SQLDescribeCol() - Describe column attributes

The following example shows an application that uses SQLDescribeCol() to retrieve descriptor information about table columns.

/* ... */
/*******************************************************************
** process_stmt
** - allocates a statement handle
** - executes the statement
** - determines the type of statement
**   - if no result columns exist, therefore non-select statement
**      - if rowcount > 0, assume statement was UPDATE, INSERT, DELETE
**     else
**      - assume a DDL, or Grant/Revoke statement
**     else
**      - must be a select statement.
**      - display results
** - frees the statement handle
*******************************************************************/
int
process_stmt(SQLHENV henv,
             SQLHDBC hdbc,
             SQLCHAR * sqlstr)
{
    SQLHSTMT        hstmt;
    SQLSMALLINT     nresultcols;
    SQLINTEGER      rowcount;
    SQLRETURN       rc;
    /* Allocate a statement handle */
    SQLAllocHandle( SQL_ HANDLE_STMT, hdbc, &hstmt);
    /* Execute the SQL statement in "sqlstr"    */
    rc = SQLExecDirect(hstmt, sqlstr, SQL_NTS);
    if (rc != SQL_SUCCESS)
        if (rc == SQL_NO_DATA_FOUND) {
            printf("\nStatement executed without error, however,\n");
            printf("no data was found or modified\n");
            return (SQL_SUCCESS);
        } else
            CHECK_HANDLE( SQL_HANDLE_STMT, hstmt, rc);
    rc = SQLNumResultCols(hstmt, &nresultcols);
    /* Determine statement type */
    if (nresultcols == 0) {     /* statement is not a select statement */
        rc = SQLRowCount(hstmt, &rowcount);
        if (rowcount > 0) {     /* assume statement is UPDATE, INSERT, DELETE */
            printf("Statement executed, %ld rows affected\n", rowcount);
        } else {                /* assume statement is GRANT, REVOKE or a DLL
                                 * statement */
            printf("Statement completed successful\n");
        }
    } else {                    /* display the result set */
        display_results(hstmt, nresultcols);
    }                           /* end determine statement type */
    rc = SQLFreeHandle(SQL_HANDLE_STMT, hstmt); /* Free statement handle */
    return (0);
}                               /* end process_stmt */
/*******************************************************************
** display_results
**  - for each column
**      - get column name
**      - bind column
**  - display column headings
**  - fetch each row
**      - if value truncated, build error message
**      - if column null, set value to "NULL"
**      - display row
**      - print truncation message
**  - free local storage
*******************************************************************/
display_results(SQLHSTMT hstmt,
                SQLSMALLINT nresultcols)
{
    SQLCHAR         colname[32];
    SQLSMALLINT     coltype;
    SQLSMALLINT     colnamelen;
    SQLSMALLINT     nullable;
    SQLINTEGER      collen[MAXCOLS];
    SQLUINTEGER     precision;
    SQLSMALLINT     scale;
    SQLINTEGER      outlen[MAXCOLS];
    SQLCHAR        *data[MAXCOLS];
    SQLCHAR         errmsg[256];
    SQLRETURN       rc;
    SQLINTEGER      i;
    SQLINTEGER      x;
    SQLINTEGER      displaysize;
    for (i = 0; i < nresultcols; i++) {
        SQLDescribeCol(hstmt, i + 1, colname, sizeof(colname),
                       &colnamelen, &coltype, &precision, &scale, NULL);
        collen[i] = precision; /* Note, assignment of unsigned int to signed */
        /* Get display length for column */
        SQLColAttribute(hstmt, i + 1, SQL_COLUMN_DISPLAY_SIZE, NULL, 0,
                         NULL, &displaysize);
        /*
         * Set column length to max of display length, and column name
         * length. Plus one byte for null terminator
         */
        collen[i] = max(displaysize, strlen((char *) colname)) + 1;
        printf("%-*.*s", collen[i], collen[i], colname);
        /* Allocate memory to bind column                             */
        data[i] = (SQLCHAR *) malloc(collen[i]);
        /* Bind columns to program vars, converting all types to CHAR */
        rc = SQLBindCol(hstmt, i + 1, SQL_C_CHAR, data[i], collen[i], &outlen[i]);
    }
    printf("\n");
    /* Display result rows                                            */
    while ((rc = SQLFetch(hstmt)) != SQL_NO_DATA_FOUND) {
        errmsg[0] = '\0';
        for (i = 0; i < nresultcols; i++) {
            /* Build a truncation message for any columns truncated */
            if (outlen[i] >= collen[i]) {
                sprintf((char *) errmsg + strlen((char *) errmsg),
                        "%ld chars truncated, col %d\n",
                        outlen[i] - collen[i] + 1, i + 1);
                sprintf((char *) errmsg + strlen((char *) errmsg),
                        "Bytes to return = %ld size of buffer\n",
                        outlen[i], collen[i]);
            }
            if (outlen[i] == SQL_NULL_DATA)
                printf("%-*.*s", collen[i], collen[i], "NULL");
            else
                printf("%-*.*s", collen[i], collen[i], data[i]);
        }                       /* for all columns in this row  */
        printf("\n%s", errmsg); /* print any truncation messages */
    }                           /* while rows to fetch */
    /* Free data buffers                                              */
    for (i = 0; i < nresultcols; i++) {
        free(data[i]);
    }
}                               /* end display_results */
/* ... */