View source for Template:cpp/io/c/scanf format

The '''format''' string consists of
* non-whitespace {{#if:{{{wcs|}}}|wide|multibyte}} characters except {{c|%}}: each such character in the format string consumes exactly one identical character from the input stream, or causes the function to fail if the next character on the stream does not compare equal.
* whitespace characters: any single whitespace character in the format string consumes all available consecutive whitespace characters from the input (determined as if by calling {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/iswspace}}|{{ltt|{{get lang}}/string/byte/isspace}}}} in a loop). Note that there is no difference between {{c|"\n"}}, {{c|" "}}, {{c|"\t\t"}}, or other whitespace in the format string.
* conversion specifications. Each conversion specification has the following format:

:* introductory {{c|%}} character

:* {{mark optional}} assignment-suppressing character {{c|*}}. If this option is present, the function does not assign the result of the conversion to any receiving argument.

:* {{mark optional}} integer number (greater than zero) that specifies ''maximum field width'', that is, the maximum number of characters that the function is allowed to consume when doing the conversion specified by the current conversion specification. Note that {{c|%s}} and {{c|%[}} may lead to buffer overflow if the width is not provided.

:* {{mark optional}} ''length modifier'' that specifies the size of the receiving argument, that is, the actual destination type. This affects the conversion accuracy and overflow rules. The default destination type is different for each conversion type (see table below).

:* conversion format specifier

The following format specifiers are available:

{| class="wikitable" style="text-align:center; font-size:85%; max-width:90em;"
|-
! rowspan=1 | Conversion<br> specifier
! rowspan=1 | Explanation
! colspan=9 | Argument type
|-
! colspan=2 style="text-align:right; font-weight:normal;" | <div style="display:inline-block; vertical-align:middle; text-align:center">'''Length modifier →'''</div>
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! {{tt|hh}}
{{#ifeq:{{get lang}}|c|{{mark c99}}|{{mark c++11}}}}
! {{tt|h}}
! style="font-weight:normal;" | (none)
! {{ttb|l}}
! {{ttb|ll}}
{{#ifeq:{{get lang}}|c|{{mark c99}}|{{mark c++11}}}}
! {{ttb|j}}
{{#ifeq:{{get lang}}|c|{{mark c99}}|{{mark c++11}}}}
! {{ttb|z}}
{{#ifeq:{{get lang}}|c|{{mark c99}}|{{mark c++11}}}}
! {{ttb|t}}
{{#ifeq:{{get lang}}|c|{{mark c99}}|{{mark c++11}}}}
! {{ttb|L}}
|-
! {{tt|%}}
| style="text-align:left;" | matches literal {{ttb|%}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
|-
! {{tt|c}}
| style="text-align:left;" |
: matches a '''character''' or a sequence of '''characters'''
If a width specifier is used, matches exactly ''width'' {{#if:{{{wcs|}}}|wide}} characters (the argument must be a pointer to an array with sufficient room). Unlike %s and %[, does not append the null character to the array.
| rowspan=3 {{n/a}}
| rowspan=3 {{n/a}}
| rowspan=3 | {{vertical|{{c|char*}}}}
| rowspan=3 | {{vertical|{{c|wchar_t*}}}}
| rowspan=3 {{n/a}}
| rowspan=3 {{n/a}}
| rowspan=3 {{n/a}}
| rowspan=3 {{n/a}}
| rowspan=3 {{n/a}}
|-
! {{tt|s}}
| style="text-align:left;" |
: matches a sequence of non-whitespace characters (a '''string''')
If width specifier is used, matches up to ''width'' or until the first whitespace character, whichever appears first. Always stores a null character in addition to the characters matched (so the argument array must have room for at least ''width+1'' characters)
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|-
! {{tt|[}}{{spar|set}}{{tt|]}}
| style="text-align:left;" |
:matches a non-empty sequence of character from {{spar|set}} of characters.

If the first character of the set is {{ttb|^}}, then all characters not in the set are matched. If the set begins with {{ttb|]}} or {{ttb|^]}} then the {{ttb|]}} character is also included into the set. It is implementation-defined whether the character {{ttb|-}} in the non-initial position in the scanset may be indicating a range, as in {{ttb|[0-9]}}. If width specifier is used, matches only up to ''width''. Always stores a null character in addition to the characters matched (so the argument array must have room for at least ''width+1'' characters)
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|-
! {{tt|d}}
| style="text-align:left;" |
:matches a '''decimal integer'''.

The format of the number is the same as expected by {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/wcstol}}|{{ltt|{{get lang}}/string/byte/strtol}}}} with the value {{c|10}} for the {{tt|base}} argument
| rowspan=6 | {{vertical|{{c|signed char*}} or {{c|unsigned char*}}}}
| rowspan=6 | {{vertical|{{c|signed short*}} or {{c|unsigned short*}}}}
| rowspan=6 | {{vertical|{{c|signed int*}} or {{c|unsigned int*}}}}
| rowspan=6 | {{vertical|{{c|signed long*}} or {{c|unsigned long*}}}}
| rowspan=6 | {{vertical|{{c|signed long long*}} or {{c|unsigned long long*}}}}
| rowspan=6 | {{vertical|{{c|intmax_t*}} or {{c|uintmax_t*}}}}
| rowspan=6 | {{vertical|{{c|size_t*}}}}
| rowspan=6 | {{vertical|{{c|ptrdiff_t*}}}}
| rowspan=6 {{n/a}}
|-
! {{tt|i}}
| style="text-align:left;" |
:matches an '''integer'''.

The format of the number is the same as expected by {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/wcstol}}|{{ltt|{{get lang}}/string/byte/strtol}}}} with the value {{c|0}} for the {{tt|base}} argument (base is determined by the first characters parsed)
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|-
! {{tt|u}}
| style="text-align:left;" |
:matches an unsigned '''decimal integer'''.

The format of the number is the same as expected by {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/wcstoul}}|{{ltt|{{get lang}}/string/byte/strtoul}}}} with the value {{c|10}} for the {{tt|base}} argument.
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|-
! {{tt|o}}
| style="text-align:left;" |
:matches an unsigned '''octal integer'''.

The format of the number is the same as expected by {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/wcstoul}}|{{ltt|{{get lang}}/string/byte/strtoul}}}} with the value {{c|8}} for the {{tt|base}} argument
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|-
! {{tt|x}}, {{tt|X}}
| style="text-align:left;" |
:matches an unsigned '''hexadecimal integer'''.

The format of the number is the same as expected by {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/wcstoul}}|{{ltt|{{get lang}}/string/byte/strtoul}}}} with the value {{c|16}} for the {{tt|base}} argument
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|-
! {{tt|n}}
| style="text-align:left;" |
:returns the '''number of characters read so far'''.

No input is consumed. Does not increment the assignment count. If the specifier has assignment-suppressing operator defined, the behavior is undefined
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|-
! {{tt|a}}, {{tt|A}}{{#ifeq:{{get lang}}|c|{{mark c99}}|{{mark c++11}}}}<br> {{tt|e}}, {{tt|E}}<br> {{tt|f}}, {{tt|F}}{{#ifeq:{{get lang}}|c|{{mark c99}}|}}<br> {{tt|g}}, {{tt|G}}
| style="text-align:left;" |
:matches a '''floating-point number'''.

The format of the number is the same as expected by {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/wcstof}}|{{ltt|{{get lang}}/string/byte/strtof}}}}
| {{n/a}}
| {{n/a}}
| {{vertical|{{c|float*}}}}
| {{vertical|{{c|double*}}}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{vertical|{{c|long double*}}}}
|-
! {{tt|p}}
| style="text-align:left;" |
:matches implementation defined character sequence defining a '''pointer'''.

{{tt|printf}} family of functions should produce the same sequence using {{ttb|%p}} format specifier
| {{n/a}}
| {{n/a}}
| {{vertical|{{c|void**}}}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
| {{n/a}}
|}
For every conversion specifier other than {{c|n}}, the longest sequence of input characters which does not exceed any specified ﬁeld width and which either is exactly what the conversion specifier expects or is a prefix of a sequence it would expect, is what's consumed from the stream. The ﬁrst character, if any, after this consumed sequence remains unread. If the consumed sequence has length zero or if the consumed sequence cannot be converted as specified above, the matching failure occurs unless end-of-ﬁle, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

All conversion specifiers other than {{c|[}}, {{c|c}}, and {{c|n}} consume and discard all leading whitespace characters (determined as if by calling {{#if:{{{wcs|}}}|{{ltt|{{get lang}}/string/wide/iswspace}}|{{ltt|{{get lang}}/string/byte/isspace}}}}) before attempting to parse the input. These consumed characters do not count towards the specified maximum field width.

{{#if:{{{wcs|}}}
|If the length specifier {{c|l}} is not used, the conversion specifiers {{c|c}}, {{c|s}}, and {{c|[}} perform wide-to-multibyte character conversion as if by calling {{ltt|{{get lang}}/string/multibyte/wcrtomb}} with an {{ltt|{{get lang}}/string/multibyte/mbstate_t}} object initialized to zero before the first character is converted.
|The conversion specifiers {{c|lc}}, {{c|ls}}, and {{c|l[}} perform multibyte-to-wide character conversion as if by calling {{ltt|{{get lang}}/string/multibyte/mbrtowc}} with an {{ltt|{{get lang}}/string/multibyte/mbstate_t}} object initialized to zero before the first character is converted.
}}

The conversion specifiers {{c|s}} and {{c|[}} always store the null terminator in addition to the matched characters. The size of the destination array must be at least one greater than the specified field width. The use of {{c|%s}} or {{c|%[}}, without specifying the destination array size, is as unsafe as {{#ifeq:{{get lang}}|c|{{lc|gets}}|{{lc|std::gets}}}}.

The correct conversion specifications for the [[{{get lang}}/types/integer|fixed-width integer types]] ({{lc|int8_t}}, etc) are defined in the header {{#ifeq:{{get lang}}|c|[[c/types/integer|{{tt|1=<inttypes.h>}}]]|{{header|cinttypes}}}} (although {{ltt|{{get lang}}/types/integer|SCNdMAX}}, {{ltt|{{get lang}}/types/integer|SCNuMAX}}, etc is synonymous with {{c|%jd}}, {{c|%ju}}, etc).

There is a [[{{get lang}}/language/eval_order|sequence point]] after the action of each conversion specifier; this permits storing multiple fields in the same "sink" variable.

When parsing an incomplete floating-point value that ends in the exponent with no digits, such as parsing {{c|"100er"}} with the conversion specifier {{c|%f}}, the sequence {{c|"100e"}} (the longest prefix of a possibly valid floating-point number) is consumed, resulting in a matching error (the consumed sequence cannot be converted to a floating-point number), with {{c|"r"}} remaining. Some existing implementations do not follow this rule and roll back to consume only {{c|"100"}}, leaving {{c|"er"}}, e.g. [https://sourceware.org/bugzilla/show_bug.cgi?id=1765 glibc bug 1765]