| FSCANF(3P) | POSIX Programmer's Manual | FSCANF(3P) |
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
fscanf, scanf, sscanf — convert formatted input
#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, ...);
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.1‐2017 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.
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 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 (<space>, <tab>, <newline>, <vertical-tab>, or <form-feed>); 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:
The fscanf() functions shall execute each directive of the format in turn. 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 characters shall be 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 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 characters (as specified by isspace()) 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 %, 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
shall accept an optional assignment-allocation character 'm', which
shall cause 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 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:
If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.
The following conversion specifiers are valid:
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.
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. Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a wchar_t.
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.
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.
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. Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a wchar_t.
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 characters, 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 (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.
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(), gets(), fscanf(), or scanf() using stream that returns data not supplied by a prior call to ungetc().
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 and errno shall be set to indicate the error. If a read error occurs, the error indicator for the stream shall be set.
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:
In addition, the fscanf() function may fail if:
The following sections are informative.
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'.
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);
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.
This function is aligned with the ISO/IEC 9899:1999 standard, and in doing so a few ``obvious'' things were not included. Specifically, 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/IEC 9899:1999 standard, it has not been done here. Applications needing this could use the corresponding wide-character functions to achieve the desired results.
None.
Section 2.5, Standard I/O Streams, fprintf(), getc(), setlocale(), strtod(), strtol(), strtoul(), wcrtomb()
The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale, <inttypes.h>, <langinfo.h>, <stdio.h>, <wchar.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
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