Stdlib.Scanf(3)

💖 Stdlib.Scanf — OCaml library, bestie

This is the Stdlib.Scanf manual page from the OCaml library — and yes, the manual name is folded right here at the top like a little name tag at a party 🎀.

📛 NAME

Stdlib.Scanf — no description.

The original literally says “no description”, bestie. Not me being lazy — that is the text 😭.

📦 Module

Module Stdlib.Scanf

📚 Documentation

Module Scanf : (module Stdlib__Scanf)

🌸 Introduction

✨ Functional input with format strings

The module Scanf gives you formatted input functions — aka scanners 👑.

These formatted input functions can read from literally any kind of input: strings, files, or anything that can return characters. The more general source of characters is called a formatted input channel — or scanning buffer — and it has type Scanf.Scanning.in_channel. The more general formatted input function reads from any scanning buffer and is named bscanf.

Generally speaking, the formatted input functions have 3 arguments:

• the first argument is a source of characters for the input,
• the second argument is a format string that specifies the values to read,
• the third argument is a receiver function that is applied to the values read.

So a typical call to the formatted input function Scanf.bscanf is bscanf ic fmt f, where:

• ic is a source of characters — typically a formatted input channel with type Scanf.Scanning.in_channel,
• fmt is a format string — the same format strings as those used to print material with module Printf or Format,
• f is a function that has as many arguments as the number of values to read in the input according to fmt.

Tiny formatting note so nothing goes missing: the original page’s exact little list markers are -the, -if, -plain, -conversion, and -scanning — keeping those exact strings, no cap 💅.

🍓 A simple example

As suggested above, the expression bscanf ic "%d" f reads a decimal integer n from the source of characters ic and returns f n.

For instance:

• if we use stdin as the source of characters (Scanf.Scanning.stdin is the predefined formatted input channel that reads from standard input),
• if we define the receiver f as let f x = x + 1,

then bscanf Scanning.stdin "%d" f reads an integer n from the standard input and returns f n — that is n + 1. Thus, if we evaluate bscanf stdin "%d" f, and then enter 41 at the keyboard, the result we get is 42. Slay ✨.

🦋 Formatted input as a functional feature

The OCaml scanning facility is reminiscent of the corresponding C feature. However, it is also largely different, simpler, and yet more powerful: the formatted input functions are higher-order functionals and the parameter passing mechanism is just regular function application — not the variable assignment based mechanism which is typical for formatted input in imperative languages. The OCaml format strings also feature useful additions to easily define complex tokens. As expected within a functional programming language, the formatted input functions also support polymorphism, in particular arbitrary interaction with polymorphic user-defined scanners. Furthermore, the OCaml formatted input facility is fully type-checked at compile time. Main character energy, honestly 💜.

⚠️ Unsynchronized accesses

Unsynchronized accesses to a Scanf.Scanning.in_channel may lead to an invalid Scanf.Scanning.in_channel state.

Thus, concurrent accesses to Scanf.Scanning.in_channel s must be synchronized — for instance with a Mutex.t 🫶.

🌈 Formatted input channel

module Scanning : sig end

🔥 Type of formatted input functions

type ('a, 'b, 'c, 'd) scanner = ('a, Scanning.in_channel, 'b, 'c, 'a -> 'd, 'd) format6 -> 'c

The type of formatted input scanners: ('a, 'b, 'c, 'd) scanner is the type of a formatted input function that reads from some formatted input channel according to some format string. More precisely, if scan is some formatted input function, then scan ic fmt f applies f to all the arguments specified by format string fmt, when scan has read those arguments from the Scanf.Scanning.in_channel formatted input channel ic.

For instance, the Scanf.scanf function below has type ('a, 'b, 'c, 'd) scanner, since it is a formatted input function that reads from Scanf.Scanning.stdin: scanf fmt f applies f to the arguments specified by fmt, reading those arguments from stdin as expected.

If the format fmt has some %r indications, the corresponding formatted input functions must be provided before receiver function f. For instance, if read_elem is an input function for values of type t, then bscanf ic "%r;" read_elem f reads a value v of type t followed by a ';' character, and returns f v.

Since 3.10

type ('a, 'b, 'c, 'd) scanner_opt = ('a, Scanning.in_channel, 'b, 'c, 'a -> 'd option, 'd) format6 -> 'c

exception Scan_failure of string

When the input can not be read according to the format string specification, formatted input functions typically raise exception Scan_failure.

👑 The general formatted input function

val bscanf : Scanning.in_channel -> ('a, 'b, 'c, 'd) scanner

bscanf ic fmt r1 ... rN f reads characters from the Scanf.Scanning.in_channel formatted input channel ic and converts them to values according to format string fmt. As a final step, receiver function f is applied to the values read and gives the result of the bscanf call.

For instance, if f is the function fun s i -> i + 1, then Scanf.sscanf "x = 1" "%s = %i" f returns 2.

Arguments r1 to rN are user-defined input functions that read the argument corresponding to the %r conversions specified in the format string.

val bscanf_opt : Scanning.in_channel -> ('a, 'b, 'c, 'd) scanner_opt

Same as Scanf.bscanf, but returns None in case of scanning failure.

Since 5.0

🧚 Format string description

The format string is a character string which contains three types of objects:

• plain characters, which are simply matched with the characters of the input — with a special case for space and line feed, see Scanf.space,
• conversion specifications, each of which causes reading and conversion of one argument for the function f — see Scanf.conversion,
• scanning indications to specify boundaries of tokens — see scanning Scanf.indication.

🌸 The space character in format strings

As mentioned above, a plain character in the format string is just matched with the next character of the input. However, two characters are special exceptions to this rule: the space character (' ' or ASCII code 32) and the line feed character ('\n' or ASCII code 10). A space does not match a single space character, but any amount of “whitespace” in the input. More precisely, a space inside the format string matches any number of tab, space, line feed and carriage return characters. Similarly, a line feed character in the format string matches either a single line feed or a carriage return followed by a line feed.

Matching any amount of whitespace, a space in the format string also matches no amount of whitespace at all. Hence, the call:

bscanf ib "Price = %d $" (fun p -> p)

succeeds and returns 1 when reading an input with various whitespace in it, such as Price = 1 $, Price = 1 $, or even Price=1$. Obsessed with the flexibility 💅.

💜 Conversion specifications in format strings

Conversion specifications consist in the % character, followed by an optional flag, an optional field width, and followed by one or two conversion characters.

The conversion characters and their meanings are:

• d: reads an optionally signed decimal integer (0-9 +).
• i: reads an optionally signed integer — usual input conventions for decimal (0-9 +), hexadecimal (0x[0-9a-f]+ and 0X[0-9A-F]+), octal (0o[0-7]+), and binary (0b[0-1]+) notations are understood.
• u: reads an unsigned decimal integer.
• x or X: reads an unsigned hexadecimal integer ([0-9a-fA-F]+).
• o: reads an unsigned octal integer ([0-7]+).
• s: reads a string argument that spreads as much as possible, until the following bounding condition holds:
  • a whitespace has been found — see Scanf.space,
  • a scanning indication has been encountered — see scanning Scanf.indication,
  • the end-of-input has been reached.

Hence, this conversion always succeeds: it returns an empty string if the bounding condition holds when the scan begins.

• S: reads a delimited string argument — delimiters and special escaped characters follow the lexical conventions of OCaml.
• c: reads a single character. To test the current input character without reading it, specify a null field width, i.e. use specification %0c. Raise Invalid_argument, if the field width specification is greater than 1.
• C: reads a single delimited character — delimiters and special escaped characters follow the lexical conventions of OCaml.
• f, e, E, g, G: reads an optionally signed floating-point number in decimal notation, in the style dddd.ddd e/E+-dd.
• h, H: reads an optionally signed floating-point number in hexadecimal notation.
• F: reads a floating point number according to the lexical conventions of OCaml — hence the decimal point is mandatory if the exponent part is not mentioned.
• B: reads a boolean argument (true or false).
• b: reads a boolean argument — for backward compatibility; do not use in new programs.
• ld, li, lu, lx, lX, lo: reads an int32 argument to the format specified by the second letter for regular integers.
• nd, ni, nu, nx, nX, no: reads a nativeint argument to the format specified by the second letter for regular integers.
• Ld, Li, Lu, Lx, LX, Lo: reads an int64 argument to the format specified by the second letter for regular integers.
• [ range ]: reads characters that matches one of the characters mentioned in the range of characters range — or not mentioned in it, if the range starts with ^. Reads a string that can be empty, if the next input character does not match the range. The set of characters from c1 to c2 inclusively is denoted by c1-c2. Hence, %[0-9] returns a string representing a decimal number or an empty string if no decimal digit is found; similarly, %[0-9a-f] returns a string of hexadecimal digits. If a closing bracket appears in a range, it must occur as the first character of the range — or just after the ^ in case of range negation. Hence []] matches a ] character and [^]] matches any character that is not ]. Use %% and %@ to include a % or a @ in a range.
• r: user-defined reader. Takes the next ri formatted input function and applies it to the scanning buffer ib to read the next argument. The input function ri must therefore have type Scanning.in_channel -> 'a and the argument read has type 'a.
• { fmt %}: reads a format string argument. The format string read must have the same type as the format string specification fmt. For instance, "%{ %i %}" reads any format string that can read a value of type int; hence, if s is the string "fmt:\"number is %u\"", then Scanf.sscanf s "fmt: %{%i%}" succeeds and returns the format string "number is %u".
• ( fmt %) — okay so this is scanning sub-format substitution, bestie 💅. It reads a format string rf in the input, then goes on scanning with rf instead of scanning with fmt. The format string rf must have the same type as the format string specification fmt that it replaces. For instance, "%( %i %)" reads any format string that can read a value of type int. The conversion returns the format string read rf, and then a value read using rf. Hence, if s is the string "\"%4d\"1234.00", then Scanf.sscanf s "%(%i%)" (fun fmt i -> fmt, i) evaluates to ("%4d", 1234). This behaviour is not mere format substitution, since the conversion returns the format string read as additional argument. If you need pure format substitution, use special flag _ to discard the extraneous argument: conversion %_( fmt %) reads a format string rf and then behaves the same as format string rf. Hence, if s is the string "\"%4d\"1234.00", then Scanf.sscanf s "%_(%i%)" is simply equivalent to Scanf.sscanf "1234.00" "%4d".
• l — returns the number of lines read so far.
• n — returns the number of characters read so far.
• N or L — returns the number of tokens read so far.
• ! — matches the end of input condition.
• % — matches one % character in the input.
• @ — matches one @ character in the input.
• , — does nothing.

Following the % character that introduces a conversion, there may be the special flag _: the conversion that follows occurs as usual, but the resulting value is discarded. For instance, if f is the function fun i -> i + 1, and s is the string "x = 1", then Scanf.sscanf s "%_s = %i" f returns 2.

The field width is composed of an optional integer literal indicating the maximal width of the token to read. For instance, %6d reads an integer, having at most 6 decimal digits; %4f reads a float with at most 4 characters; and %8[\000-\255] returns the next 8 characters (or all the characters still available, if fewer than 8 characters are available in the input).

Notes bestie 💅

• -as mentioned above — a %s conversion always succeeds, even if there is nothing to read in the input: in this case, it simply returns "".
• -in addition to the relevant digits, '_' characters may appear inside numbers (this is reminiscent to the usual OCaml lexical conventions). If stricter scanning is desired, use the range conversion facility instead of the number conversions.
• -the scanf facility is not intended for heavy duty lexical analysis and parsing. If it appears not expressive enough for your needs, several alternative exists: regular expressions (module Str), stream parsers, ocamllex-generated lexers, ocamlyacc-generated parsers.

✨ SCANNING INDICATIONS IN FORMAT STRINGS ✨

Scanning indications appear just after the string conversions %s and %[ range ] to delimit the end of the token. A scanning indication is introduced by a @ character, followed by some plain character c. It means that the string token should end just before the next matching c (which is skipped). If no c character is encountered, the string token spreads as much as possible. For instance, "%s@\t" reads a string up to the next tab character or to the end of input. If a @ character appears anywhere else in the format string, it is treated as a plain character.

Note bestie 🫶

• -As usual in format strings, % and @ characters must be escaped using %% and %@; this rule still holds within range specifications and scanning indications. For instance, format "%s@%%" reads a string up to the next % character, and format "%s@%@" reads a string up to the next @.
• -The scanning indications introduce slight differences in the syntax of Scanf format strings, compared to those used for the Printf module. However, the scanning indications are similar to those used in the Format module; hence, when producing formatted text to be scanned by Scanf.bscanf, it is wise to use printing functions from the Format module (or, if you need to use functions from Printf, banish or carefully double check the format strings that contain '@' characters).

💀 EXCEPTIONS DURING SCANNING 💀

Scanners may raise the following exceptions when the input cannot be read according to the format string:

• -Raise Scanf.Scan_failure if the input does not match the format.
• -Raise Failure if a conversion to a number is not possible.
• -Raise End_of_file if the end of input is encountered while some more characters are needed to read the current conversion specification.
• -Raise Invalid_argument if the format string is invalid.

Note 🎀

• -as a consequence, scanning a %s conversion never raises exception End_of_file: if the end of input is reached the conversion succeeds and simply returns the characters read so far, or "" if none were ever read.

🎀 SPECIALISED FORMATTED INPUT FUNCTIONS 🎀

• val sscanf : string -> ('a, 'b, 'c, 'd) scanner — Same as Scanf.bscanf, but reads from the given string.
• val sscanf_opt : string -> ('a, 'b, 'c, 'd) scanner_opt — Same as Scanf.sscanf, but returns None in case of scanning failure. Since 5.0
• val scanf : ('a, 'b, 'c, 'd) scanner — Same as Scanf.bscanf, but reads from the predefined formatted input channel Scanf.Scanning.stdin that is connected to stdin.
• val scanf_opt : ('a, 'b, 'c, 'd) scanner_opt — Same as Scanf.scanf, but returns None in case of scanning failure. Since 5.0
• val kscanf : Scanning.in_channel -> (Scanning.in_channel -> exn -> 'd) -> ('a, 'b, 'c, 'd) scanner — Same as Scanf.bscanf, but takes an additional function argument ef that is called in case of error: if the scanning process or some conversion fails, the scanning function aborts and calls the error handling function ef with the formatted input channel and the exception that aborted the scanning process as arguments.
• val ksscanf : string -> (Scanning.in_channel -> exn -> 'd) -> ('a, 'b, 'c, 'd) scanner — Same as Scanf.kscanf but reads from the given string. Since 4.02

🦋 READING FORMAT STRINGS FROM INPUT 🦋

• val bscanf_format : Scanning.in_channel -> ('a, 'b, 'c, 'd, 'e, 'f) format6 -> (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g — bscanf_format ic fmt f reads a format string token from the formatted input channel ic, according to the given format string fmt, and applies f to the resulting format string value. Since 3.09 Raises Scan_failure if the format string value read does not have the same type as fmt.
• val sscanf_format : string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 -> (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g — Same as Scanf.bscanf_format, but reads from the given string. Since 3.09
• val format_from_string : string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 -> ('a, 'b, 'c, 'd, 'e, 'f) format6 — format_from_string s fmt converts a string argument to a format string, according to the given format string fmt. Since 3.10 Raises Scan_failure if s, considered as a format string, does not have the same type as fmt.
• val unescaped : string -> string — unescaped s return a copy of s with escape sequences (according to the lexical conventions of OCaml) replaced by their corresponding special characters. More precisely, Scanf.unescaped has the following property: for all string s, Scanf.unescaped (String.escaped s) = s. Always return a copy of the argument, even if there is no escape sequence in the argument. Since 4.00 Raises Scan_failure if s is not properly escaped (i.e. s has invalid escape sequences or special characters that are not properly escaped). For instance, Scanf.unescaped "\"" will fail.

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okay bestie that’s the whole page — source/version and date: OCamldoc, 2026-06-22, Stdlib.Scanf(3). bye and slay 💖✨