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macOS sscanf returns nothing

M

MrFusion

macrumors 6502a
Original poster
Jun 8, 2005
613
0
West-Europe
I found this code fragment with google, but the output is "Input [__]".
input_buf contains "test\n" according to the debugger, which is what I entered.
Any idea why this isn't working?

My goal is to ask the user a question (via the command line) and evaluate the response.
Thanks

Code: 
	char input_buf[64] = {0};
	char data[64] = {0};
	
	printf("Enter something: ");
	while( fgets(input_buf, sizeof(input_buf), stdin) == NULL )
	{
		/* parse the input entered */
		sscanf(input_buf, "%s", data);
	}
	
	printf("Input [_%s_]\n", data);
 
MrFusion said:
Code: 
	char input_buf[64] = {0};
	char data[64] = {0};
	
	printf("Enter something: ");
	while( fgets(input_buf, sizeof(input_buf), stdin) [COLOR="Red"]== NULL[/COLOR] )
	{
		/* parse the input entered */
		sscanf(input_buf, "%s", data);
	}
	
	printf("Input [_%s_]\n", data);
Click to expand...

Where exactly did you find that code? I want to know what URL to avoid.

Look carefully at the red-hilited piece. Under what circumstances does fgets() return NULL? What does it return under other circumstances? Which of those do you want to loop under?
 
chown33 said:
Where exactly did you find that code? I want to know what URL to avoid.

Look carefully at the red-hilited piece. Under what circumstances does fgets() return NULL? What does it return under other circumstances? Which of those do you want to loop under?
Click to expand...
It's as though the program is telling the computer to keep parsing when there's nothing to parse. If the computer reads a nonempty string, it won't execute the sscanf.

Why would anyone put the integer zero, not the numeral 0, into a string? To put the null character into the array, I need to type '\0', not a zero.
 
Bill McEnaney said:
It's as though the program is telling the computer to keep parsing when there's nothing to parse. If the computer reads a nonempty string, it won't execute the sscanf.

Why would anyone put the integer zero, not the numeral 0, into a string? To put the null character into the array, I need to type '\0', not a zero.
Click to expand...

That just initialises the array to all 0 so that you know it doesn't contain garbage. Unless of course I have misunderstood which bit you are talking about :p, which is quite likely at the moment.
 
chown33 said:
Which of those do you want to loop under?
Click to expand...

The loop doesn't make sense to me even if you do fix it. Isn't this the kind of construct you might use to read a multi-line file into a buffer since it will exit on eof (once "fixed")? You want to exit the loop once you've got good input, right? Not keep going replacing each line until you error out.

Obviously I'm missing something.

EDIT: (Also the use of fgets just to turn around and use sscanf to copy it to another array just seems wrong. I too would like to know where this came from to avoid going there).

B
 
Last edited:
Bill McEnaney said:
Why would anyone put the integer zero, not the numeral 0, into a string?
Click to expand...

The numeral 0 is this character constant: '0'. Its integer value is 0x30, which is decidedly different from integer zero.

To put the null character into the array, I need to type '\0', not a zero.
Click to expand...
The character constant '\0' has an integer value of 0.

C is not strictly typed when it comes to characters. For all intents and purposes, the char type is equivalent to a small integer type. It can perform all operations exactly as if it were a small integer. It can even be signed, despite the fact that no charset actually has negative codes.
 
chown33 said:
Where exactly did you find that code? I want to know what URL to avoid.
Click to expand...

I found it here.

Look carefully at the red-hilited piece. Under what circumstances does fgets() return NULL? What does it return under other circumstances? Which of those do you want to loop under?
Click to expand...

I have no idea.

I first tried fscan, but that didn't wait at all for any user input.

MrFusion said:
I found this code fragment with google, but the output is "Input [__]".
input_buf contains "test\n" according to the debugger, which is what I entered.
Any idea why this isn't working?

My goal is to ask the user a question (via the command line) and evaluate the response.
Thanks

Code: 
	char input_buf[64] = {0};
	char data[64] = {0};
	
	printf("Enter something: ");
	while( fgets(input_buf, sizeof(input_buf), stdin) == NULL )
	{
		/* parse the input entered */
		sscanf(input_buf, "%s", data);
	}
	
	printf("Input [_%s_]\n", data);
Click to expand...

The consensus is that this is crappy code. Fine by me. But what would then be the recommended way to ask the user for input on the command line? I know about argv[], but I mean during runtime. I tried google, but didn't find a decent example.
 
Last edited by a moderator:
MrFusion said:
The consensus is that this is crappy code. Fine by me. But what would then be the recommended way to ask the user for input on the command line? I know about argv[], but I mean during runtime. I tried google, but didn't find a decent example.
Click to expand...

http://www.daniweb.com/forums/thread93390.html has some discussion about the differences between scanf and fgets. (First Google hit).

Code: 
char data[64];
 
if ( fgets ( data, sizeof(data), stdin ) != NULL )
  printf("Input [_%s_]\n", data);

B
 
Last edited:
MrFusion said:
I found it here.



I have no idea.

I first tried fscan, but that didn't wait at all for any user input.
Click to expand...

man pages said:
RETURN VALUES
Upon successful completion, fgets() and gets() return a pointer to the string. If end-of-file occurs before any characters are read, they
return NULL and the buffer contents remain unchanged.
Click to expand...

As you can see, fgets returns NULL only if fgets fails to get any characters. In your case, fgets capture your string from stdin and returns the pointer, and thus the while loop is never executed.
 
MrFusion said:
I have no idea.
Click to expand...

Google for N1256.pdf. Download the document that you find. Search for the function that you have no idea about and read what it says. You have no chance of ever writing code that will work unless you know what you are doing, and reading the documentation is the way to learn this.


MrFusion said:
I found this code fragment with google, but the output is "Input [__]".
input_buf contains "test\n" according to the debugger, which is what I entered.
Any idea why this isn't working?
Click to expand...

Wouldn't it be better to ask: How can I find out why this isn't working? I'd change the code as follows:

Code: 
	char input_buf[64] = {0};
	char data[64] = {0};
	
	printf("Enter something: ");
	for(;;) {
            char* fgets_result = fgets(input_buf, sizeof(input_buf), stdin);
            if (fgets_result != NULL )
                break;
	    /* parse the input entered */
	    sscanf(input_buf, "%s", data);
	}
	
	printf("Input [_%s_]\n", data);

Now set a few breakpoints, start with debugger, and see what happens. Isn't that a lot easier than asking people for help?
 
Last edited:
gnasher729 said:
Google for N1256.pdf. Download the document that you find. Search for the function that you have no idea about and read what it says. You have no chance of ever writing code that will work unless you know what you are doing, and reading the documentation is the way to learn this.
Click to expand...

Great advice! If you know what you have to look for or if you are familiar with these low level c routines.
Being rushed to fix this problem also didn't help.

Anyway, this works:
Code: 
	char input_buf[64] = {0};
	char data[64] = {0};
	char data2[64] = {0};
	char stop[] = "stop";
	
	printf("Enter something: ");
	while (TRUE)
	{
		char* fgets_result = fgets(input_buf, sizeof(input_buf), stdin); //retrieve data from stdin (= command line). fgets_result points to input_buf
		sscanf(input_buf, "%s %s", data, data2);	//parses inputbuf according to middle argument and dumps results in the char arrays. View this as a reversed printf.
		if (strncmp(data2,stop,sizeof(stop)) == 0) //only the first x=sizeof(stop) characters are compared. Also works if data2 is longer, even though the first x characters are a match.
			break;
		printf("Intermediate input [_%s_] [_%s_] \n", data, data2);
		printf("Enter something else: ");
	}
	printf("Input [_%s_] [_%s_] \n", data, data2);

Thanks for pointing out the direction.
 
chown33 said:
The numeral 0 is this character constant: '0'. Its integer value is 0x30, which is decidedly different from integer zero.
Click to expand...
I know that the character zero isn't the integer zero.


chown33 said:
The character constant '\0' has an integer value of 0.

C is not strictly typed when it comes to characters. For all intents and purposes, the char type is equivalent to a small integer type. It can perform all operations exactly as if it were a small integer. It can even be signed, despite the fact that no charset actually has negative codes.
Click to expand...
That's why I can add a character to an integer. If I wanted to ensure that a string variable's value wasn't garbage, I would have initialized that variable with a blank, which you can also treat as an integer. Better yet(?), if I wanted its value to be the null string, I would have typed two consecutive double quotation marks with nothing between them.
 
Last edited:
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