mod.c</TT
> by the preprocessor.
</P><PRE
CLASS="PROGRAMLISTING"
>#include "test_mod.h"
#include <stdio.h>
void
db_connect()
{
EXEC SQL CONNECT TO testdb1;
}
void
db_test()
{
EXEC SQL BEGIN DECLARE SECTION;
char dbname[1024];
EXEC SQL END DECLARE SECTION;
EXEC SQL SELECT current_database() INTO :dbname;
printf("current_database = %s\n", dbname);
}
void
db_disconnect()
{
EXEC SQL DISCONNECT ALL;
}</PRE
><P>
</P
></DD
><DT
><TT
CLASS="FILENAME"
>test_mod.h</TT
></DT
><DD
><P
> A header file with declarations of the functions in the C
module (<TT
CLASS="FILENAME"
>test_mod.pgc</TT
>). It is included by
<TT
CLASS="FILENAME"
>test_cpp.cpp</TT
>. This file has to have an
<TT
CLASS="LITERAL"
>extern "C"</TT
> block around the declarations,
because it will be linked from the C++ module.
</P><PRE
CLASS="PROGRAMLISTING"
>#ifdef __cplusplus
extern "C" {
#endif
void db_connect();
void db_test();
void db_disconnect();
#ifdef __cplusplus
}
#endif</PRE
><P>
</P
></DD
><DT
><TT
CLASS="FILENAME"
>test_cpp.cpp</TT
></DT
><DD
><P
> The main code for the application, including
the <CODE
CLASS="FUNCTION"
>main</CODE
> routine, and in this example a
C++ class.
</P><PRE
CLASS="PROGRAMLISTING"
>#include "test_mod.h"
class TestCpp
{
public:
TestCpp();
void test();
~TestCpp();
};
TestCpp::TestCpp()
{
db_connect();
}
void
TestCpp::test()
{
db_test();
}
TestCpp::~TestCpp()
{
db_disconnect();
}
int
main(void)
{
TestCpp *t = new TestCpp();
t->test();
return 0;
}</PRE
><P>
</P
></DD
></DL
></DIV
><P>
</P
><P
> To build the application, proceed as follows. Convert
<TT
CLASS="FILENAME"
>test_mod.pgc</TT
> into <TT
CLASS="FILENAME"
>test_mod.c</TT
> by
running <TT
CLASS="COMMAND"
>ecpg</TT
>, and generate
<TT
CLASS="FILENAME"
>test_mod.o</TT
> by compiling
<TT
CLASS="FILENAME"
>test_mod.c</TT
> with the C compiler:
</P><PRE
CLASS="PROGRAMLISTING"
>ecpg -o test_mod.c test_mod.pgc
cc -c test_mod.c -o test_mod.o</PRE
><P>
</P
><P
> Next, generate <TT
CLASS="FILENAME"
>test_cpp.o</TT
> by compiling
<TT
CLASS="FILENAME"
>test_cpp.cpp</TT
> with the C++ compiler:
</P><PRE
CLASS="PROGRAMLISTING"
>c++ -c test_cpp.cpp -o test_cpp.o</PRE
><P>
</P
><P
> Finally, link these object files, <TT
CLASS="FILENAME"
>test_cpp.o</TT
>
and <TT
CLASS="FILENAME"
>test_mod.o</TT
>, into one executable, using the C++
compiler driver:
</P><PRE
CLASS="PROGRAMLISTING"
>c++ test_cpp.o test_mod.o -lecpg -o test_cpp</PRE
><P>
</P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="ecpg-lo.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="ecpg-sql-commands.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Large Objects</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="ecpg.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Embedded SQL Commands</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>