scanf

scanf is a function that reads data with specified format from a given string stream source, originated from C programming language, and is present in many other programming languages.

The scanf function prototype is:

int scanf (char *format, ...);

The function returns the total number of items successfully matched, which can be less than the number requested. If the input stream is exhausted or reading from it otherwise fails before any items are matched, EOF is returned.

So far as is traceable, "scanf" stands for "scan format", because it scans the input for valid tokens and parses them according to a specified format.

Usage

The scanf function is found in C, in which it reads input for numbers and other datatypes from standard input (often a console or command prompt).

The following shows code in C that reads a variable number of unformatted decimal integers from the console and prints out each of them on a separate line:

#include <stdio.h>

int main(void) {
  int n;
  while (scanf("%d", &n) == 1)
    printf ("%d\n", n);
  return 0;
}

After being processed by the program above, a messy list of integers such as

456 123 789     456 12
456 1
      2378

will appear neatly as:

To print out a line:

 #include <stdio.h>
 
 int main(void) {
   char word[20];
   scanf("%19s", word);
   printf("%s\n", word);
   return 0;
 }

No matter what the datatype the programmer wants the program to read, the arguments (such as &n above) must be pointers pointing to memory. Otherwise, the function will not perform correctly because it will be attempting to overwrite the wrong sections of memory, rather than pointing to the memory location of the varaible you are attempting to get input for.

As scanf is designated to read only from standard input, many programming languages with interfaces, such as PHP, have derivatives such as sscanf and fscanf but not scanf itself.

Derivatives of scanf

Depending on the actual source of input, programmers can utilize different derivatives of scanf. Two common examples are sscanf and fscanf.

sscanf

The sscanf derivative reads input from a user-specified string source instead of the console (known as standard input, stdin). It has prototypes as follows:

(C or C++)

int sscanf (const char *source, const char *format, ...);

(PHP)

int sscanf (const string source, const string format [, mixed args...])

Unlike scanf and fscanf, sscanf does not remove the read items away from the input flush. Therefore, every time the function is executed, the string source is read again from the very beginning.

fscanf

The fscanf derivative reads input from a specified file stream. The prototypes are as follows:

(C or C++)

int fscanf (FILE *file, const char *format, ...);

(PHP)

int fscanf (resource file, const string format [, mixed args...])

The fscanf derivative works like the original scanf function - parts of the input, once read, will not be read again until the file is closed and reopened.

vscanf, vsscanf, and vfscanf

int vscanf (const char *format, va_list args);
int vsscanf (const char *source, const char *format, va_list args);
int vfscanf (FILE *file, const char *format, va_list args);

These are like the same functions without the v prefix, except they take their arguments from a va_list. (See stdarg.h.) These variants may be used in variable-argument functions.

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 octal when preceded by "0". For example, the string "031" would be read as 31 using '%d', and 25 using '%i'.
'%u' : Scan for decimal unsigned int, unsigned short, and unsigned char (Note that in the C99 standard the input value "-" sign is optional, so if a negative number is read, no errors will arise and the result will be the two's complement. See strtoul()).
'%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 a hexadecimal number. '%x' uses lower-case letters and '%X' uses upper-case.
'%o' : Scan an integer as an octal number.
'%s' : Scan a character string. The scan terminates at whitespace. Without a length modifier, this conversion specifier is dangerous because it can lead to buffer overflows. A null character is stored at the end of the string.
'%c' : Scan a character (char). No null character is added.
'(space)': Space scans for whitespace characters.

The above can be used in compound with numeric modifiers and the 'l', 'L' modifiers which stand for "long" in between the percent symbol and the letter. There can also be numeric values between the percent symbol and the letters, preceding the "long" modifiers if any, that specifies the number of characters to be scanned. An optional asterisk (*) right after the percent symbol denotes that the datum read by this format specifier is not to be stored in a variable. No argument behind the format string should be included for this dropped variable.

The "ll" and "ff" modifiers in printf are not present in scanf, causing differences between modes of input and output.

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. The last step can be done by sscanf, for example.

Security

Like printf, scanf is vulnerable to format string attacks. Great care should be taken to ensure that the formatting string includes limitations for string and array sizes. In most cases the input string size from a user is arbitrary, it can not be determined before the scanf function is executed. This means that uses of '%s' placeholders without length specifiers are inherently insecure and exploitable for buffer overflows. Another potential problem is to allow dynamic formatting strings, for example formatting strings stored in configuration files or other user controlled files. In this case the allowed input length of string sizes can not be specified unless the formatting string is checked beforehand and limitations are enforced. Related to this are additional or mismatched formatting placeholders which do not match the actual vararg list. These placeholders might be partically extracted from the stack, contain undesirable or even insecure pointers depending on the particular implementation of varargs.