Provided by: ocaml-man_5.3.0-2_all 
NAME
Stdlib.Scanf - no description
Module
Module Stdlib.Scanf
Documentation
Module Scanf
: (module Stdlib__Scanf)
Introduction
Functional input with format strings
The module Scanf provides formatted input functions or scanners.
The formatted input functions can read from any kind of input, including strings, files, or anything that
can return characters. The more general source of characters is named a formatted input channel (or
scanning buffer) and 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.
Hence, 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 .
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 .
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 the 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.
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$ .
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 (see scanning Scanf.indication ) has been encountered,
-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 %) : scanning sub-format substitution. 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:
-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:
-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.
OCamldoc 2025-01-27 Stdlib.Scanf(3o)