Scanf format string


A scanf format string is a control parameter used in various functions to specify the layout of an input string. The functions can then divide the string and translate into values of appropriate data types. String scanning functions are often supplied in standard libraries.
The term "scanf" comes from the C library, which popularized this type of function, but such functions predate C, and other names are used, such as readf in ALGOL 68. scanf format strings, which provide formatted input, are complementary to printf format strings, which provide formatted output. These provide simple functionality and fixed format compared to more sophisticated and flexible parsers or template engines, but are sufficient for many purposes.

History

's portable input/output library, including scanf, officially became part of Unix in Version 7.

Usage

The scanf function, which is found in C, reads input for numbers and other datatypes from standard input.
The following C code reads a variable number of unformatted decimal integers from the standard input stream and prints each of them out on separate lines:

  1. include
int main

After being processed by the program above, an irregularly spaced list of integers such as
456 123 789 456 12
456 1
2378
will appear consistently spaced as:
456
123
789
456
12
456
1
2378
To print out a word:

  1. include
int main

No matter what the data type the programmer wants the program to read, the arguments 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 variable you are attempting to get input for.
In the last example an address-of operator is not used for the argument: as word is the name of an array of char, as such it is equivalent to a pointer to the first element of the array. While the expression &word would numerically evaluate to the same value, semantically, it has an entirely different meaning in that it stands for the address of the whole array rather than an element of it. This fact needs to be kept in mind when assigning scanf output to strings.
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.

Format string specifications

The formatting placeholders in scanf are more or less the same as that in printf, its reverse function. As in printf, the POSIX extension is defined.
There are rarely constants in a format string, mainly because a program is usually not designed to read known data, although scanf does accept these if explicitly specified. The exception is one or more whitespace characters, which discards all whitespace characters in the input.
Some of the most commonly used placeholders follow:
The above can be used in compound with numeric modifiers and the l, L modifiers which stand for "long" and "long 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 ff modifier in printf is not present in scanf, causing differences between modes of input and output. The ll and hh modifiers are not present in the C90 standard, but are present in the C99 standard.
An example of a format string is
The above format string scans the first seven characters as a decimal integer, then reads the remaining as a string until a space, newline, or tab is found, then consumes whitespace until the first non-whitespace character is found, then consumes that character, and finally scans the remaining characters as a double. 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 discarded before new input can be read. An alternative method, which avoids this, is to use fgets and then examine the string read in. The last step can be done by sscanf, for example.
In the case of the many float type characters, many implementations choose to collapse most into the same parser. Microsoft MSVCRT does it with, while glibc does so with all four.

Vulnerabilities

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 and cannot 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 partially extracted from the stack, contain undesirable or even insecure pointers depending on the particular implementation of varargs.