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 size modifier h, l (ell), ll (ell ell), or L indicating the size of the receiving object. The conversion characters d, i, and n must be preceded by h if the corresponding argument is a pointer to short int rather than a pointer to int, by l (ell) if it is a pointer to long int, or by ll (ell ell) if it is a pointer to long long int. Similarly, the conversion characters o, u, and x must be preceded by h if the corresponding argument is a pointer to unsigned short int rather than a pointer to unsigned int, by l (ell) if it is a pointer to unsigned long int, or by ll (ell ell) if it is a pointer to unsigned long long int. The conversion characters e, f, and g must be preceded by l (ell) if the corresponding argument is a pointer to double rather than a pointer to float, or by L if it is a pointer to long double. Finally, the conversion characters c, s, and [ must be precede by l (ell) if the corresponding argument is a pointer to wchar_t rather than a pointer to a character type. If an h, l (ell), ll (ell ell), or L appears with any other conversion character, the behavior is undefined.
• A conversion character that specifies the type of conversion to be applied. The valid conversion characters are described below.

The scanf() functions execute 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.

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(3C)) are skipped, unless the conversion specification includes a [, c, C, or n conversion character.

An item is read from the input, unless the conversion specification includes an n conversion character. 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 character) 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 character, 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 following conversion characters are valid:

d

Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of strtol(3C) with the value 10 for the base argument. In the absence of a size modifier, the corresponding argument must be 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 corresponding argument must be a pointer to int.

o

Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of strtoul(3C) with the value 8 for the base argument. In the absence of a size modifier, the corresponding argument must be a pointer to unsigned int.

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 corresponding argument must be a pointer to unsigned int.

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 corresponding argument must be a pointer to unsigned int.

e,f,g

Matches an optionally signed floating-point number, whose format is the same as expected for the subject sequence of strtod(3C). In the absence of a size modifier, the corresponding argument must be a pointer to float.

If the printf(3C) family of functions generates character string representations for infinity and NaN (a 7858 symbolic entity encoded in floating-point format) to support the ANSI/IEEE Std 754: 1985 standard, the scanf() family of functions will recognize them as input.

s

Matches a sequence of bytes that are not white-space characters. The corresponding argument must be 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 will be added automatically.

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(3C) function, with the conversion state described by an mbstate_t object initialized to zero before the first character is converted. The corresponding argument must be a pointer to an array of wchar_t large enough to accept the sequence and the terminating null wide-character, which will be added automatically.

[

Matches a non-empty sequence of characters from a set of expected characters (the scanset). The normal skip over white-space characters is suppressed in this case. The corresponding argument must be 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 will be added automatically.

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. The corresponding argument must be a pointer to an array of wchar_t large enough to accept the sequence and the terminating null wide-character, which will be added automatically.

The conversion specification includes all subsequent characters in the format string up to and including the matching right square bracket (]). The characters between the square brackets (the scanlist) comprise the scanset, unless the character after the left square bracket is a circumflex (^), in which case the scanset contains all characters 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, it indicates a range of characters to be matched.

c

Matches a sequence of characters of the number specified by the field width (1 if no field width is present in the conversion specification). The corresponding argument must be a pointer to the initial byte of an array of char, signed char, or unsigned char large enough to accept the sequence. No null byte is added. The normal skip over white-space characters is suppressed in this case.

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. The corresponding argument must be a pointer to an array of wchar_t large enough to accept the resulting sequence of wide-characters. No null wide-character is added.

p

Matches the set of sequences that is the same as the set of sequences that is produced by the %p conversion of the corresponding printf(3C) functions. The corresponding argument must be a pointer to a pointer to void. If the input item is a value converted earlier during the same program execution, the pointer that results will compare equal to that value; otherwise the behavior of the %p conversion is undefined.

n

No input is consumed. The corresponding argument must be a pointer to the integer into which is to be written the number of bytes read from the input so far by this call to the scanf() functions. Execution of a %n conversion specification does not increment the assignment count returned at the completion of execution of the function.

C

Same as lc.

S

Same as ls.

%

Matches a single %; no conversion or assignment occurs. The complete conversion specification must be %%.

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

The conversion characters E, G, and X are also valid and behave the same as, respectively, e, g, and x.

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 newline 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 st_atime field of the file associated with stream for update. The st_atime field will be marked for update by the first successful execution of fgetc(3C), fgets(3C), fread(3C), fscanf(), getc(3C), getchar(3C), gets(3C), or scanf() using stream that returns data not supplied by a prior call to ungetc(3C).

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

will assign to n the value 3, to i the value 25, to x the value 5.432, and name will contain the string Hamster.

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

with input:

56789 0123 56a72

will assign 56 to i, 789.0 to x, skip 0123, and place the string 56\0 in name. The next call to getchar(3C) will return the character a.