NAME

fscanf, scanf, sscanf — convert formatted input

SYNOPSIS

#include <stdio.h>

int fscanf(FILE *restrict stream, const char *restrict format, ...);
int scanf(const char *restrict format, ...);
int sscanf(const char *restrict s, const char *restrict format, ...);


DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The fscanf() function shall read from the named input stream. The scanf() function shall read from the standard input stream stdin. The sscanf() function shall read from the string s. Each function reads bytes, interprets them according to a format, and stores the results in its arguments. Each expects, as arguments, a control string format described below, and a set of pointer arguments indicating where the converted input should be stored. The result is undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments shall be evaluated but otherwise ignored.

^[CX] Conversions can be applied to the nth argument after the format in the argument list, rather than to the next unused argument. In this case, the conversion specifier character % (see below) is replaced by the sequence "%n$", where n is a decimal integer in the range [1,{NL_ARGMAX}]. This feature provides for the definition of format strings that select arguments in an order appropriate to specific languages. In format strings containing the "%n$" form of conversion specifications, it is unspecified whether numbered arguments in the argument list can be referenced from the format string more than once.

The format can contain either form of a conversion specification—that is, % or "%n$"—but the two forms cannot be mixed within a single format string. The only exception to this is that %% or %* can be mixed with the "%n$" form. When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N-1)th, are pointers.

The fscanf() function in all its forms shall allow detection of a language-dependent radix character in the input string. The radix character is defined in the current locale (category LC_NUMERIC ). In the POSIX locale, or in a locale where the radix character is not defined, the radix character shall default to a <period> ('.').

The application shall ensure that the format is a character string, beginning and ending in its initial shift state, if any, composed of zero or more directives. Each directive is composed of one of the following: one or more white-space bytes; an ordinary character (neither '%' nor a white-space byte); or a conversion specification. Each conversion specification is introduced by the character '%' ^[CX]  or the character sequence "%n$",  after which the following appear in sequence:

• An optional assignment-suppressing character '*'.

• An optional non-zero decimal integer that specifies the maximum field width.

• ^[CX] An optional assignment-allocation character 'm'.

• An option length modifier that specifies the size of the receiving object.

• A conversion specifier character that specifies the type of conversion to be applied. The valid conversion specifiers are described below.

The fscanf() functions shall execute each directive of the format in turn. When all directives have been executed, or if a directive fails (as detailed below), the function shall return. Failures are described as input failures (due to the unavailability of input bytes) or matching failures (due to inappropriate input).

A directive composed of one or more white-space bytes shall be executed by reading input up to the first non-white-space byte, which shall remain unread, or until no more bytes can be read. The directive shall never fail.

A directive that is an ordinary character shall be executed as follows: the next byte shall be read from the input and compared with the byte that comprises the directive; if the comparison shows that they are not equivalent, the directive shall fail, and the differing and subsequent bytes shall remain unread. Similarly, if end-of-file, an encoding error, or a read error prevents a character from being read, the directive shall fail.

A directive that is a conversion specification defines a set of matching input sequences, as described below for each conversion character. A conversion specification shall be executed in the following steps.

Input white-space bytes shall be skipped, unless the conversion specification includes a [, c, C, or n conversion specifier.

An item shall be read from the input, unless the conversion specification includes an n conversion specifier. An input item shall be defined as the longest sequence of input bytes (up to any specified maximum field width, which may be measured in characters or bytes dependent on the conversion specifier) which is an initial subsequence of a matching sequence. The first byte, if any, after the input item shall remain unread. If the length of the input item is 0, the execution of the conversion specification shall fail; this condition is a matching failure, unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

Except in the case of a % conversion specifier, the input item (or, in the case of a %n conversion specification, the count of input bytes) shall be converted to a type appropriate to the conversion character. If the input item is not a matching sequence, the execution of the conversion specification fails; this condition is a matching failure. Unless assignment suppression was indicated by a '*', the result of the conversion shall be placed in the object pointed to by the first argument following the format argument that has not already received a conversion result if the conversion specification is introduced by %, ^[CX]  or in the nth argument if introduced by the character sequence "%n$". If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the space provided, the behavior is undefined.

^[CX] The c, s, and [ conversion specifiers shall accept an optional assignment-allocation character 'm', which shall cause a memory buffer to be allocated to hold the conversion results. If the conversion specifier is s or [, the allocated buffer shall include space for a terminating null character (or wide character). In such a case, the argument corresponding to the conversion specifier should be a reference to a pointer variable that will receive a pointer to the allocated buffer. The system shall allocate a buffer as if malloc() had been called. The application shall be responsible for freeing the memory after usage. If there is insufficient memory to allocate a buffer, the function shall set errno to [ENOMEM] and a conversion error shall result. If the function returns EOF, any memory successfully allocated for parameters using assignment-allocation character 'm' by this call shall be freed before the function returns.

The length modifiers and their meanings are:

hh

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to signed char or unsigned char.

h

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to short or unsigned short.

l (ell)

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long or unsigned long; that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to double; or that a following c, s, or [ conversion specifier applies to an argument with type pointer to wchar_t. ^[CX]  If the 'm' assignment-allocation character is specified, the conversion applies to an argument with the type pointer to a pointer to wchar_t.

ll (ell-ell)

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long long or unsigned long long.

j

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to intmax_t or uintmax_t.

z

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to size_t or the corresponding signed integer type.

t

Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to ptrdiff_t or the corresponding unsigned type.

L

Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to long double.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

The following conversion specifiers are valid:

d

Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of strtol() with the value 10 for the base argument. In the absence of a size modifier, the application shall ensure that the corresponding argument is a pointer to int.

i

Matches an optionally signed integer, whose format is the same as expected for the subject sequence of strtol() with 0 for the base argument. In the absence of a size modifier, the application shall ensure that the corresponding argument is a pointer to int.

o

Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of strtoul() with the value 8 for the base argument. In the absence of a size modifier, the application shall ensure that the corresponding argument is a pointer to unsigned.

u

Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of strtoul() with the value 10 for the base argument. In the absence of a size modifier, the application shall ensure that the corresponding argument is a pointer to unsigned.

x

Matches an optionally signed hexadecimal integer, whose format is the same as expected for the subject sequence of strtoul() with the value 16 for the base argument. In the absence of a size modifier, the application shall ensure that the corresponding argument is a pointer to unsigned.

a, e, f, g

Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as expected for the subject sequence of strtod(). In the absence of a size modifier, the application shall ensure that the corresponding argument is a pointer to float.

If the fprintf() family of functions generates character string representations for infinity and NaN (a symbolic entity encoded in floating-point format) to support IEEE Std 754-1985, the fscanf() family of functions shall recognize them as input.

s

Matches a sequence of bytes that are not white-space bytes. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to the initial byte of an array of char, signed char, or unsigned char large enough to accept the sequence and a terminating null character code, which shall be added automatically. ^[CX]  Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a char.

If an l (ell) qualifier is present, the input is a sequence of characters that begins in the initial shift state. Each character shall be converted to a wide character as if by a call to the mbrtowc() function, with the conversion state described by an mbstate_t object initialized to zero before the first character is converted. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to an array of wchar_t large enough to accept the sequence and the terminating null wide character, which shall be added automatically. ^[CX]  Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a wchar_t.

[

Matches a non-empty sequence of bytes from a set of expected bytes (the scanset). The normal skip over white-space bytes shall be suppressed in this case. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to the initial byte of an array of char, signed char, or unsigned char large enough to accept the sequence and a terminating null byte, which shall be added automatically. ^[CX]  Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a char.

If an l (ell) qualifier is present, the input is a sequence of characters that begins in the initial shift state. Each character in the sequence shall be converted to a wide character as if by a call to the mbrtowc() function, with the conversion state described by an mbstate_t object initialized to zero before the first character is converted. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to an array of wchar_t large enough to accept the sequence and the terminating null wide character, which shall be added automatically.
^[CX] Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a wchar_t.

The conversion specification includes all subsequent bytes in the format string up to and including the matching <right-square-bracket> (']'). The bytes between the square brackets (the scanlist) comprise the scanset, unless the byte after the <left-square-bracket> is a <circumflex> ('^'), in which case the scanset contains all bytes that do not appear in the scanlist between the <circumflex> and the <right-square-bracket>. If the conversion specification begins with "[]" or "[^]", the <right-square-bracket> is included in the scanlist and the next <right-square-bracket> is the matching <right-square-bracket> that ends the conversion specification; otherwise, the first <right-square-bracket> is the one that ends the conversion specification. If a '-' is in the scanlist and is not the first character, nor the second where the first character is a '^', nor the last character, the behavior is implementation-defined.

c

Matches a sequence of bytes of the number specified by the field width (1 if no field width is present in the conversion specification). No null byte is added. The normal skip over white-space bytes shall be suppressed in this case. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to the initial byte of an array of char, signed char, or unsigned char large enough to accept the sequence. ^[CX]  Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a char.

If an l (ell) qualifier is present, the input shall be a sequence of characters that begins in the initial shift state. Each character in the sequence is converted to a wide character as if by a call to the mbrtowc() function, with the conversion state described by an mbstate_t object initialized to zero before the first character is converted. No null wide character is added. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to an array of wchar_t large enough to accept the resulting sequence of wide characters. ^[CX]  Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a wchar_t.

p

Matches an implementation-defined set of sequences, which shall be the same as the set of sequences that is produced by the %p conversion specification of the corresponding fprintf() functions. The application shall ensure that the corresponding argument is a pointer to a pointer to void. The interpretation of the input item is implementation-defined. If the input item is a value converted earlier during the same program execution, the pointer that results shall compare equal to that value; otherwise, the behavior of the %p conversion specification is undefined.

n

No input is consumed. The application shall ensure that the corresponding argument is a pointer to the integer into which shall be written the number of bytes read from the input so far by this call to the fscanf() functions. Execution of a %n conversion specification shall not increment the assignment count returned at the completion of execution of the function. No argument shall be converted, but one shall be consumed. If the conversion specification includes an assignment-suppressing character or a field width, the behavior is undefined.

C

^[XSI] Equivalent to lc.

S

^[XSI] Equivalent to ls.

%

Matches a single '%' character; no conversion or assignment occurs. The complete conversion specification shall be %%.

If a conversion specification is invalid, the behavior is undefined.

The conversion specifiers A, E, F, G, and X are also valid and shall be equivalent to a, e, f, g, and x, respectively.

If end-of-file is encountered during input, conversion shall be terminated. If end-of-file occurs before any bytes matching the current conversion specification (except for %n) have been read (other than leading white-space bytes, where permitted), execution of the current conversion specification shall terminate with an input failure. Otherwise, unless execution of the current conversion specification is terminated with a matching failure, execution of the following conversion specification (if any) shall be terminated with an input failure.

Reaching the end of the string in sscanf() shall be equivalent to encountering end-of-file for fscanf().

If conversion terminates on a conflicting input, the offending input is left unread in the input. Any trailing white-space bytes (including <newline> characters) shall be left unread unless matched by a conversion specification. The success of literal matches and suppressed assignments is only directly determinable via the %n conversion specification.

^[CX] The fscanf() and scanf() functions may mark the last data access timestamp of the file associated with stream for update. The last data access timestamp shall be marked for update by the first successful execution of fgetc(), fgets(), fread(), getc(), getchar(), getdelim(), getline(), fscanf(), or scanf() using stream that returns data not supplied by a prior call to ungetc().

RETURN VALUE

Upon successful completion, these functions shall return the number of successfully matched and assigned input items; this number can be zero in the event of an early matching failure. If the input ends before the first conversion (if any) has completed, and without a matching failure having occurred, EOF shall be returned. If an error occurs before the first conversion (if any) has completed, and without a matching failure having occurred, EOF shall be returned ^[CX]  and errno shall be set to indicate the error.  If an error occurs, the error indicator for the stream shall be set.

ERRORS

For the conditions under which the fscanf() functions fail and may fail, refer to fgetc or fgetwc.

In addition, the fscanf() function shall fail if:

[EILSEQ]

^[CX] Input byte sequence does not form a valid character.

[ENOMEM]

Insufficient storage space is available.

In addition, the fscanf() function may fail if:

[EINVAL]

^[CX] There are insufficient arguments.

EXAMPLES

The call:

int i, n; float x; char name[50];
n = scanf("%d%f%s", &i, &x, name);

with the input line:

25 54.32E-1 Hamster

assigns to n the value 3, to i the value 25, to x the value 5.432, and name contains the string "Hamster".

The call:

int i; float x; char name[50];
(void) scanf("%2d%f%*d %[0123456789]", &i, &x, name);

with input:

56789 0123 56a72

assigns 56 to i, 789.0 to x, skips 0123, and places the string "56\0" in name. The next call to getchar() shall return the character 'a'.

Reading Data into an Array

The following call uses fscanf() to read three floating-point numbers from standard input into the input array.

float input[3]; fscanf (stdin, "%f %f %f", input, input+1, input+2);

APPLICATION USAGE

If the application calling fscanf() has any objects of type wint_t or wchar_t, it must also include the <wchar.h> header to have these objects defined.

For functions that allocate memory as if by malloc(), the application should release such memory when it is no longer required by a call to free(). For fscanf(), this is memory allocated via use of the 'm' assignment-allocation character.

RATIONALE

The set of characters allowed in a scanset is limited to single-byte characters. In other similar places, multi-byte characters have been permitted, but for alignment with the ISO C standard, it has not been done here. Applications needing this could use the corresponding wide-character functions to achieve the desired results.

FUTURE DIRECTIONS

None.

SEE ALSO

2.5 Standard I/O Streams, fprintf, getc, setlocale, strtod, strtol, strtoul, wcrtomb

XBD 7. Locale, <inttypes.h>, <langinfo.h>, <stdio.h>, <wchar.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Aligned with ISO/IEC 9899:1990/Amendment 1:1995 (E). Specifically, the l (ell) qualifier is now defined for the c, s, and [ conversion specifiers.

The DESCRIPTION is updated to indicate that if infinity and NaN can be generated by the fprintf() family of functions, then they are recognized by the fscanf() family.

Issue 6

The Open Group Corrigenda U021/7 and U028/10 are applied. These correct several occurrences of "characters" in the text which have been replaced with the term "bytes".

The normative text is updated to avoid use of the term "must" for application requirements.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

• The prototypes for fscanf(), scanf(), and sscanf() are updated.

• The DESCRIPTION is updated.

• The hh, ll, j, t, and z length modifiers are added.

• The a, A, and F conversion characters are added.

The DESCRIPTION is updated to use the terms "conversion specifier" and "conversion specification" consistently.

Issue 7

Austin Group Interpretation 1003.1-2001 #170 is applied.

SD5-XSH-ERN-9 is applied, correcting fscanf() to scanf() in the DESCRIPTION.

SD5-XSH-ERN-132 is applied, adding the assignment-allocation character 'm'.

Functionality relating to the %n$ form of conversion specification is moved from the XSI option to the Base.

Changes are made related to support for finegrained timestamps.

The APPLICATION USAGE section is updated to clarify that memory is allocated as if by malloc().

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0185 [302], XSH/TC1-2008/0186 [90], and XSH/TC1-2008/0187 [14] are applied. XSH/TC1-2008/0186 [90] changes the second sentence in the RETURN VALUE section to align with expected wording changes in the next revision of the ISO C standard.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0135 [936] is applied.

Issue 8

Austin Group Defect 1163 is applied, clarifying the handling of white space in the format string.

Austin Group Defect 1173 is applied, clarifying the description of the assignment-allocation character 'm'.

Austin Group Defect 1302 is applied, aligning these functions with the ISO/IEC 9899:2018 standard.

Austin Group Defect 1330 is applied, removing obsolescent interfaces.

Austin Group Defect 1375 is applied, changing "terminating null character" to "terminating null character (or wide character)".

Austin Group Defect 1562 is applied, clarifying that it is the application's responsibility to ensure that the format is a character string, beginning and ending in its initial shift state, if any.

Austin Group Defect 1624 is applied, changing the RETURN VALUE section.