Format string specifications

The formatting placeholders in scanf are more or less the same as that in printf, its reverse function.

There are rarely constants (i.e. characters that are not formatting placeholders) in a format string, mainly because a program is usually not designed to read known data. The exception is one or more whitespace characters, which discards all whitespace characters in the input.

Some of the most commonly used placeholders follow:

• %d : Scan an integer as a signed decimal number.
• %i : Scan an integer as a signed number. Similar to %d, but interprets the number as hexadecimal when preceded by 0x and octalwhen preceded by 0. For example, the string 031 would be read as 31 using %d, and 25 using %i. The flag h in %hi indicates conversion to a short and hh conversion to a char.
• %u : Scan for decimal unsigned int (Note that in the C99 standard the input value minus sign is optional, so if a minus sign is read, no errors will arise and the result will be the two's complement of a negative number, likely a very large value. See strtoul().) Correspondingly, %hu scans for an unsigned short and %hhu for an unsigned char.
• %f : Scan a floating-point number in normal (fixed-point) notation.
• %g, %G : Scan a floating-point number in either normal or exponential notation. %g uses lower-case letters and %G uses upper-case.
• %x, %X : Scan an integer as an unsigned hexadecimal number.
• %o : Scan an integer as an octal number.
• %s : Scan a character string. The scan terminates at whitespace. A null character is stored at the end of the string, which means that the buffer supplied must be at least one character longer than the specified input length.
• %c : Scan a character (char). No null character is added.
• whitespace: Any whitespace characters trigger a scan for zero or more whitespace characters. The number and type of whitespace characters do not need to match in either direction.
• %lf : Scan as a double floating-point number.
• %Lf : Scan as a long double floating-point number.

An example of a format string is

"%7d%s %c%lf"

The above format string scans the first seven characters as a decimal integer, then reads the remaining as a string until a space, new line or tab is found, then scans the first non-whitespace character following and a double-precision floating-point number afterwards.

Error handling

scanf is usually used in situations when the program cannot guarantee that the input is in the expected format. Therefore a robust program must check whether the scanf call succeeded and take appropriate action. If the input was not in the correct format, the erroneous data will still be on the input stream and must be read and discarded before new input can be read. An alternative method of reading input, which avoids this, is to use fgets and then examine the string read in.

Whitespace character

The most common whitespace characters may be typed via the space bar or the tab key. Depending on context, a line-break generated by the return or enter key may be considered white space as well.

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