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scanf

Summary
#include <stdio.h>

int scanf (
  const char *fmtstr    /* format string */
  <[>, argument<]>...);   /* additional arguments */
Description

The scanf function reads data from the input stream using the getchar routine. Data input are stored in the locations specified by argument according to the format string fmtstr. Each argument must be a pointer to a variable that corresponds to the type defined in fmtstr. The type controls the interpretation of the input data. The fmtstr may be composed of one or more whitespace characters, non-whitespace characters, and format specifications.

  • Whitespace characters, blank (' '), tab ('\t'), or newline ('\n'), cause scanf to skip whitespace characters in the input stream. A single whitespace character in the format string matches 0 or more whitespace characters in the input stream.
  • Non-whitespace characters, with the exception of the percent sign ('%'), cause scanf to read but not store a matching character from the input stream. The scanf function terminates if the next character in the input stream does not match the specified non-whitespace character.
  • Format specifications begin with a percent sign ('%') and cause scanf to read and convert characters from the input stream to the specified type values. The converted value is stored to an argument from the parameter list. Characters following a percent sign that are not recognized as a format specification are treated as ordinary characters. For example, %% matches a single percent sign in the input stream.

The format string is read from left to right. Characters that are not part of the format specifications must match characters in the input stream. These characters are read from the input stream but are discarded and not stored. If a character in the input stream conflicts with the format string, scanf terminates. Any conflicting characters remain in the input stream.

The first format specification encountered in the format string references the first argument after fmtstr. The scanf function converts input characters and stores the value using the format specification. The second format specification accesses the second argument after fmtstr, and so on. If there are more arguments than format specifications, the extra arguments are ignored. Results are unpredictable if there are not enough arguments for the format specifications.

Values in the input stream are called input fields and are delimited by whitespace characters. When converting input fields, scanf ends a conversion for an argument when a whitespace character or another unrecognized character is encountered.

Format specifications have the following format:

% <[>*<]> <[>width<]> <[>{b|h|l}<]> type

Each field in the format specification can be a single character or a number which specifies a particular format option.

The type field is where a single character specifies whether input characters are interpreted as a character, string, or number. This field can be any one of the characters in the following table.

Character Argument Type Input Format
d int * Signed decimal number.
i int * Signed decimal, hexadecimal, or octal integer.
u unsigned int * Unsigned decimal number.
o unsigned int * Unsigned octal number.
x unsigned int * Unsigned hexadecimal number.
e float * Floating-point number.
f float * Floating-point number.
g float * Floating-point number.
c char * A single character.
s char * A string of characters terminated by whitespace.

An asterisk ('*') as the first character of a format specification causes the input field to be scanned but not stored. The asterisk suppresses assignment of the format specification.

The width field is a non-negative number that specifies the maximum number of characters read from the input stream. No more than width characters are read and converted for the corresponding argument. However, fewer than width characters may be read if a whitespace or other unrecognized character is encountered first.

The optional characters b, h, and l may immediately precede the type character to respectively specify char, short, or long versions of the integer types d, i, u, o, and x. If l is applied to the floating-point types f, e, and g, a pointer to a double is required as an argument.

Note

  • This function is implementation-specific and is based on the operation of the _getkey and putchar functions. These functions, as provided in the standard library, read and write characters using the microcontroller's serial port. Custom functions may use other I/O devices.
Return Value

The scanf function returns the number of input fields that were successfully converted. An EOF is returned if an error is encountered.

See Also

gets, printf, putchar, puts, sprintf, sscanf, vprintf, vsprintf

Example
#include <stdio.h>

void tst_scanf (void) {
  char a;
  int b;
  long c;

  unsigned char x;
  unsigned int y;
  unsigned long z;

  float f,g;
  char d, buf [10];
  int argsread;

  printf ("Enter a signed byte, int, and long\n");
  argsread = scanf ("%bd %d %ld", &a, &b, &c);
  printf ("%d arguments read\n", argsread);

  printf ("Enter an unsigned byte, int, and long\n");
  argsread = scanf ("%bu %u %lu", &x, &y, &z);
  printf ("%d arguments read\n", argsread);

  printf ("Enter a character and a string\n");
  argsread = scanf ("%c %9s", &d, buf);
  printf ("%d arguments read\n", argsread);

  printf ("Enter two floating-point numbers\n");
  argsread = scanf ("%f %f", &f, &g);
  printf ("%d arguments read\n", argsread);
}
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