Implementation status: partially implemented

Synopsis

#include <stdio.h>

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

Description

The functions read formatted input from the given stream.

The fscanf() function reads from the named input stream. The scanf() function reads from the standard input stream stdin. The sscanf() function reads 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 is evaluated but otherwise ignored.
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 allows 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 defaults to a ( '.' ).

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 characters ( , , , , or ); an ordinary character (neither '%' nor a white-space character); or a conversion specification. Each conversion specification is introduced by the character '%' 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.

  • 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 executes each directive of the format in turn. If a directive fails, as detailed below, the function returns. 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 characters is executed by reading input until no more valid input can be read, or up to the first byte which is not a white-space character, which remains unread.

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

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

Input white-space characters (as specified by isspace()) are skipped, unless the conversion specification includes a [, c, C, or n conversion specifier.

An item is read from the input, unless the conversion specification includes an n conversion specifier. An input item is 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 remains unread. If the length of the input item is 0, the execution of the conversion specification fails; 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) is 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 is 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 %, 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.

The %c, %s, and %[ conversion specifiers accept an optional assignment-allocation character 'm', which causes a memory buffer to be allocated to hold the string converted including a terminating null character. In such a case, the argument corresponding to the conversion specifier should be a reference to a pointer variable that receives a pointer to the allocated buffer. The system allocates a buffer as if malloc() had been called. The application is responsible for freeing the memory after usage. If there is insufficient memory to allocate a buffer, the function sets errno to [ENOMEM] and a conversion error result. If the function returns EOF, any memory successfully allocated for parameters using assignment-allocation character 'm' by this call is 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] [Option Start] If the 'm' assignment-allocation character is specified, the conversion applies to an argument with the type pointer to a pointer to wchar_t. [Option End]
  • 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 ensures 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 ensures 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 ensures 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 ensures 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 ensures 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 ensures 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, the fscanf() family of functions recognizes them as input.

  • s - Matches a sequence of bytes that are not white-space characters. If the 'm' assignment-allocation character is not specified, the application ensures 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 is added automatically. Otherwise, the application ensures 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 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. If the 'm' assignment-allocation character is not specified, the application ensures 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 is added automatically. Otherwise, the application ensures 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 characters is suppressed in this case. If the 'm' assignment-allocation character is not specified, the application ensures 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 is added automatically. Otherwise, the application ensures 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 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. If the 'm' assignment-allocation character is not specified, the application ensures 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 is added automatically.
    Otherwise, the application ensures 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 ( ']' ). The bytes between the square brackets (the scanlist) comprise the scanset, unless the byte after the is a ( '^' ), in which case the scanset contains all bytes that do not appear in the scanlist between the and the . If the conversion specification begins with "[]" or "[^]", the is included in the scanlist and the next is the matching that ends the conversion specification; otherwise, the first 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.

  • 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 characters is suppressed in this case. If the 'm' assignment-allocation character is not specified, the application ensures 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. Otherwise, the application ensures 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 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 ensures that the corresponding argument is a pointer to an array of wchar_t large enough to accept the resulting sequence of wide characters. Otherwise, the application ensures that the corresponding argument is a pointer to a pointer to a wchar_t.

  • p - Matches an implementation-defined set of sequences, which is the same as the set of sequences that is produced by the %p conversion specification of the corresponding fprintf() functions. The application ensures 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 ensures that the corresponding argument is a pointer to the integer into which is written the number of bytes read from the input so far by this call to the fscanf() functions. Execution of a %n conversion specification does not increment the assignment count returned at the completion of execution of the function. No argument is converted, but one is consumed. If the conversion specification includes an assignment-suppressing character or a field width, the behavior is undefined.
  • C - Equivalent to lc.
  • S - Equivalent to ls.
  • % - Matches a single '%' character; no conversion or assignment occurs. The complete conversion specification is %%.

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

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

If end-of-file is encountered during input, conversion is 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 characters, where permitted), execution of the current conversion specification terminates 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) is terminated with an input failure.

Reaching the end of the string in sscanf() is 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 (including characters) is 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.

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 is marked for update by the first successful execution of fgetc(), fgets(), fread(), getc(), getchar(), getdelim(), getline(), gets(), fscanf(), or scanf() using stream that returns data not supplied by a prior call to ungetc().

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.

Return value

Upon successful completion, these functions 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 is returned. If an error occurs before the first conversion (if any) has completed, and without a matching failure having occurred, EOF is returned and errno is set to indicate the error. If a read error occurs, the error indicator for the stream is set.

Errors

[EAGAIN] The O_NONBLOCK flag is set for the file descriptor underlying stream and the thread would be delayed in the fgetc() operation.
[EBADF] The file descriptor underlying stream is not a valid file descriptor open for reading.
[EINTR] The read operation was terminated due to the receipt of a signal, and no data was transferred.
[EIO] A physical I/O error has occurred, or the process is in a background process group attempting to read from its controlling terminal, and either the calling thread is blocking SIGTTIN or the process is ignoring SIGTTIN or the process group of the process is orphaned.
[EOVERFLOW] The file is a regular file and an attempt was made to read at or beyond the offset maximum associated with the corresponding stream.
[ENOMEM] Insufficient storage space is available.
[ENXIO] A request was made of a nonexistent device, or the request was outside the capabilities of the device.
[EILSEQ] Input byte sequence does not form a valid character.
[ENOMEM] Insufficient storage space is available.
[EINVAL] There are insufficient arguments.

Implementation tasks

  • Implement wide character handling.
  • Implement error handling.