Runtime error when running my code

Question

When I compile my C codes sometimes I get this error message.

Mycode.exe has stopped working..
A problem caused the program to stop working correctly.
Windows will close the program and notify you if a solution available.

My C code :

#include<stdio.h> 

main(){ 
    char a; 
    a="S"; 
    printf("%s",a); 
}

So what is the reason for this problem?
Syntax error, Runtime error or another reason?

It means (a) you're not just compiling and linking your program; you're also running it. And (b) your program is invoking *undefined behaviour* that is fortunately causing early termination by the OS. The code you just posted does just that. That code suggests a [good book on the C language](https://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list?s=1|3.4128) is a sound investment. — WhozCraig, Jul 20 '16 at 08:39
As I said, your code is compiling *and* running. It's just plain bad code that invokes undefined behavior. Thus the suggestion for a book on the language. (and that code belongs in *your question*; not in a comment down here. And for goodness sake, *turn up your warning levels on your compiler*. — WhozCraig, Jul 20 '16 at 08:47
Please turn on/up compiler warnings! Your code should give 2 warnings, one for assignment of incompatible types (assigning a string to a char) and one for improper format code (printing a char with "%s"). — Klas Lindbäck, Jul 20 '16 at 08:54

score 1 · Accepted Answer · edited Jul 20 '16 at 09:29

When you invoke printf with %s it means that printf will start printing at the given address and end when a null terminator is reached, because you are giving printf a char and not a pointer to a char, it tries to use the value written in a to start printing from.

a is a char taking up a space of one Byte while an address is 8 Bytes in a 64 bit system, so basically printf takes the value in 'a' and the the next 7 Bytes(which are random 'garbage') and tries to use it as an address to stop printing.

that is why it sometimes works as you said, sometimes those random addresses are fine to start printing from, but sometimes they are addresses that you are not authorized to access like areas of memory used by the OS or kernel.

to solve the problem you need to make a a char * and not a char, and assign it with a string.

Govind Madhu · Answer 2 · 2016-07-20T08:59:12.737

-1

Change your code to

#include<stdio.h> 
int main()
{ 
  char a; 
  a='S'; 
  printf("%c",a);
  return 0;
 }

and it will work fine.

edited Jul 20 '16 at 08:59

answered Jul 20 '16 at 08:54

Govind Madhu

182
1
9

although your code fixes one compiler warning, it does not fix the code – dvhh Jul 20 '16 at 08:57
Did not notice the double quotes until now. Fixed that as well. – Govind Madhu Jul 20 '16 at 08:59
but are you sure that was the intended code ? maybe @gayantha-akalanka wanted to print a string, and not a single character only. – dvhh Jul 20 '16 at 09:03
@dvhh I'm going by the sample code he provided. He is using a character variable and not an array declared as char. – Govind Madhu Jul 20 '16 at 09:08
I want to print only a single character.so think @Govind Madhu your suggestion is ok..thank you very much guys! – Gayantha Akalanka Jul 20 '16 at 09:57
Unfortunately what you have done is given him a fish as opposed to teach him to fish as `monkeyStix` did in the accepted answer. The point of SO is not to blindly answer a question but explain why your answer solves the problem. – KevinDTimm Jul 20 '16 at 13:03
@KevinDTimm It's ok.I followed both answers.But i have another question.@Govind Madhu@KevinDTimm What happen if i use double quotes in a="S"? ( I got $ sign as the output) – Gayantha Akalanka Jul 20 '16 at 14:25
@GayanthaAkalanka - that is covered in `monkeyStix` answer – KevinDTimm Jul 20 '16 at 14:40
@KevinDTimm Gotcha! Thank you.. – Gayantha Akalanka Jul 20 '16 at 14:42

Runtime error when running my code

2 Answers2