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Overview</a></div> +<div class="fancy-toc2"><a href="#advantages">1.1. Advantages</a></div> +<div class="fancy-toc2"><a href="#disadvantages">1.2. Disadvantages</a></div> +<div class="fancy-toc1"><a href="#formatting_details">2. Formatting Details</a></div> +<div class="fancy-toc2"><a href="#substitution_types">2.1. Substitution Types</a></div> +<div class="fancy-toc2"><a href="#the_optional_length_field">2.2. The Optional Length Field</a></div> +<div class="fancy-toc2"><a href="#the_optional_width_field">2.3. The Optional Width Field</a></div> +<div class="fancy-toc2"><a href="#the_optional_precision_field">2.4. The Optional Precision Field</a></div> +<div class="fancy-toc2"><a href="#the_options_flags_field">2.5. The Options Flags Field</a></div> +<div class="fancy-toc1"><a href="#implementation_and_history">3. Implementation And History</a></div> +</div> +</div> +<script> +function toggle_toc(){ +var sub = document.getElementById("toc_sub") +var mk = document.getElementById("toc_mk") +if( sub.style.display!="block" ){ +sub.style.display = "block"; +mk.innerHTML = "▼"; +} else { +sub.style.display = "none"; +mk.innerHTML = "►"; +} +} +</script> +</div> + + + + +<h1 id="overview"><span>1. </span>Overview</h1> + +<p>SQLite contains its own implementation of the string formatting routine "printf()", +accessible via the following interfaces: + +</p><ul> +<li> <a href="lang_corefunc.html#format">format()</a> → an SQL function returning the formatted string +</li><li> <a href="c3ref/mprintf.html">sqlite3_mprintf()</a> → Store the formatted string in memory obtained + <a href="c3ref/free.html">sqlite3_malloc64()</a>. +</li><li> <a href="c3ref/mprintf.html">sqlite3_snprintf()</a> → Store the formatted string in a static buffer +</li><li> <a href="c3ref/str_append.html">sqlite3_str_appendf()</a> → Append formatted text to a dynamic string +</li><li> <a href="c3ref/mprintf.html">sqlite3_vmprintf()</a> → Varargs version of sqlite3_mprintf() +</li><li> <a href="c3ref/mprintf.html">sqlite3_vsnprintf()</a> → Varargs version of sqlite3_snprintf() +</li><li> <a href="c3ref/str_append.html">sqlite3_str_vappendf()</a> → Varargs version of sqlite3_str_appendf() +</li></ul> + +<p>The same core string formatter is also used internally by SQLite. + +</p><h2 id="advantages"><span>1.1. </span>Advantages</h2> + +<p>Why does SQLite have its own private built-in printf() implementation? +Why not use the printf() implementation from the standard C library? +Several reasons: + +</p><p> +</p><ol> +<li><p> +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. + +</p></li><li><p> +We know of no way to use the standard library printf() C interface to +implement the <a href="lang_corefunc.html#format">format() SQL function</a> feature of SQLite. The built-in +printf() implementation can be easily adapted to that task, however. + +</p></li><li><p> +The printf() in SQLite supports new non-standard substitution +types (<a href="printf.html#percentq">%q</a>, <a href="printf.html#percentq">%Q</a>, <a href="printf.html#percentw">%w</a>, and <a href="printf.html#percentz">%z</a>), and enhanced substitution +behavior (%s and <a href="printf.html#percentz">%z</a>) that are useful both internally to SQLite +and to applications using SQLite. +Standard library printf()s cannot normally be extended in this way. + +</p></li><li><p> +Via the <a href="c3ref/mprintf.html">sqlite3_mprintf()</a> and <a href="c3ref/mprintf.html">sqlite3_vmprintf()</a> interfaces, +the built-in printf() implementation supports the ability to render an +arbitrary-length string into a memory buffer obtained from <a href="c3ref/free.html">sqlite3_malloc64()</a>. +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(). + +</p></li><li><p> +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. + +</p></li><li><p> +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. + +</p></li><li><p> +Using a built-in printf() implementation means that SQLite has one +fewer dependency on the host environment, making it more portable. +</p></li></ol> + +<h2 id="disadvantages"><span>1.2. </span>Disadvantages</h2> + +<p> +In fairness, having a built-in implementation of printf() also comes with +some disadvantages. To wit: + +</p><ol> +<li><p> +The built-in printf() implementation uses extra code space +(about 7800 bytes on GCC 5.4 with -Os). + +</p></li><li><p> +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 <i>exactly</i> as a decimal +value, but for many doubles the exact decimal representation requires +more than 16 or 26 significant digits. +The SQLite printf() function only renders the first 16 or 26 significant digits +because that can be done efficient and because 16 decimal digits it is sufficient +to distinguish every possible double value. Use the <a href="floatingpoint.html#decext">decimal extension</a> to get +the exact decimal equivalent of double value for the rare cases where that is +required. + +</p></li><li><p> +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. + +</p></li><li><p> +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. +</p></li></ol> + +<p> +In spite of the disadvantages, the developers believe that having a built-in +printf() implementation inside of SQLite is a net positive. + +</p><h1 id="formatting_details"><span>2. </span>Formatting Details</h1> + +<p>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: + +</p><blockquote> +<b>%</b><i>[flags][width][</i><b>.</b><i>precision][length]type</i> +</blockquote> + +<p>All substitutions begin with a single "%" and end with a single type character. +The other elements of the substitution are optional. + +</p><p>To include a single "%" character in the output, put two consecutive +"%" characters in the template. + +</p><h2 id="substitution_types"><span>2.1. </span>Substitution Types</h2> + +<p>The following chart shows the substitution types supported by SQLite: + +</p><center> +<table border="1" cellpadding="10" width="80%"> +<tr> +<th>Substitution Type</th><th>Meaning +</th></tr><tr> +<td>% +</td><td>Two "%" characters in a row are translated into a single "%" in the output, + without substituting any values. +</td></tr><tr> +<td>d, i +</td><td>The argument is a signed integer which is displayed in decimal. +</td></tr><tr> +<td>u +</td><td>The argument is an unsigned integer which is displayed in decimal. +</td></tr><tr> +<td>f +</td><td>The argument is a double which is displayed in decimal. +</td></tr><tr> +<td>e, E +</td><td>The argument is a double which is displayed in exponential notation. + The exponent character is 'e' or 'E' depending on the type. +</td></tr><tr> +<td>g, G +</td><td>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. +</td></tr><tr> +<td>x, X +</td><td>The argument is an integer which is displayed in hexadecimal. + Lower-case hexadecimal is used for %x and upper-case is used + for %X +</td></tr><tr> +<td>o +</td><td>The argument is an integer which is displayed in octal. +</td></tr><tr> +<td>s, z +</td><td> +<a name="percentz"></a> + + 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, <a href="c3ref/free.html">sqlite3_free()</a> 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.<br><br> + 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. +</td></tr><tr> +<td>c +</td><td>For the C-language interfaces, the argument is an integer which + is interpreted as a character. For the <a href="lang_corefunc.html#format">format() SQL function</a> the + argument is a string from which the first character is extracted and + displayed. +</td></tr><tr> +<td>p +</td><td>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 <a href="lang_corefunc.html#format">format() SQL function</a> works like %x. +</td></tr><tr> +<td>n +</td><td>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. +</td></tr><tr> +<td>q, Q +</td><td><a name="percentq"></a> + + 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. + <br><br>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. + <br><br>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. + <br><br> + The %q and %Q substitutions are SQLite enhancements, not found in + most other printf() implementations. +</td></tr><tr> +<td>w +</td><td><a name="percentw"></a> + + 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. + <br><br> + The %w substitution is an SQLite enhancements, not found in + most other printf() implementations. +</td></tr></table> +</center> + +<h2 id="the_optional_length_field"><span>2.2. </span>The Optional Length Field</h2> + +<p>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 +<a href="lang_corefunc.html#format">format() SQL function</a> which always uses 64-bit values. The following +table shows the length specifiers allowed by SQLite: + +</p><center> +<table border="1" cellpadding="10" width="80%"> +<tr> +<th>Length Specifier +</th><th>Meaning +</th></tr><tr> +<td><i>(default)</i> +</td><td>An "int" or "unsigned int". 32-bits on all modern systems. +</td></tr><tr> +<td>l +</td><td>A "long int" or "long unsigned int". Also 32-bits on all modern systems. +</td></tr><tr> +<td>ll +</td><td>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. +</td></tr></table> +</center> + +<p>Only the "ll" length modifier ever makes a difference for SQLite. And +it only makes a difference when using the C-language interfaces. + +</p><h2 id="the_optional_width_field"><span>2.3. </span>The Optional Width Field</h2> + +<p>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. + +</p><p>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. + +</p><p>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. + +</p><p>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. + +</p><h2 id="the_optional_precision_field"><span>2.4. </span>The Optional Precision Field</h2> + +<p>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 "%". + +</p><p>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. + +</p><p>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. + +</p><p>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. + +</p><p>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. + +</p><p>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. + +</p><h2 id="the_options_flags_field"><span>2.5. </span>The Options Flags Field</h2> + +<p>Flags consist of zero or more characters that immediately follow the +"%" that introduces the substitution. The various flags and their meanings +are as follows: + +</p><center> +<table border="1" cellpadding="10" width="80%"> +<tr> +<th>Flag +</th><th>Meaning +</th></tr><tr> +<td><b>-</b> +</td><td>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. +</td></tr><tr> +<td><b>+</b> +</td><td>For signed numeric substitutions, include a "+" sign before positive numbers. +A "-" sign always appears before negative numbers regardless of flag settings. +</td></tr><tr> +<td><i>(space)</i> +</td><td>For signed numeric substitutions, prepend a single space before positive +numbers. +</td></tr><tr> +<td><b>0</b> +</td><td>(The zero-padding option) +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. +Infinity and NaN (Not-A-Number) floating point values are normally rendered +as "Inf" and "NaN", respectively, but with the zero-padding option enabled +they are rendered as "9.0e+999" and "null". In other words, with the zero-padding +option, floating-point Infinity and NaN are rendered as valid SQL and JSON literals. +</td></tr><tr> +<td><b>#</b> +</td><td>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. +</td></tr><tr> +<a name="comma"></a> + +<td><b>,</b> +</td><td>The comma option causes comma separators to be added to the output of numeric +substitutions (%d, %f, and similar) before every 3rd digits to the left of +the decimal point. No commas are added for digits to the right of the decimal point. +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. +</td></tr><tr> +<td><b>!</b> +</td><td>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.<br><br> +The alternate-form-2 flag is a non-standard extension that appears in no +other printf() implementations, as far as we know. +</td></tr></table> +</center> + +<h1 id="implementation_and_history"><span>3. </span>Implementation And History</h1> + +<p> +The core string formatting routine is the sqlite3VXPrintf() function found in the +<a href="https://sqlite.org/src/file/src/printf.c">printf.c</a> 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 (<a href="crew.html">Hipp</a>) 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 <a href="https://sqlite.org/src/timeline?c=f9372072a6">2000-10-08</a> +for SQLite version 1.0.9. + +</p><p> +The <a href="https://www.fossil-scm.org/">Fossil Version Control System</a> uses its own +printf() implementation that is derived from an early version of the SQLite +printf() implementation, but those two implementations have since diverged. + +</p><p> +The <a href="c3ref/mprintf.html">sqlite3_snprintf()</a> 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. +</p><p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/printf.in?m=4880d4e1ee">2023-06-29 23:18:37</a> UTC </small></i></p> + |