NAME
sqlite3_mprintf,
sqlite3_vmprintf,
sqlite3_snprintf,
sqlite3_vsnprintf
—
Formatted String Printing Functions
SYNOPSIS
char *
sqlite3_mprintf(
const char*,
...);
char *
sqlite3_vmprintf(
const char*,
va_list);
char *
sqlite3_snprintf(
int,
char*,
const char*,
...);
char *
sqlite3_vsnprintf(
int,
char*,
const char*,
va_list);
DESCRIPTION
These routines are work-alikes of the "printf()" family of functions
from the standard C library. These routines understand most of the common
K&R formatting options, plus some additional non-standard formats,
detailed below. Note that some of the more obscure formatting options from
recent C-library standards are omitted from this implementation.
The sqlite3_mprintf() and sqlite3_vmprintf() routines write their results into
memory obtained from sqlite3_malloc(). The strings returned by these two
routines should be released by sqlite3_free(). Both routines return a NULL
pointer if sqlite3_malloc() is unable to allocate enough memory to hold the
resulting string.
The sqlite3_snprintf() routine is similar to "snprintf()" from the
standard C library. The result is written into the buffer supplied as the
second parameter whose size is given by the first parameter. Note that the
order of the first two parameters is reversed from snprintf(). This is an
historical accident that cannot be fixed without breaking backwards
compatibility. Note also that sqlite3_snprintf() returns a pointer to its
buffer instead of the number of characters actually written into the buffer.
We admit that the number of characters written would be a more useful return
value but we cannot change the implementation of sqlite3_snprintf() now
without breaking compatibility.
As long as the buffer size is greater than zero, sqlite3_snprintf() guarantees
that the buffer is always zero-terminated. The first parameter "n"
is the total size of the buffer, including space for the zero terminator. So
the longest string that can be completely written will be n-1 characters.
The sqlite3_vsnprintf() routine is a varargs version of sqlite3_snprintf().
These routines all implement some additional formatting options that are useful
for constructing SQL statements. All of the usual printf() formatting options
apply. In addition, there is are "%q", "%Q",
"%w" and "%z" options.
The %q option works like %s in that it substitutes a nul-terminated string from
the argument list. But %q also doubles every '´' character. %q is
designed for use inside a string literal. By doubling each '´' character
it escapes that character and allows it to be inserted into the string.
For example, assume the string variable zText contains text as follows:
char *zText = "It's a happy day!";
One can use this text in an SQL statement as follows:
char *zSQL = sqlite3_mprintf("INSERT INTO table VALUES('%q')", zText);
sqlite3_exec(db, zSQL, 0, 0, 0); sqlite3_free(zSQL);
Because the %q format string is used, the '´' character in zText is escaped
and the SQL generated is as follows:
INSERT INTO table1 VALUES('It''s a happy day!')
This is correct. Had we 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.
The %Q option works like %q except it also adds single quotes around the outside
of the total string. Additionally, if the parameter in the argument list is a
NULL pointer, %Q substitutes the text "NULL" (without single
quotes). So, for example, one could say:
char *zSQL = sqlite3_mprintf("INSERT INTO table VALUES(%Q)", zText);
sqlite3_exec(db, zSQL, 0, 0, 0); sqlite3_free(zSQL);
The code above will render a correct SQL statement in the zSQL variable even if
the zText variable is a NULL pointer.
The "%w" formatting option is like "%q" except that it
expects to be contained within double-quotes instead of single quotes, and it
escapes the double-quote character instead of the single-quote character. The
"%w" formatting option is intended for safely inserting table and
column names into a constructed SQL statement.
The "%z" formatting option works like "%s" but with the
addition that after the string has been read and copied into the result,
sqlite3_free() is called on the input string.
SEE ALSO
sqlite3_malloc(3)