What is the difference between `%d` and `%i` when used as format specifiers in `printf` and `scanf`? 

What is the difference between conversion specifiers %i and %d in formatted IO functions (*printf / *scanf)

Why is it that `scanf()` needs the `l` in &quot;`%lf`&quot; when reading a `double`, when `printf()` can use &quot;`%f`&quot; regardless of whether its argument is a `double` or a `float`?

Example code:

    double d;
    scanf(&quot;%lf&quot;, &amp;d);
    printf(&quot;%f&quot;, d);


Why does scanf() need &quot;%lf&quot; for doubles, when printf() is okay with just &quot;%f&quot;?

Using the following code:

    char *name = malloc(sizeof(char) + 256); 

	printf(&quot;What is your name? &quot;);
	scanf(&quot;%s&quot;, name);
	
	printf(&quot;Hello %s. Nice to meet you.\n&quot;, name);

A user can enter their name but when they enter a name with a space like `Lucas Aardvark`, `scanf()` just cuts off everything after `Lucas`. How do I make `scanf()` allow spaces

How do you allow spaces to be entered using scanf?

I&#39;m a little bit confused about something. I was under the impression that the correct way of reading a C string with `scanf()` went along the lines of

(never mind the possible buffer overflow, it&#39;s just a simple example)

    char string[256];
    scanf( &quot;%s&quot; , string );

However, the following seems to work too,

    scanf( &quot;%s&quot; , &amp;string );

Is this just my compiler (gcc), pure luck, or something else?

Why does reading into a string buffer with scanf work both with and without the ampersand (&amp;)?

I have the following program 

    #include &lt;stdio.h&gt;
    
    int main(void)
    {
        unsigned short int length = 10; 
                
        printf(&quot;Enter length : &quot;);
        scanf(&quot;%u&quot;, &amp;length);
    
        printf(&quot;value is %u \n&quot;, length);
    
        return 0;
    }

Which when compiled using `gcc filename.c` issued the following warning (in the `scanf()` line). 

`warning: format ‘%u’ expects argument of type ‘unsigned int *’, but argument 2 has type ‘short unsigned int *’ [-Wformat]`

I then referred the `C99 specification - 7.19.6 Formatted input/output functions` and couldn&#39;t understand the correct format specifier when using the length modifiers (like `short`, `long`, etc) with `unsigned` for `int` data type.

Is `%u` the correct specifier `unsigned short int`? If so why am I getting the above mentioned warning?! 

EDIT: 
Most of the time, I was trying `%uh` and it was still giving the warning. 

What is the format specifier for unsigned short int?

I have very frequently seen people discouraging others from using `scanf` and saying that there are better alternatives. However, all I end up seeing is either *&quot;don&#39;t use `scanf`&quot;* or *&quot;here&#39;s a correct format string&quot;*, and never any examples of the *&quot;better alternatives&quot;* mentioned.

For example, let&#39;s take this snippet of code:

    scanf(&quot;%c&quot;, &amp;c);

This reads the whitespace that was left in the input stream after the last conversion. The usual suggested solution to this is to use:

    scanf(&quot; %c&quot;, &amp;c);

or to not use `scanf`.

Since `scanf` is bad, what are some ANSI C options for converting input formats that `scanf` can usually handle (such as integers, floating-point numbers, and strings) without using `scanf`?

What can I use for input conversion instead of scanf?

In C:
I&#39;m trying to get char from the user with `scanf` and when I run it the program don&#39;t wait for the user to type anything...

This is the code:

	char ch;
	printf(&quot;Enter one char&quot;);
	scanf(&quot;%c&quot;, &amp;ch);
	printf(&quot;%c\n&quot;,ch);

Why is not working?

How to do scanf for single char in C

I have the following program:

    int main(int argc, char *argv[])
    {
      int a, b;
      char c1, c2;
      printf(&quot;Enter something: &quot;);
      scanf(&quot;%d&quot;,&amp;a); // line 1
      printf(&quot;Enter other something: &quot;);
      scanf(&quot;%d&quot;, &amp;b); // line 2

      printf(&quot;Enter a char: &quot;);
      scanf(&quot;%c&quot;,&amp;c1); // line 3
      printf(&quot;Enter another char: &quot;);
      scanf(&quot;%c&quot;, &amp;c2); // line 4

      printf(&quot;Done&quot;); // line 5

      system(&quot;PAUSE&quot;);

      return 0;
    }

As I read in the C book, the author says that `scanf()` left a newline character in the buffer, therefore, the program does not stop at line 4 for user to enter the data, rather it stores the new line character in c2 and moves to line 5.

Is that right?

However, does this only happen with `char` data types? Because I did not see this problem with `int` data types as in line 1, 2, 3. Is it right?


scanf() leaves the newline character in the buffer

I&#39;m using Ubuntu and I&#39;m also using Geany and CodeBlock as my IDE.
What I&#39;m trying to do is reading a string (like `&quot;Barack Obama&quot;`) and put it in a variable:

&lt;!-- language: c --&gt;

    #include &lt;stdio.h&gt;
    
    int main(void)
    {
    	char name[100];

    	printf(&quot;Enter your name: &quot;);
    	scanf(&quot;%s&quot;, name);
    	printf(&quot;Your Name is: %s&quot;, name);

    	return 0;
    }

Output:

&lt;!-- language: none --&gt;

    Enter your name: Barack Obama
    Your Name is: Barack

How can I make the program read the whole name?

Reading string from input with space character?


I use this code:

    while ( scanf(&quot;%s&quot;, buf) == 1 ){

What would be the best way to prevent possible buffer overflow so that it can be passed strings of random lengths? 

I know I can limit the input string by calling for example:

    while ( scanf(&quot;%20s&quot;, buf) == 1 ){

But I&#39;d prefer to be able to process whatever the user inputs.
Or can&#39;t this be done safely using scanf and I should use fgets?


How to prevent scanf causing a buffer overflow in C?

If I don&#39;t know how long the word is, I cannot write `char m[6];`,  
The length of the word is maybe ten or twenty long.
How can I use `scanf` to get input from the keyboard?

    #include &lt;stdio.h&gt;
    int main(void)
    {
        char  m[6];
        printf(&quot;please input a string with length=5\n&quot;);
        scanf(&quot;%s&quot;,&amp;m);
        printf(&quot;this is the string: %s\n&quot;, m);
        return 0;
    }


please input a string with length=5  
input: hello  
this is the string: hello  


How can I read an input string of unknown length?

I want to know the disadvantages of `scanf()`.

In many sites, I have read that using `scanf` might cause buffer overflows. What is the reason for this? Are there any other drawbacks with `scanf`?

Disadvantages of scanf

I&#39;m looking for an equivalent to `sscanf()` in Python. I want to parse `/proc/net/*` files, in C I could do something like this:

    int matches = sscanf(
            buffer,
            &quot;%*d: %64[0-9A-Fa-f]:%X %64[0-9A-Fa-f]:%X %*X %*X:%*X %*X:%*X %*X %*d %*d %ld %*512s\n&quot;,
            local_addr, &amp;local_port, rem_addr, &amp;rem_port, &amp;inode);

I thought at first to use `str.split`, however it doesn&#39;t split on the given characters, but the `sep` string as a whole:

    &gt;&gt;&gt; lines = open(&quot;/proc/net/dev&quot;).readlines()
    &gt;&gt;&gt; for l in lines[2:]:
    &gt;&gt;&gt;     cols = l.split(string.whitespace + &quot;:&quot;)
    &gt;&gt;&gt;     print len(cols)
    1

Which should be returning 17, as explained above.

Is there a Python equivalent to `sscanf` (not RE), or a string splitting function in the standard library that splits on any of a range of characters that I&#39;m not aware of?

sscanf in Python

gcc 4.4.4 c89

What is better to convert a string to an integer value.

I have tried 2 different methods atoi and sscanf. Both work as expected.

    char digits[3] = &quot;34&quot;;
    int device_num = 0;
    
    if(sscanf(digits, &quot;%d&quot;, &amp;device_num) == EOF) {
        fprintf(stderr, &quot;WARNING: Incorrect value for device\n&quot;);
        return FALSE;
    }

or using atoi

    device_num = atoi(digits);

I was thinking that the sscanf would be better as you can check for errors. However, atoi doesn&#39;t doing any checking.


What is the difference between sscanf or atoi to convert a string to an integer?

I have been working on a small exercise for my CIS class and am very confused by the methods C uses to read from a file. All that I really need to do is read through a file line by line and use the information gathered from each line to do a few manipulations. I tried using the getline method and others with no luck. 
My code is currently as follows:
   

    int main(char *argc, char* argv[]){
          const char *filename = argv[0];
          FILE *file = fopen(filename, &quot;r&quot;);
          char *line = NULL;
        
          while(!feof(file)){
            sscanf(line, filename, &quot;%s&quot;);
            printf(&quot;%s\n&quot;, line);
          }
        return 1;
    }
Right now I am getting a seg fault with the sscanf method and I am not sure why. I am a total C noob and just wondering if there was some big picture thing that I was missing. 
Thanks

Going through a text file line by line in C

As you can see from the code snippet below, I have declared one `char` variable and one `int` variable. When the code gets compiled, it must identify the data types of variables `str` and `i`.

Why do I need to tell again during scanning my variable that it&#39;s a string or integer variable by specifying `%s` or `%d` to `scanf`? Isn&#39;t the compiler mature enough to identify that when I declared my variables?

    #include &lt;stdio.h&gt;
    
    int main ()
    {
      char str [80];
      int i;
    
      printf (&quot;Enter your family name: &quot;);
      scanf (&quot;%s&quot;,str);  
      printf (&quot;Enter your age: &quot;);
      scanf (&quot;%d&quot;,&amp;i);
      
      return 0;
    }

Why do I have to specify data type each time in C to printf() and scanf()?

I want to read a string entered by the user. I don&#39;t know the length of the string. As there are no strings in C I declared a pointer:

    char * word;

and used `scanf` to read input from the keyboard:

    scanf(&quot;%s&quot; , word) ;

but I got a segmentation fault.

How can I read input from the keyboard in C when the length is unknown ?

How to read string from keyboard using C?

I try to read-in 2 values using scanf() in C, but the values the system writes into memory are not equal to my entered values. Here is the code:

    double a,b;
    printf(&quot;--------\n&quot;); //seperate lines
    scanf(&quot;%ld&quot;,&amp;a);
    printf(&quot;--------\n&quot;); 
    scanf(&quot;%ld&quot;,&amp;b);
    printf(&quot;%d %d&quot;,a,b);

If I enter 1 and 2, CMD returns a correct, but b = -858993460 Here is what I already tried:
using float or int instead of double, using scanf_s, using scanf(&quot;%d or %f for %i or %li or %lf or %e or %g ), using fflush(stdin) to clear keyboard buffer, reading in b first, trying like all possible combinations. I found out that there is a problem with the length of double on 32 bit OS, so that you are forced to use scanf(&quot;%lf&quot;, &amp;f) to read in a double. No matter what I do, second value is always wrong.

I use MS VS express 2012 for Desktop on Windows 7 32 bit OS.

Reading in double values with scanf in c

If I have an integer variable I can use `sscanf` as shown below by using the format specifier %d.

    sscanf (line, &quot;Value of integer: %d\n&quot;, &amp;my_integer);

Where can I find format specifiers for `uint8_t`, `uint16_t`, `uint32_t` and `uint64_t`?

uint64_t has probably %lu.

Format specifiers for uint8_t, uint16_t, ...?

I&#39;ve a small C-program which just reads numbers from stdin, one at each loop cycle. If the user inputs some NaN, an error should be printed to the console and the input prompt should return again. On input of &quot;0&quot;, the loop should end and the number of given positive/negative values should be printed to the console. Here&#39;s the program:

    #include &lt;stdio.h&gt;
    
    int main()
    {
        int number, p = 0, n = 0;
    
        while (1) {
            printf(&quot;-&gt; &quot;);
            if (scanf(&quot;%d&quot;, &amp;number) == 0) {
                printf(&quot;Err...\n&quot;);
                continue;
            }
            
            if (number &gt; 0) p++;
            else if (number &lt; 0) n++;
            else break; /* 0 given */
        }

        printf(&quot;Read %d positive and %d negative numbers\n&quot;, p, n);
        return 0;
    }

My problem is, that on entering some non-number (like &quot;a&quot;), this results in an infinite loop writing &quot;-&gt; Err...&quot; over and over. I guess it&#39;s a scanf() issue and I know this function could be replace by a safer one, but this example is for beginners, knowing just about printf/scanf, if-else and loops.

I&#39;ve already read the answers to the question[`scanf()` skips every other `while` loop in C][1] and skimmed through other questions, but nothing really answer this specific problem.

  [1]: https://stackoverflow.com/questions/1669821/scanf-skips-every-other-while-loop-in-c

Why is scanf() causing infinite loop in this code?

When `sscanf()` or another function from the `scanf` family is given a sequence of digits whose converted value exceeds the maximum value of the target integer type,  

* should the conversion be considered to have failed? 
* is the behavior defined at all?

 

All Scanf Solutions on Gang of Coders

Total of 21 Scanf Solutions

What is the difference between conversion specifiers %i and %d in formatted IO functions (printf / scanf)

Why does scanf() need "%lf" for doubles, when printf() is okay with just "%f"?