Pro C Tutorial

Introduction

Embedded SQL is a method of combining the computing power of a high-level language like C/C++ and the database manipulation capabilities of SQL. It allows you to execute any SQL s

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Statement from an application program. Oracle's embedded SQL environment is called Pro* C.

How to Compile a Pro*C program

A Pro*C program is compiled in two steps.

First, the Pro*C precompiler recognizes the SQL statements embedded in the program, and replaces them with appropriate calls to the functions in the SQL runtime library.

proc file.pc
this will build file.c and file.lis.

The output is pure C/C++ code with all the pure C/C++ portions intact. Then, a regular C/C++ compiler is used to compile the code and produces the executable.

gcc -o file file.c

Pro*C Syntax

SQL

All SQL statements need to start with EXEC SQL and end with a semicolon ";". You can place the SQL statements anywhere within a C/C++ block, with the restriction that the declarative statements do not come after the executable statements.

Example

    {
        int a;
        /* ... */
        EXEC SQL SELECT salary INTO :a
                 FROM Employee
                 WHERE SSN=876543210;
        /* ... */
        printf("The salary is %d\n", a);
        /* ... */
    }

Preprocessor Directives

The C/C++ preprocessor directives that work with Pro*C are #include and #if. Pro*C does not recognize #define. For example, the following code is invalid:

Example

    #define THE_SSN 876543210
    /* ... */
    EXEC SQL SELECT salary INTO :a
             FROM Employee
             WHERE SSN = THE_SSN;    /* INVALID */

Statement Labels

You can connect C/C++ labels with SQL.

Example

EXEC SQL WHENEVER SQLERROR GOTO error_in_SQL;
    /* ... */
error_in_SQL:
    /* do error handling */

Host Variables

Basics

Host variables are the key to the communication between the host program and the database. A host variable expression must resolve to an lvalue (i.e., it can be assigned). You can declare host variables according to C syntax, as you declare regular C variables. The host variable declarations can be placed wherever C variable declarations can be placed. The C datatypes that can be used with Oracle include:

  • char
  • char[n]
  • int
  • short
  • long
  • float
  • double
  • VARCHAR[n] - This is a pseudo-type recognized by the Pro*C precompiler. It is used to represent blank-padded, variable-length strings. Pro*C precompiler will convert it into a structure with a 2-byte length field and a n-byte character array.

You cannot use register storage-class specifier for the host variables.

A host variable reference must be prefixed with a colon ":" in SQL statements, but should not be prefixed with a colon in C statements. When specifying a string literal via a host variable, the single quotes must be omitted; Pro*C understands that you are specifying a string based on the declared type of the host variable. C function calls and most of the pointer arithmetic expressions cannot be used as host variable references even though they may indeed resolve to lvalues. The following code illustrates both legal and illegal host variable references:

Example

int deptnos[3] = { 000, 111, 222 };
int get_deptno() { return deptnos[2]; }
int *get_deptnoptr() { return &(deptnos[2]); }
 
int main() 
{
    int x; char *y; int z;
    /* ... */
    EXEC SQL INSERT INTO emp(empno, ename, deptno)
        VALUES(:x, :y, :z);         /* LEGAL */
    EXEC SQL INSERT INTO emp(empno, ename, deptno)
        VALUES(:x + 1,              /* LEGAL: the reference is to x */
               'Big Shot',          /* LEGAL: but not really a host var */
               :deptnos[2]);        /* LEGAL: array element is fine */
    EXEC SQL INSERT INTO emp(empno, ename, deptno)
        VALUES(:x, :y,
               :(*(deptnos+2)));    /* ILLEGAL: although it has an
lvalue */
    EXEC SQL INSERT INTO emp(empno, ename, deptno)
        VALUES(:x, :y,
               :get_deptno());      /* ILLEGAL: no function calls */
    EXEC SQL INSERT INTO emp(empno, ename, deptno)
        VALUES(:x, :y,
               :(*get_depnoptr())); /* ILLEGAL: although it has an lvalue */
    /* ... */
}

Pointers

You can define pointers using the regular C syntax, and use them in embedded SQL statements. As usual, prefix them with a colon:

Example
int *x;
/* … */
EXEC SQL SELECT xyz INTO :x FROM …;

The result of this SELECT statement will be written into *x, not x.

Structures

Structures can be used as host variables.

Example

  typedef struct {
        char name[21];    /* one greater than column length; for '\0' */
        int SSN;
    } Emp;
    /* ... */
    Emp bigshot;
    /* ... */
    EXEC SQL INSERT INTO emp (ename, eSSN)
        VALUES (:bigshot);

Arrays

Host arrays can be used in the following way:

    int emp_number[50];
    char name[50][11];
    /* ... */
    EXEC SQL INSERT INTO emp(emp_number, name)
        VALUES (:emp_number, :emp_name);

which will insert all the 50 tuples in one go.

Arrays can only be single dimensional. The example char name[50][11] would seem to contradict that rule. However, Pro*C actually considers name a one-dimensional array of strings rather than a two-dimensional array of characters. You can also have arrays of structures.

When using arrays to store the results of a query, if the size of the host array (say n) is smaller than the actual number of tuples returned by the query, then only the first n result tuples will be entered into the host array.

Indicator Variables

Indicator variables are essentially "NULL flags" attached to host variables. You can associate every host variable with an optional indicator variable. An indicator variable must be defined as a 2-byte integer (using the type short) and, in SQL statements, must be prefixed by a colon and immediately follow its host variable. Or, you may use the keyword INDICATOR in between the host variable and indicator variable.

Example

    short indicator_var;
    EXEC SQL SELECT xyz INTO :host_var:indicator_var
        FROM ...;
    /* ... */
    EXEC SQL INSERT INTO R
        VALUES(:host_var INDICATOR :indicator_var, ...);

Datatype Equivalencing

Oracle recognizes two kinds of datatypes: internal and external. Internal datatypes specify how Oracle stores column values in database tables. External datatypes specify the formats used to store values in input and output host variables. At precompile time, a default Oracle external datatype is assigned to each host variable. Datatype equivalencing allows you to override this default equivalencing and lets you control the way Oracle interprets the input data and formats the output data.

The equivalencing can be done on a variable-by-variable basis using the VAR statement. The syntax is:

EXEC SQL VAR <host_var> IS <type_name> [ (<length>) ];

For example, suppose you want to select employee names from the emp table, and then pass them to a routine that expects C-style '\0'-terminated strings. You need not explicitly '\0'-terminate the names yourself. Simply equivalence a host variable to the STRING external datatype, as follows:

char emp_name[21];
EXEC SQL VAR emp_name IS STRING(21);

The length of the ename column in the emp table is 20 characters, so you allot emp_name 21 characters to accommodate the '\0'-terminator. STRING is an Oracle external datatype specifically designed to interface with C-style strings. When you select a value from the ename column into emp_name, Oracle will automatically '\0'-terminate the value for you.

You can also equivalence user-defined datatypes to Oracle external datatypes using the TYPE statement. The syntax is:

EXEC SQL TYPE <user_type> IS <type_name> [ (<length>) ] [REFERENCE];

You can declare a user-defined type to be a pointer, either explicitly, as a pointer to a scalar or structure, or implicitly as an array, and then use this type in a TYPE statement. In these cases, you need to use the REFERENCE clause at the end of the statement, as shown below:

typedef unsigned char *my_raw;
EXEC SQL TYPE my_raw IS VARRAW(4000) REFERENCE;
my_raw buffer;
/* … */
buffer = malloc(4004);

Here we allocated more memory than the type length (4000) because the precompiler also returns the length, and may add padding after the length in order to meet the alignment requirement on your system.

Dynamic SQL

While embedded SQL is fine for fixed applications, sometimes it is important for a program to dynamically create entire SQL statements. With dynamic SQL, a statement stored in a string variable can be issued. PREPARE turns a character string into a SQL statement, and EXECUTE executes that statement. Consider the following example.

char *s = "INSERT INTO emp VALUES(1234, 'jon', 3)";
EXEC SQL PREPARE q FROM :s;
EXEC SQL EXECUTE q;

Alternatively, PREPARE and EXECUTE may be combined into one statement:

char *s = "INSERT INTO emp VALUES(1234, 'jon', 3)";
EXEC SQL EXECUTE IMMEDIATE :s;

Transactions

Oracle PRO*C supports transactions as defined by the SQL standard. A transaction is a sequence of SQL statements that Oracle treats as a single unit of work. A transaction begins at your first SQL statement. A transaction ends when you issue "EXEC SQL COMMIT" (to make permanent any database changes during the current transaction) or "EXEC SQL ROLLBACK" (to undo any changes since the current transaction began). After the current transaction ends with your COMMIT or ROLLBACK statement, the next executable SQL statement will automatically begin a new transaction.

If your program exits without calling EXEC SQL COMMIT, all database changes will be discarded.

Error Handling

After each executable SQL statement, your program can find the status of execution either by explicit checking of SQLCA, or by implicit checking using the WHENEVER statement.

SQLCA

SQLCA (SQL Communications Area) is used to detect errors and status changes in your program. This structure contains components that are filled in by Oracle at runtime after every executable SQL statement.

To use SQLCA you need to include the header file sqlca.h using the #include directive. In case you need to include sqlca.h at many places, you need to first undefine the macro SQLCA with #undef SQLCA. The relevant chunk of sqlca.h follows:

#ifndef SQLCA
#define SQLCA 1
 
struct sqlca {
    /* ub1 */ char sqlcaid[8];
    /* b4 */ long sqlabc;
    /* b4 */ long sqlcode;
    struct {
        /* ub2 */ unsigned short sqlerrml;
        /* ub1 */ char sqlerrmc[70];
    } sqlerrm;
    /* ub1 */ char sqlerrp[8];
    /* b4 */ long sqlerrd[6];
    /* ub1 */ char sqlwarn[8];
    /* ub1 */ char sqlext[8];
};
/* ... */

WHENEVER Statement

This statement allows you to do automatic error checking and handling. The syntax is:

EXEC SQL WHENEVER <condition> <action>;

Oracle automatically checks SQLCA for <condition>, and if such condition is detected, your program will automatically perform <action>.

<condition> can be any of the following:

  • SQLWARNING - sqlwarn[0] is set because Oracle returned a warning
  • SQLERROR - sqlcode is negative because Oracle returned an error
  • NOT FOUND - sqlcode is positive because Oracle could not find a row that meets your WHERE condition, or a SELECT INTO or FETCH returned no rows

<action> can be any of the following:

  • CONTINUE - Program will try to continue to run with the next statement if possible
  • DO - Program transfers control to an error handling function
  • GOTO <label> - Program branches to a labeled statement
  • STOP - Program exits with an exit() call, and uncommitted work is rolled back

Some examples of the WHENEVER statement:

EXEC SQL WHENEVER SQLWARNING DO print_warning_msg();
EXEC SQL WHENEVER NOT FOUND GOTO handle_empty;

Here is a more concrete example:

/* code to find student name given id */
/* ... */
for (;;) {
    printf("Give student id number : ");
    scanf("%d", &id);
    EXEC SQL WHENEVER NOT FOUND GOTO notfound;
    EXEC SQL SELECT studentname INTO :st_name
             FROM   student
             WHERE  studentid = :id;
    printf("Name of student is %s.\n", st_name);
    continue;
notfound:
    printf("No record exists for id %d!\n", id);
}
/* ... */

Note that the WHENEVER statement does not follow regular C scoping rules. Scoping is valid for the entire program. For example, if you have the following statement somewhere in your program (such as before a loop):

EXEC SQL WHENEVER NOT FOUND DO break;

All SQL statements that occur after this line in the file would be affected. Make sure you use the following line to cancel the effect of WHENEVER when it is no longer needed (such as after your loop):

EXEC SQL WHENEVER NOT FOUND CONTINUE;

Bibliography
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