Michael said:
I have the following code piece of code:
#include <stdio.h>
#include <string.h>
void reverse(char* begin, char* end) {
char tmp;
while(begin < end) {
tmp = *begin;
*begin = *end;
*end = tmp;
++begin;
--end;
}
}
int main(int argc, char* argv[]) {
char a_str[8] = "abc def";
a_str[7] = '\0';
printf("str: %s\n",a_str);
reverse(a_str,a_str + strlen(a_str) - 1);
printf("str: %s\n",a_str);
char* b_str = "abc def";
printf("str: %s\n",b_str);
reverse(b_str,b_str + strlen(b_str) - 1);
printf("str: %s\n",b_str);
return 0;
}
when I use a_str for the parameter to the function reverse
everthing is ok, however when b_str is used the program
crashes when the line containing *begin = *end is executed.
I've always thought that the definitions of a_str and b_str
were more or less equivelent. Can someone explain where I
have gone wrong.
The char* b_str declaration will not work unless you have a c99
compiler. If it does work it will declare an initialized string
somewhere in memory that is not modifiable.
Chapter and verse, please, where the C standard states that string
literals are stored in "memory that is not modifiable". Or even
mentions the possibility of such a type of memory.
The memory may or may not be modifiable. The C standard says nothing
about it. The type of a string literal is "array of char", and
specifically NOT "array of const char". This is almost certainly
because of the indisputable fact that string literals existed in the
language long before the 'const' type qualifier and corresponding
keyword did.
Attempting to modify a string literal in C produces undefined behavior
not because the type is const, but because the C standard specifically
states that it produces undefined behavior.
On some implementations, the string literal would actually be
modified, that being one possible consequence of undefined behavior.
On some implementations, the writes would generate no errors in
themselves, but the string literal would be unchanged, that being
another possible consequence of undefined behavior.
And on still other implementations, the program will "crash" when it
attempts to modify the string literal, that being yet another possible
consequence of undefined behavior.
--
Jack Klein
Home:
http://JK-Technology.Com
FAQs for
comp.lang.c
http://c-faq.com/
comp.lang.c++
http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html
