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SQLite's Built-in printf()

# 1. Overview

SQLite contains its own implementation of the string formatting routine "printf()", accessible via the following interfaces:

- format() → an SQL function returning the formatted string
- sqlite3_mprintf() → Store the formatted string in memory obtained sqlite3_malloc64().
- sqlite3_snprintf() → Store the formatted string in a static buffer
- sqlite3_str_appendf() → Append formatted text to a dynamic string
- sqlite3_vmprintf() → Varargs version of sqlite3_mprintf()
- sqlite3_vsnprintf() → Varargs version of sqlite3_snprintf()
- sqlite3_str_vappendf() → Varargs version of sqlite3_str_appendf()

The same core string formatter is also used internally by SQLite.

## 1.1. Advantages

Why does SQLite have its own private built-in printf() implementation? Why not use the printf() implementation from the standard C library? Several reasons:

1. By using its own built-in implementation, SQLite guarantees that the output will be the same on all platforms and in all LOCALEs. This is important for consistency and for testing. It would be problematic if one machine gave an answer of "5.25e+08" and another gave an answer of "5.250e+008". Both answers are correct, but it is better when SQLite always gives the same answer.

2. We know of no way to use the standard library printf() C interface to implement the format() SQL function feature of SQLite. The built-in printf() implementation can be easily adapted to that task, however.

3. The printf() in SQLite supports new non-standard substitution types (%q, %Q, %w, and %z), and enhanced substitution behavior (%s and %z) that are useful both internally to SQLite and to applications using SQLite. Standard library printf()s cannot normally be extended in this way.

4. Via the sqlite3_mprintf() and sqlite3_vmprintf() interfaces, the built-in printf() implementation supports the ability to render an arbitrary-length string into a memory buffer obtained from sqlite3_malloc64(). This is safer and less error prone than trying to precompute an upper size limit on the result string, allocate an appropriately sized buffer, and then calling snprintf().

5. The SQLite-specific printf() supports a new flag (!) called the "alternate-form-2" flag. The alternate-form-2 flag changes the processing of floating-point conversions in subtle ways so that the output is always an SQL-compatible text representation of a floating-point number - something that is not possible to achieve with standard-library printf(). For string substitutions, the alternate-form-2 flag causes the width and precision to be measured in characters instead of bytes, which simplifies processing of strings containing multi-byte UTF8 characters.

6. The built-in SQLite has compile-time options such as SQLITE_PRINTF_PRECISION_LIMIT that provide defense against denial-of-service attacks for application that expose the printf() functionality to untrusted users.

7. Using a built-in printf() implementation means that SQLite has one fewer dependency on the host environment, making it more portable.

## 1.2. Disadvantages

In fairness, having a built-in implementation of printf() also comes with some disadvantages. To wit:

1. The built-in printf() implementation uses extra code space (about 7800 bytes on GCC 5.4 with -Os).

2. The floating-point to text conversion subfunction for the built-in printf() is limited in precision to 16 significant digits or 26 significant digits if the "!" alternate-form-2 flag is used. Every IEEE-754 double can be represented exactly as a decimal floating-point value, but some doubles require more than 16 or 26 significant digits.

3. The order of the buffer pointer and buffer size parameters in the built-in snprintf() implementation is reversed from the order used in standard-library implementations.

4. The built-in printf() implementation does not handle posix positional referencing modifiers that allow the order of arguments to printf() to be different from the order of the %-substitutions. In the built-in printf(), the order of the arguments must exactly match the order of the %-substitutions.

In spite of the disadvantages, the developers believe that having a built-in printf() implementation inside of SQLite is a net positive.

# 2. Formatting Details

The format string for printf() is a template for the generated string. Substitutions are made whenever a "%" character appears in the format string. The "%" is followed by one or more additional characters that describe the substitution. Each substitution has the following format:

> %[flags][width][.precision][length]type

All substitutions begin with a single "%" and end with a single type character. The other elements of the substitution are optional.

To include a single "%" character in the output, put two consecutive "%" characters in the template.

## 2.1. Substitution Types

The following chart shows the substitution types supported by SQLite:

Substitution Type | Meaning
--- | ---
% | Two "%" characters in a row are translated into a single "%" in the output, without substituting any values.
d, i | The argument is a signed integer which is displayed in decimal.
u | The argument is an unsigned integer which is displayed in decimal.
f | The argument is a double which is displayed in decimal.
e, E | The argument is a double which is displayed in exponential notation. The exponent character is 'e' or 'E' depending on the type.
g, G | The argument is a double which is displayed in either normal decimal notation or if the exponent is not close to zero, in exponential notation.
x, X | The argument is an integer which is displayed in hexadecimal. Lower-case hexadecimal is used for %x and upper-case is used for %X
o | The argument is an integer which is displayed in octal.
s, z | The argument is either a zero-terminated string that is displayed, or a null pointer which is treated as an empty string. For the %z type in C-language interfaces, sqlite3_free() is invoked on the string after it has been copied into the output. The %s and %z substitutions are identical for the SQL printf() function, with a NULL argument treated as an empty string. The %s substitution is universal among printf functions, but the %z substitution and safe treatment of null pointers are SQLite enhancements, not found in other printf() implementations.
c | For the C-language interfaces, the argument is an integer which is interpreted as a character. For the format() SQL function the argument is a string from which the first character is extracted and displayed.
p | The argument is a pointer which is displayed as a hexadecimal address. Since the SQL language has no concept of a pointer, the %p substitution for the format() SQL function works like %x.
n | The argument is a pointer to an integer. Nothing is displayed for this substitution type. Instead, the integer to which the argument points is overwritten with the number of characters in the generated string that result from all format symbols to the left of the %n.
q, Q | The argument is a zero-terminated string. The string is printed with all single quote (') characters doubled so that the string can safely appear inside an SQL string literal. The %Q substitution type also puts single-quotes on both ends of the substituted string. If the argument to %Q is a null pointer then the output is an unquoted "NULL". In other words, a null pointer generates an SQL NULL, and a non-null pointer generates a valid SQL string literal. If the argument to %q is a null pointer then no output is generated. Thus a null-pointer to %q is the same as an empty string. For these substitutions, the precision is the number of bytes or characters taken from the argument, not the number of bytes or characters that are written into the output. The %q and %Q substitutions are SQLite enhancements, not found in most other printf() implementations.
w | This substitution works like %q except that it doubles all double-quote characters (") instead of single-quotes, making the result suitable for using with a double-quoted identifier name in an SQL statement. The %w substitution is an SQLite enhancements, not found in most other printf() implementations.

## 2.2. The Optional Length Field

The length of the argument value can be specified by one or more letters that occur just prior to the substitution type letter. In SQLite, the length only matter for integer types. The length is ignored for the format() SQL function which always uses 64-bit values. The following table shows the length specifiers allowed by SQLite:

Length Specifier | Meaning
--- | ---
(default) | An "int" or "unsigned int". 32-bits on all modern systems.
l | A "long int" or "long unsigned int". Also 32-bits on all modern systems.
ll | A "long long int" or "long long unsigned" or an "sqlite3_int64" or "sqlite3_uint64" value. These are 64-bit integers on all modern systems.

Only the "ll" length modifier ever makes a difference for SQLite. And it only makes a difference when using the C-language interfaces.

## 2.3. The Optional Width Field

The width field specifies the minimum width of the substituted value in the output. If the string or number that is written into the output is shorter than the width, then the value is padded. Padding is on the left (the value is right-justified) by default. If the "-" flag is used, then the padding is on the right and the value is left-justified.

The width is measured in bytes by default. However, if the "!" flag is present then the width is in characters. This only makes a difference for multi-byte utf-8 characters, and those only occur on string substitutions.

If the width is a single "*" character instead of a number, then the actual width value is read as an integer from the argument list. If the value read is negative, then the absolute value is used for the width and the value is left-justified as if the "-" flag were present.

If the value being substituted is larger than the width, then full value is added to the output. In other words, the width is the minimum width of the value as it is rendered in the output.

## 2.4. The Optional Precision Field

The precision field, if it is present, must follow the width separated by a single "." character. If there is no width, then the "." that introduces the precision immediately follows either the flags (if there are any) or the initial "%".

For string substitutions (%s, %z, %q, %Q, or %w) the precision is the number of byte or character used from the argument. The number is bytes by default but is characters if the "!" flag is present. If there is no precision, then the entire string is substituted. Examples: "%.3s" substitutes the first 3 bytes of the argument string. "%!.3s" substitutes the first three characters of the argument string.

For integer substitutions (%d, %i, %x, %X, %o, and %p) the precision specifies minimum number of digits to display. Leading zeros are added if necessary, to expand the output to the minimum number of digits.

For floating-point substitutions (%e, %E, %f, %g, %G) the precision specifies the number of digits to display to the right of the decimal point.

For the character substitution (%c) a precision N greater than 1 causes the character to be repeated N times. This is a non-standard extension found only in SQLite.

If the precision is a single "*" character instead of a number, then the actual precision value is read as an integer from the argument list.

## 2.5. The Options Flags Field

Flags consist of zero or more characters that immediately follow the "%" that introduces the substitution. The various flags and their meanings are as follows:

Flag | Meaning
--- | ---
- | Left-justify the value in the output. The default is to right-justify. If the width is zero or is otherwise less than the length of the value being substituted, then there is no padding and the "-" flag is a no-op.
+ | For signed numeric substitutions, include a "+" sign before positive numbers. A "-" sign always appears before negative numbers regardless of flag settings.
(space) | For signed numeric substitutions, prepend a single space before positive numbers.
0 | Prepend as many "0" characters to numeric substitutions as necessary to expand the value out to the specified width. If the width field is omitted, then this flag is a no-op.
# | This is the "alternate-form-1" flag. For %g and %G substitutions, this causes trailing zeros to be removed. This flag forces a decimal point to appear for all floating-point substitutions. For %o, %x, and %X substitutions, the alternate-form-1 flag cause the value to be prepended with "0", "0x", or "0X", respectively.
, | This flag causes comma-separators to be added to the output of %d and %i substitutions, between every 3 digits from the left. This can help humans to more easily discern the magnitude of large integer values. For example, the value 2147483647 would be rendered as "2147483647" using "%d" but would appear as "2,147,483,647" with "%,d". This flag is a non-standard extension.
! | This is the "alternate-form-2 flag. For string substitutions, this flag causes the width and precision to be understand in terms of characters rather than bytes. For floating point substitutions, the alternate-form-2 flag increases the maximum number of significant digits displayed from 16 to 26, forces the display of the decimal point and causes at least one digit to appear after the decimal point. The alternate-form-2 flag is a non-standard extension that appears in no other printf() implementations, as far as we know.

# 3. Implementation And History

The core string formatting routine is the sqlite3VXPrintf() function found in the printf.c source file. All the various interfaces invoke (sometimes indirectly) this one core function. The sqlite3VXPrintf() function began as code written by the first author of SQLite (Hipp) when he was a graduate student at Duke University in the late 1980s. Hipp kept this printf() implementation in his personal toolbox until he started working on SQLite in 2000. The code was incorporated into the SQLite source tree on 2000-10-08 for SQLite version 1.0.9.

The Fossil Version Control System uses its own printf() implementation that is derived from an early version of the SQLite printf() implementation, but those two implementations have since diverged.

The sqlite3_snprintf() function has its buffer pointer and buffer size arguments reversed from what is found in the standard C library snprintf() routine. This is because there was no snprintf() routine in the standard C library when Hipp was first implementing his version, and he chose a different order than the designers of the standard C library.

This page last modified on 2022-02-15 13:32:30 UTC