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scanf Width Specification

width is a positive decimal integer controlling the maximum number of characters to be read from stdin. No more than width characters are converted and stored at the corresponding argument. Fewer than width characters may be read if a white-space character (space, tab, or newline) or a character that cannot be converted according to the given format occurs before width is reached.

The optional prefixes h, l, I64, and L indicate the size of the argument (long or short, single-byte character or wide character, depending upon the type character that they modify). These format-specification characters are used with type characters in scanf or wscanf functions to specify interpretation of arguments as shown in the following table. The type prefixes h, l, I64, and L are Microsoft extensions and are not ANSI compatible. The type characters and their meanings are described in the Type Characters for scanf functions table.

Note   The h, l, and L prefixes are Microsoft extensions when used with data of type char.

Size Prefixes for scanf and wscanf Format-Type Specifiers

To specify Use prefix With type specifier
double l e, E, f, g, or G
long double (same as double) L e, E, f, g, or G
long int l d, i, o, x, or X
long unsigned int l u
short int h d, i, o, x, or X
short unsigned int h u
__int64 I64 d, i, o, u, x, or X
Single-byte character with scanf h c or C
Single-byte character with wscanf h c or C
Wide character with scanf l c or C
Wide character with wscanf l c, or C
Single-byte – character string with scanf h s or S
Single-byte – character string with wscanf h s or S
Wide-character string with scanf l s or S
Wide-character string with wscanf l s or S

The following examples use h and l with scanf functions and wscanf functions:

scanf( "%ls", &x );     // Read a wide-character string
wscanf( "%lC", &x );    // Read a single-byte character

To read strings not delimited by space characters, a set of characters in brackets ([ ]) can be substituted for the s (string) type character. The corresponding input field is read up to the first character that does not appear in the bracketed character set. If the first character in the set is a caret (^), the effect is reversed: The input field is read up to the first character that does appear in the rest of the character set.

Note that %[a-z] and %[z-a] are interpreted as equivalent to %[abcde...z]. This is a common scanf function extension, but note that the ANSI standard does not require it.

To store a string without storing a terminating null character ('\0'), use the specification %nc where n is a decimal integer. In this case, the c type character indicates that the argument is a pointer to a character array. The next n characters are read from the input stream into the specified location, and no null character ('\0') is appended. If n is not specified, its default value is 1.

The scanf function scans each input field, character by character. It may stop reading a particular input field before it reaches a space character for a variety of reasons:

  • The specified width has been reached.
  • The next character cannot be converted as specified.
  • The next character conflicts with a character in the control string that it is supposed to match.
  • The next character fails to appear in a given character set.

For whatever reason, when the scanf function stops reading an input field, the next input field is considered to begin at the first unread character. The conflicting character, if there is one, is considered unread and is the first character of the next input field or the first character in subsequent read operations on stdin.

See Also

scanf, wscanf | Run-Time Routines and .NET Framework Equivalents

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