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Scanf format string which stands for " scan f ormatted" refers to a control parameter used by a class of functions in the string-processing libraries of various programming languages. The format string specifies a method for reading a string into an arbitrary number of varied data type parameter s. The input string is by default read from the standard input , but variants exist that read the input from other sources. The term "scanf" is due to the C language, which popularized this type of function, but these functions predate C, and other names are used, such as "readf" in ALGOL Scanf format strings, which provide formatted input parsing , are complementary to printf format strings , which provide formatted output templating.
In both cases these provide simple functionality and fixed format compared to more sophisticated and flexible parsers or template engines, but are sufficient for many purposes. The scanf function, which is found in C , reads input for numbers and other datatypes from standard input often a command line interface or similar kind of a text user interface. The following shows code in C that reads a variable number of unformatted decimal integers from the standard input stream and prints out each of them on a separate line:.
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. 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.
The formatting placeholders in scanf are more or less the same as that in printf , its reverse function. There are rarely constants i. The exception is one or more whitespace characters, which discards all whitespace characters in the input. 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.
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.
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 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.
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. 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.
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