CODEWITHSUNDEEP
clinuxstring
Julen
Julen

Reputation: 1144

Output from a printf call is not geting displayed on screen

I'm getting a string from the keyboard, in C and Linux (Ubuntu 14.04): 

char string[STR];    
fgets(string, STR, stdin);

And I've noticed that strlen() is not working.

So, I've decided to do the following:

int i = 0;    
while (string[i] != '\0') {
        printf("\nchar: %c\n", string[i]);
        i++;
}
printf("\nFinished");

I type "hello" as input, and the output is the following:

char: h

char: e

char: l

char: l

char: o

char:

"Finished" is never printed.

I can't see which is the problem. I suspect it's because the input read from keyboard doesn't have '\0' terminated?

Yesterday I was working with Windows, and was working fine with strings.

EDIT: STR is defined as 128.

Upvotes: 0

Views: 117

Answers (3)

Ziezi
Ziezi

Reputation: 6457

1.If STR's values is smaller than the input string's (length + termination character: '\0'), the last character of your input string isn't stored in char string[STR];. The function strlen() uses '\0' to determine the end of the string, that is why without it, it "doesn't work" and your program runs in an infinite loop.

2.Buffering may be the other cause, as @l3x perfectly explains.

Upvotes: 1

pmg
pmg

Reputation: 108978

Yes it does. fgets() stores a '\0' as the last byte written (may not be the last position in the array).

You need to check the return value of fgets() and most functions with a prototype in <stdio.h>.

char x[100]; // unitialized, x[99] may be anything
if (!fgets(x, sizeof x, stdin)) /* error; contents of x are not trustworthy */;
x[strlen(x)] == '\0'; // true, always true
                      // provided the previous statement did not generate an error

Upvotes: 1

P.P
P.P

Reputation: 121377

From the code snippets you posted, there's no obvious problem. My guess is that this is part of a larger program and what you observe is due to a output buffering. The standard output stream is typically line buffered and will be flushed periodically (as the buffer becomes full).

So adding \n at the end of printf might fix it if stdout is attached to a terminal device. If you are redirecting it to a file (./a.out >output) then this won't fix it either. So try calling fflush():

fflush(stdout);

after the printf call.

I say "part of larger program" because at the end of normal process termination, all the buffered output will be flushed. So there wouldn't be any need for manual flushing. On abnormal process termination, the buffered output may be lost.

Another way is to disable the stdout buffering completely using sebtuf():

setbuf(stdout, 0);

Upvotes: 4

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