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A sequence of white-space characters (space, tab, newline, etc.; see isspace(3)). This directive matches any amount of white space, including none, in the input.

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An ordinary character (i.e., one other than white space or '%'). This character must exactly match the next character of input.

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A conversion specification, which commences with a '%' (percent) character. A sequence of characters from the input is converted according to this specification, and the result is placed in the corresponding pointer argument. If the next item of input does not match the conversion specification, the conversion fails—this is a matching failure.

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An optional '*' assignment-suppression character: scanf() reads input as directed by the conversion specification, but discards the input. No corresponding pointer argument is required, and this specification is not included in the count of successful assignments returned by scanf().

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For decimal conversions, an optional quote character ('). This specifies that the input number may include thousands’ separators as defined by the LC_NUMERIC category of the current locale. (See setlocale(3).) The quote character may precede or follow the '*' assignment-suppression character.

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An optional 'm' character. This is used with string conversions (%s, %c, %[), and relieves the caller of the need to allocate a corresponding buffer to hold the input: instead, scanf() allocates a buffer of sufficient size, and assigns the address of this buffer to the corresponding pointer argument, which should be a pointer to a char * variable (this variable does not need to be initialized before the call). The caller should subsequently free(3) this buffer when it is no longer required.

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An optional decimal integer which specifies the maximum field width. Reading of characters stops either when this maximum is reached or when a nonmatching character is found, whichever happens first. Most conversions discard initial white space characters (the exceptions are noted below), and these discarded characters don’t count toward the maximum field width. String input conversions store a terminating null byte ('\0') to mark the end of the input; the maximum field width does not include this terminator.

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An optional type modifier character. For example, the l type modifier is used with integer conversions such as %d to specify that the corresponding pointer argument refers to a long rather than a pointer to an int.

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A conversion specifier that specifies the type of input conversion to be performed.

h

Indicates that the conversion will be one of d, i, o, u, x, X, or n and the next pointer is a pointer to a short or unsigned short (rather than int).

hh

As for h, but the next pointer is a pointer to a signed char or unsigned char.

j

As for h, but the next pointer is a pointer to an intmax_t or a uintmax_t. This modifier was introduced in C99.

l

Indicates either that the conversion will be one of d, i, o, u, x, X, or n and the next pointer is a pointer to a long or unsigned long (rather than int), or that the conversion will be one of e, f, or g and the next pointer is a pointer to double (rather than float). Specifying two l characters is equivalent to L. If used with %c or %s, the corresponding parameter is considered as a pointer to a wide character or wide-character string respectively.

L

Indicates that the conversion will be either e, f, or g and the next pointer is a pointer to long double or the conversion will be d, i, o, u, or x and the next pointer is a pointer to long long.

q

equivalent to L. This specifier does not exist in ANSI C.

t

As for h, but the next pointer is a pointer to a ptrdiff_t. This modifier was introduced in C99.

z

As for h, but the next pointer is a pointer to a size_t. This modifier was introduced in C99.

%

Matches a literal '%'. That is, %% in the format string matches a single input '%' character. No conversion is done (but initial white space characters are discarded), and assignment does not occur.

d

Matches an optionally signed decimal integer; the next pointer must be a pointer to int.

i

Matches an optionally signed integer; the next pointer must be a pointer to int. The integer is read in base 16 if it begins with 0x or 0X, in base 8 if it begins with 0, and in base 10 otherwise. Only characters that correspond to the base are used.

o

Matches an unsigned octal integer; the next pointer must be a pointer to unsigned int.

u

Matches an unsigned decimal integer; the next pointer must be a pointer to unsigned int.

x

Matches an unsigned hexadecimal integer (that may optionally begin with a prefix of 0x or 0X, which is discarded); the next pointer must be a pointer to unsigned int.

X

Equivalent to x.

f

Matches an optionally signed floating-point number; the next pointer must be a pointer to float.

e

Equivalent to f.

g

Equivalent to f.

E

Equivalent to f.

a

(C99) Equivalent to f.

s

Matches a sequence of non-white-space characters; the next pointer must be a pointer to the initial element of a character array that is long enough to hold the input sequence and the terminating null byte ('\0'), which is added automatically. The input string stops at white space or at the maximum field width, whichever occurs first.

c

Matches a sequence of characters whose length is specified by the maximum field width (default 1); the next pointer must be a pointer to char, and there must be enough room for all the characters (no terminating null byte is added). The usual skip of leading white space is suppressed. To skip white space first, use an explicit space in the format.

[

Matches a nonempty sequence of characters from the specified set of accepted characters; the next pointer must be a pointer to char, and there must be enough room for all the characters in the string, plus a terminating null byte. The usual skip of leading white space is suppressed. The string is to be made up of characters in (or not in) a particular set; the set is defined by the characters between the open bracket [ character and a close bracket ] character. The set excludes those characters if the first character after the open bracket is a circumflex (^). To include a close bracket in the set, make it the first character after the open bracket or the circumflex; any other position will end the set. The hyphen character - is also special; when placed between two other characters, it adds all intervening characters to the set. To include a hyphen, make it the last character before the final close bracket. For instance, [^]0-9-] means the set "everything except close bracket, zero through nine, and hyphen". The string ends with the appearance of a character not in the (or, with a circumflex, in) set or when the field width runs out.

p

Matches a pointer value (as printed by %p in printf(3)); the next pointer must be a pointer to a pointer to void.

n

Nothing is expected; instead, the number of characters consumed thus far from the input is stored through the next pointer, which must be a pointer to int. This is not a conversion and does not increase the count returned by the function. The assignment can be suppressed with the * assignment-suppression character, but the effect on the return value is undefined. Therefore %*n conversions should not be used.

EAGAIN

The file descriptor underlying stream is marked nonblocking, and the read operation would block.

EBADF

The file descriptor underlying stream is invalid, or not open for reading.

EILSEQ

Input byte sequence does not form a valid character.

EINTR

The read operation was interrupted by a signal; see signal(7).

EINVAL

Not enough arguments; or format is NULL.

ENOMEM

Out of memory.

ERANGE

The result of an integer conversion would exceed the size that can be stored in the corresponding integer type.

*

It may also be applied to %c conversion specifiers (e.g., %3mc).

*

It avoids ambiguity with respect to the %a floating-point conversion specifier (and is unaffected by gcc -std=c99 etc.).