Trying to reverse a string and getting a bus error

Question

I am trying to reverse a string (but that's not the problem that I have). The problem is trying to change the value of the string array given a certain index. However, every time I try to change the value at the index, I get a bus error. Namely, Bus error: 10. I'm not sure what this means. Also, I tried str[0] = "a" but this also gives me a bus error. Any suggestions to fix this?

#include <iostream>
using namespace std; 

void reverse(char* str){
    str[0] = 'a'; 

}


int main(){

    char* str = "hello"; 
    reverse(str); 

}

The answer is the same as the answer to this one, just recently asked: http://stackoverflow.com/questions/31816473 but also http://stackoverflow.com/questions/3075049 — Moby Disk, Aug 05 '15 at 23:27
Also: The compiler should give you a warning that your code is not valid. See http://coliru.stacked-crooked.com/a/ef79aa4884d08883 — Moby Disk, Aug 05 '15 at 23:31
`str[0] = "a";` should definitely not even compile. How did it give you a runtime error? — chris, Aug 05 '15 at 23:37
You should also get errors for `str[0] = "a"`. If you don't then you really need to turn up your compiler's diagnostic level. — M.M, Aug 05 '15 at 23:49
@chris common compilers give "warning" in their default mode for conversion from pointer to integer — M.M, Aug 05 '15 at 23:50
@MattMcNabb, Both the Clang and GCC on Coliru give errors with no warning options. I don't recall seeing compilers give warnings for that except in C mode. — chris, Aug 06 '15 at 00:06
@MattMcNabb The similarity is that the first link shows code declaring a string literal without const. — Moby Disk, Aug 06 '15 at 13:36
@MobyDisk That's where the similarity ends though, the main thing about this question is writing to the string literal, which doesn't happen in the other one — M.M, Aug 06 '15 at 20:54

score 6 · Answer 1 · answered Aug 05 '15 at 23:28

6

Allocate your string as an array on the stack and not as a pointer into a possibly read-only segment of your program.

char str[] = "hello";

answered Aug 05 '15 at 23:28

Greg Bacon

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I have one question, void reverse(char* str){}, am I passing a pointer or is * acting as a reference? – aejhyun Aug 06 '15 at 00:05
In C++, arrays "decay" into pointers in certain contexts. Here, the pointer argument to `reverse` starts out as a pointer to the first element of the array in `main`. – Greg Bacon Aug 06 '15 at 00:13
But when I write cout << str << endl;, why does it print out "hello"? Shouldn't it print only the first character of the string since it points to the first element of the array? – aejhyun Aug 06 '15 at 00:32
The type of `str` in `reverse` is pointer to `char`, which is treated by `std::cout` as a string. In contrast, plain `char` is a single character. – Greg Bacon Aug 06 '15 at 00:40

score 2 · Accepted Answer · answered Aug 06 '15 at 00:46

First of all, this line should atleast give you a warning:

char* str = "hello";

you are converting a string constant to a pointer, which is not allowed. To fix your code, you should use, char str[] = "hello" in main().

When you pass this array in reverse(), it decays to char*, now the question which you asked in previous answer's comment.

But when I write cout << str << endl;, why does it print out "hello"? Shouldn't it print only the first character of the string since it points to the first element of the array?

It is because the << operator on std::cout is overloaded. If you give it a char* or const char*, it treats the operand as a pointer to (the first character of) a C-style string, and prints the contents of that string:

const char * str= "hello";
cout << str; // prints "hello"

If you give it a char value, it prints that value as a character:

cout << *str;   // prints "h"
cout << str[0]; // prints "h"

Trying to reverse a string and getting a bus error

2 Answers2