qdb_mprintf()

Updated: October 26, 2022

Print formatted output to a new string

Synopsis:

#include <qdb/qdb.h>

char *qdb_mprintf( const char* format, ... );

Arguments:

format
A pointer to a formatting string to process. This string determines what additional arguments you need to provide. For more information, see printf() in the QNX Neutrino C Library Reference.

Library:

qdb

Description:

This function is a variant of sprintf() from the standard C library. The resulting string is written into memory obtained from malloc(), so there's no possibility of buffer overflow. You must call free() to free the strings returned by this function.

Note: The qdb_statement() function also allows you to format strings in the same way and doesn't require you to free the resulting string. However, qdb_mprintf() is useful for building queries from multiple strings.
All of the usual printf() formatting options apply, but QDB adds support for these formatting options that are useful for constructing SQL statements:
%q
This option works like %s: it substitutes a null-terminated string from the argument list; however, %q also doubles each single-quote character ('). This is useful inside a string literal. By doubling each single-quote character, %q escapes that character and allows it to be inserted into the SQL statement.
%Q
This option works like %q except that it adds single quotes around the contents of the entire string. Or, if the parameter in the argument list is a NULL pointer, %Q substitutes the text NULL in place of the %Q option.
%z
This option works like %s except that after the source string (which is the argument that %z refers to) has been copied into the formatted string, the source string memory doesn't have to be freed.

Returns:

A pointer to an escaped string
Success.
NULL
An error occurred (errno is set).

Examples:

Suppose some string variable is initialized as follows:

char *zText = "It's a happy day!";

You can use this text in an SQL statement as follows:

qdb_mprintf("INSERT INTO table VALUES('%q')", zText);

Because the %q formatting option is used, the single-quote character in zText is escaped and the generated SQL is:

INSERT INTO table1 VALUES('It''s a happy day!')

This is correct. Had you used %s instead of %q, the generated SQL would have looked like this:

INSERT INTO table1 VALUES('It's a happy day!');

This second example is an SQL syntax error. As a general rule, you should always use %q instead of %s when inserting text into a string literal.

Suppose you're unsure if your text reference is NULL. You can use this reference as follows:

char *zSQL = qdb_mprintf("INSERT INTO table VALUES(%Q)", zText);

The code above will render a correct SQL statement in the zSQL variable even if the zText variable is a NULL pointer.

The %z option is handy for nested strings:

char id[] = "12345678";
char *nested = qdb_mprintf(
            "SELECT msid FROM mediastores WHERE id = %Q", id);
char *sql = qdb_mprintf(
            "DELETE FROM library WHERE msid = (%z);", nested);
qdb_exec(sql);
free(sql);

The nested string doesn't have to be freed after it gets copied into the formatted string and the SQL code within that string is executed.

Classification:

QNX Neutrino

Safety:  
Interrupt handler No
Signal handler No
Thread Yes