Inbuilt library functions for user Input | sscanf, scanf_s, fscanf_s, sscanf_s

The C Programming Language provides various Inbuilt Library Functions for User Input. In this article, we will learn about sscanf, scanf_s, fscanf_s, sscanf_s Library Functions in C.

1. sscanf() Function in C

sscanf() is used to read formatted input from the string. Both scanf() and sscanf() functions are similar, the only difference between them is that scanf() function reads input from the user from standard input like a keyboard, and sscanf() function reads input from a string and stores the input in another string.

Syntax

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

Parameters

• str: Input string from which we want to read data.
• format: A string that contains the type specifier(s).
• … (ellipsis): It indicates that the function accepts a variable number of arguments.

Return Value

• On success, the function returns the number of values successfully read.
• In the case of an input failure, before any data can be successfully read, EOF is returned.

Note: There should be at least as many of these arguments as the number of values stored by the format specifiers.

Example: C Program to Illustrate sscanf Function

3 blue balls 

2. scanf_s() Function in C

This function is specific to Microsoft compilers. It is the same as scanf, except it does not cause buffer overload. scanf_s() function is more secure than scanf() function as it provides an additional parameter to specify the buffer size that can avoid buffer overflow.

Syntax

int scanf_s(const char *format [argument]...);

Parameters

• stream: Pointer to the File object that identifies the stream.
• format: It is a string that contains the type specifier(s).
• … (ellipsis): It indicates that the function accepts a variable number of arguments.

Note: Here we can specify the buffer size and actually control the limit of the input so that the whole application don't crash due to memory overflow.

Return Value

• On success, the function returns the number of values read.
• In the case of an input failure, before any data could be successfully read, EOF is returned.

Why to use scanf_s()?

scanf just reads whatever input is provided from the console. C does not check whether the user input will fit in the variable that you’ve designated. If you have an array called color[3] and you use scanf for the string “Red”, it will work fine but if user enters more than 3 characters scanf starts writing into memory that doesn't belong to colour array.

C won't catch this or warn you and it might or might not crash the program, depending on if something tries to access and write on that memory slot that doesn’t belong to color array. This is where scanf_s comes into play. scanf_s checks that the user input will fit in the given memory space.

Note: scanf_s() will only work in Microsoft Visual Studio.

Example 1: C Program to Illustrate sscanf_s Function

Input 1

Red

Output 1

Red

Input 2

Yellow

Output 2

No Output

Example 2: C Program to Illustrate the relation between buffer size and array size.

• If the buffer size is equal to or smaller than the size of the array, then inputting bigger than or equal to the buffer size will lead to loss of data as the excess data will be truncated and the data till buffer size - 1 will be read.
• If the buffer size is bigger than the size of an array, then
  • inputting smaller than buffer size will work out but will give an error
  • inputting bigger than buffer size will lead to buffer overflow and give the same error.

3. fscanf_s() Function in C

Difference between fscanf() and fscanf_s() is same as that of scanf() and scanf_s(). fscanf_s() is secure function that require the size of each c, C, s, S and [ ] type field to be passed as an argument immediately following the variable.

Syntax

int fscanf_s( FILE *stream, const char *format ,[argument ]... );

Parameters

• str: string used to retrieve data
• format: string that contains the type specifier(s)
• … (ellipsis): It indicates that the function accepts a variable number of arguments.

Note: fscanf_s has an extra parameter to specify the buffer size and actually control the limit of the input.

Return Value

• On success, the function returns the number of values read successfully.
• In the case of an input failure, before any data could be successfully read, EOF is returned.

Note: fscanf_s will work only in MS Visual studio.

Example: C Program to Illustrate fscanf_s Function

Output

String1 |Hello|
String2 |World|
String3 |its|
Integer |2017|

4. sscanf_s() Function in C

sscanf_s() is secure function of sscanf() and secure functions require the size of each c, C, s, S and [ type field to be passed as an argument immediately following the variable.

Syntax

int sscanf_s(const char *restrict buffer, const char *restrict format, ...);

Parameters

• str: string used to retrieve data
• format: string that contains the type specifier(s)
• … (ellipsis): It indicates that the function accepts a variable number of arguments.

Note: sscanf_s has an extra parameter to specify the buffer size and actually control the limit of the input.

Return Value

• On success, the function returns the number of values read successfully.
• In the case of an input failure, before any data could be successfully read, EOF is returned.

Note: sscanf_s() will only work in Microsoft Visual Studio.

Example: C Program to Illustrate sscanf_s Function

Output

3 blue balls