ecpg.sgml

关系型数据库 Postgresql 6.5.2

<Chapter Id="ecpg">
<DocInfo>
<AuthorGroup>
<Author>
<FirstName>Linux</FirstName>
<Surname>Tolke</Surname>
</Author>
<Author>
<FirstName>Michael</FirstName>
<Surname>Meskes</Surname>
</Author>
</AuthorGroup>
<Copyright>
<Year>1996-1997</Year>
<Holder>Linus Tolke</Holder>
</Copyright>
<Copyright>
<Year>1998</Year>
<Holder>Michael Meskes</Holder>
</Copyright>
<Date>Transcribed 1998-02-12</Date>
</DocInfo>

<Title><Application>ecpg</Application> - Embedded <Acronym>SQL</Acronym> 
in <Acronym>C</Acronym></Title>

<Para>
This describes an embedded <Acronym>SQL</Acronym> in <Acronym>C</Acronym> 
package for <ProductName>Postgres</ProductName>.

It is written by <ULink url="mailto:[e-mail]">Linus Tolke</ULink>
and <ULink url="mailto:[e-mail]">Michael Meskes</ULink>.

<Note>
<Para>
Permission is granted to copy and use in the same way as you are allowed
to copy and use the rest of the <ProductName>PostgreSQL</ProductName>.
</Para>
</Note>
</para>
<Sect1>
<Title>Why Embedded <Acronym>SQL</Acronym>?</Title>

<Para>
Embedded <Acronym>SQL</Acronym> has some small advantages over other ways 
to handle <Acronym>SQL</Acronym>
queries. It takes care of all the tedious moving of information to and
from variables in your <Acronym>C</Acronym> program. 
Many <Acronym>RDBMS</Acronym> packages
support this embedded language.
</Para>

<Para> There is an ANSI-standard describing how the embedded language should
work. <Application>ecpg</Application> was designed to meet this standard 
as much as possible. So it is
possible to port programs with embedded <Acronym>SQL</Acronym> written for
other <Acronym>RDBMS</Acronym> packages to
<ProductName>Postgres</ProductName> and thus promoting the spirit of free
software.
</Para>
</sect1>
<Sect1>
<Title>The Concept</Title>

<Para>
You write your program in <Acronym>C</Acronym> with some 
special <Acronym>SQL</Acronym> things.
For declaring variables that can be used in 
<Acronym>SQL</Acronym> statements you need to
put them in a special declare section.
You use a special syntax for the <Acronym>SQL</Acronym> queries.
</Para>

<Para>
Before compiling you run the file through 
the embedded <Acronym>SQL</Acronym> <Acronym>C</Acronym>
preprocessor and it converts the <Acronym>SQL</Acronym> statements you used 
to function
calls with the variables used as arguments. Both variables that are used
as input to the <Acronym>SQL</Acronym> statements and variables that will 
contain the
result are passed.
</Para>

<Para>
Then you compile and at link time you link with a special library that
contains the functions used. These functions (actually it is mostly one
single function) fetches the information from the arguments, performs
the <Acronym>SQL</Acronym> query using the ordinary interface 
(<FileName>libpq</FileName>) and puts back
the result in the arguments dedicated for output.
</Para>

<Para>
Then you run your program and when the control arrives to 
the <Acronym>SQL</Acronym>
statement the <Acronym>SQL</Acronym> statement is performed against 
the database and you
can continue with the result.
</Para>
</sect1>

<Sect1>
<Title>How To Use <Application>egpc</Application></Title>

<Para>
This section describes how to use the <Application>egpc</Application> tool.
</Para>

<Sect2>
<Title>Preprocessor</title>

<Para>
The preprocessor is called <Application>ecpg</Application>. 
After installation it resides in
the <ProductName>Postgres</ProductName> <FileName>bin/</FileName> directory. 
</Para>
</sect2>
<Sect2>
<Title>Library</title>

<Para>
The <Application>ecpg</Application> library is called 
<FileName>libecpg.a</FileName> or
<FileName>libecpg.so</FileName>. Additionally, the library
uses the <FileName>libpq</FileName> library for communication to the 
<ProductName>Postgres</ProductName> server so you will
have to link your program with <Parameter>-lecpg -lpq</Parameter>.
</Para>

<Para>
The library has some methods that are "hidden" but that could prove very
useful sometime.

<itemizedlist>
<listitem>
<para>
<function>ECPGdebug(int <replaceable class="parameter">on</replaceable>, FILE *<replaceable class="parameter">stream</replaceable>)</function>
turns on debug logging if called with the first argument non-zero.
Debug logging is done on <replaceable class="parameter">stream</replaceable>.
 Most <Acronym>SQL</Acronym> statement logs its arguments and result.
</Para>

<Para>
The most important one (<Function>ECPGdo</Function>) 
that is called on almost all <Acronym>SQL</Acronym>
statements logs both its expanded string,
 i.e. the string
with all the input variables inserted, and the result from the
<ProductName>Postgres</ProductName> server.
 This can be very useful when searching for errors
in your <Acronym>SQL</Acronym> statements.
</Para>
</ListItem>

<listitem>
<para>
<function>ECPGstatus()</function>
This method returns TRUE if we are connected to a database and FALSE if not.
</Para>
</ListItem>
</itemizedlist>
</Para>
</sect2>

<Sect2>
<Title>Error handling</title>

<Para>
To be able to detect errors from the <ProductName>Postgres</ProductName> 
server you include a line like
<ProgramListing>
exec sql include sqlca;
</ProgramListing>
in the include section of your file. This will define a struct and a
variable with the name <Parameter>sqlca</Parameter> as following:
<ProgramListing>
struct sqlca
{
 char sqlcaid[8];
 long sqlabc;
 long sqlcode;
 struct
 {
  int sqlerrml;
  char sqlerrmc[70];
 } sqlerrm;
 char sqlerrp[8];
 long sqlerrd[6];
 /* 0: empty                                         */
 /* 1: empty                                         */
 /* 2: number of rows processed in an INSERT, UPDATE */
 /*    or DELETE statement                           */
 /* 3: empty                                         */
 /* 4: empty                                         */
 /* 5: empty                                         */
 char sqlwarn[8];
 /* 0: set to 'W' if at least one other is 'W'       */
 /* 1: if 'W' at least one character string	     */
 /*    value was truncated when it was               */
 /*    stored into a host variable.                  */
 /* 2: empty                                         */
 /* 3: empty                                         */
 /* 4: empty                                         */
 /* 5: empty                                         */
 /* 6: empty                                         */
 /* 7: empty                                         */
 char sqlext[8];
} sqlca;
</ProgramListing>
</Para>

<Para>
If an error occured in the last <Acronym>SQL</Acronym> statement 
then <Parameter>sqlca.sqlcode</Parameter>
will be non-zero. If <Parameter>sqlca.sqlcode</Parameter> is less that 0
 then this is
some kind of serious error, like the database definition does not match
the query given. If it is bigger than 0 then this is a normal error like
the table did not contain the requested row.
</Para>

<Para>
sqlca.sqlerrm.sqlerrmc will contain a string that describes the error.
The string ends with the line number
in the source file.
</Para>

<Para>
List of errors that can occur:

<VariableList>
<VarListEntry>
<Term>-12, Out of memory in line %d.</Term>
<ListItem>
<Para>
Does not normally occur. This is a sign that your virtual memory is
exhausted.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-200, Unsupported type %s on line %d.</Term>
<ListItem>
<Para>
Does not normally occur. This is a sign that the preprocessor has
generated something that the library does not know about. Perhaps you
are running incompatible versions of the preprocessor and the library.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-201, Too many arguments line %d.</Term>
<ListItem>
<Para>
This means that <ProductName>Postgres</ProductName> has returned more
arguments than we have
matching variables. Perhaps you have forgotten a couple of the host
variables in the <Command>INTO :var1,:var2</Command>-list.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-202, Too few arguments line %d.</Term>
<ListItem>
<Para>
This means that <ProductName>Postgres</ProductName> has returned fewer
arguments than we have
host variables. Perhaps you have too many host variables in the 
<Command>INTO :var1,:var2</Command>-list.
</Para> 
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-203, Too many matches line %d.</Term>
<ListItem>
<Para>
This means that the query has returned several lines but the
variables specified are no arrays. The <Command>SELECT</Command> you made
probably was not unique.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-204, Not correctly formatted int type: %s line %d.</Term>
<ListItem>
<Para>
This means that the host variable is of an <Type>int</Type> type and the field
in the <ProductName>Postgres</ProductName> database is of another type and 
contains a value that cannot be interpreted as an <Type>int</Type>. 
The library uses <Function>strtol</Function>
for this conversion.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-205, Not correctly formatted unsigned type: %s line %d.</Term>
<ListItem>
<Para>
This means that the host variable is of an <Type>unsigned int</Type> type and
the field in the <ProductName>Postgres</ProductName> database is of another 
type and contains a
value that cannot be interpreted as an <Type>unsigned int</Type>. The library
uses <Function>strtoul</Function> for this conversion.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-206, Not correctly formatted floating point type: %s line %d.</Term>
<ListItem>
<Para>
This means that the host variable is of a <Type>float</Type> type and
the field in the <ProductName>Postgres</ProductName> database is of another 
type and contains a
value that cannot be interpreted as an <Type>float</Type>. The library
uses <Function>strtod</Function> for this conversion.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-207, Unable to convert %s to bool on line %d.</Term>
<ListItem>
<Para>
This means that the host variable is of a <Type>bool</Type> type and
the field in the <ProductName>Postgres</ProductName> database is neither 't'
nor 'f'.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-208, Empty query line %d.</Term>
<ListItem>
<Para>
<ProductName>Postgres</ProductName> returned PGRES_EMPTY_QUERY, probably
because the query indeed was empty.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-220, No such connection %s in line %d.</Term>
<ListItem>
<Para>
The program tries to access a connection that does not exist.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-221, Not connected in line %d.</Term>
<ListItem>
<Para>
The program tries to access a connection that does exist but is not open.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-230, Invalid statement name %s in line %d.</Term>
<ListItem>
<Para>
The statement you are trying to use has not been prepared.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-400, Postgres error: %s line %d.</Term>
<ListItem>
<Para>
Some <ProductName>Postgres</ProductName> error. 
The message contains the error message from the
<ProductName>Postgres</ProductName> backend.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-401, Error in transaction processing line %d. </Term>
<ListItem>
<Para>
<ProductName>Postgres</ProductName> signalled to us that we cannot start,
commit or rollback the transaction.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>-402, connect: could not open database %s.</Term>
<ListItem>
<Para>
The connect to the database did not work.
</Para>
</ListItem>
</VarListEntry>

<VarListEntry>
<Term>100, Data not found line %d.</Term>
<ListItem>
<Para>
This is a "normal" error that tells you that what you are quering cannot
be found or we have gone through the cursor.
</Para>
</ListItem>
</VarListEntry>

</VariableList>
</Para>
</Sect2>
</sect1>

<Sect1>
<Title>Limitations</Title>

<Para>
What will never be included and why or what cannot be done with this
concept.

<VariableList>
<VarListEntry>
<Term>oracles single tasking possibility</Term>
<ListItem>
<Para>
Oracle version 7.0 on AIX 3 uses the OS-supported locks on the shared
memory segments and allows the application designer to link an
application in a so called single tasking way. Instead of starting one
client process per application process both the database part and the
application part is run in the same process. In later versions of oracle
this is no longer supported.
</Para>

<Para>
This would require a total redesign of the <ProductName>Postgres</ProductName> access model and
that effort can not justify the performance gained.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
</sect1>

<Sect1>
<Title>Porting From Other <Acronym>RDBMS</Acronym> Packages</Title>

<Para>
The design of <Application>ecpg</Application> follows SQL standard. So
porting from a standard RDBMS should not be a problem. Unfortunately there
is no such thing as a standard RDBMS. So <Application>ecpg</Application>
also tries to understand syntax additions as long as they do not create
conflicts with the standard. 
</Para>

<Para>
The following list shows all the known incompatibilities. If you find one
not listed please notify <ULink url="mailto:[e-mail]">Michael
Meskes</ULink>. Note, however, that we list only incompatibilities from
a precompiler of another RDBMS to <Application>ecpg</Application> and not
additional <Application>ecpg</Application> features that these RDBMS do not
have.
</Para>

<Para>
<VariableList>
<VarListEntry>
<Term>Syntax of FETCH command</Term>
<ListItem>
<Para>
The standard syntax of the FETCH command is:
</para>
<Para>
FETCH [direction] [amount] IN|FROM <Replaceable>cursor name</Replaceable>.
</Para>
<para>
<Application>ORACLE</Application>, however, does not use the keywords IN
resp. FROM. This feature cannot be added since it would create parsing
conflicts.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
</Para>
</sect1>

<Sect1>
<Title>Installation</Title>

<Para>
Since version 0.5 <Application>ecpg</Application> is distributed 
together with <ProductName>Postgres</ProductName>. So you
should get your precompiler, libraries and header files compiled and
installed by default as a part of your installation.
</Para>
</sect1>

<Sect1>
<Title>For the Developer</Title>

<Para>
This section is for those who want to develop the 
<Application>ecpg</Application> interface. It
describes how the things work. The ambition is to make this section
contain things for those that want to have a look inside and the section
on How to use it should be enough for all normal questions.