大写字符串并返回局部变量

Question

I'm trying to make a procedure that will capitalize a string, but I get junk values and a warning from gcc that I'm returning the address of a local variable. Coming from Python, I'm confused by this issue. (Don't want to use stuff in <string.h> )

 2
 3
 4char* strcaps(const char *strlower)
 5{
 6  int len = 0;
 7  while (*strlower != '\0'){
 8    len++;
 9    strlower++;
10  }
11  char newStr[len + 1];
12  int i;
13  for (i = 0; i < len; i++)
14    newStr[i] = strlower[i] - 32;
    newStr[len + 1] = '\0';    //EDIT, just put this in.
15  return newStr;
16}
17
18int main(void)
19{
20  
21  
22  
23  printf("I expect capitalized version of edgar, I got %s\n", strcaps("edgar"));
24
25
26  return 0;
27}

I'm guessing I can make a global string and then mutable that, but I'd rather keep everything inside the function strcaps...is this a case when I should use malloc?

Thanks

EDIT: Just realized that I never put a '\\0 ' at the end of the new string, although I'm not sure if that is related.

Answer 1

You have two options in C.

1) you can pass your function an pointer to the string and function will modify it and return void

In this case you can just use the original string

2) you can pass your function const char* and return char * , but then you need to malloc inside the function

If you do a malloc inside function, you can then use the string (print it or do whatever you want), at the and of the program you call free

Answer 2

is this a case when I should use malloc?

Yes, exactly.

In your case, you return pointer to local variable, which (the variable) will be destroyed after the execution of the function. This makes the pointer dangling.

Using malloc will create the string in the heap, not stack, and it will remain valid after after the return of the function.

Answer 3

First of all, you should always put a null character at the end of a string, because the standard library uses this to denote the actual end of a string, to prevent from walking over memory.

This is an implementation.

/* Makes a copy of the string as to not modify the original buffer */
char* makeCopy(char* s){

    char* buff = (char*) malloc(len(s) + 1);
    strcpy(buff, s);
    return buff;
}

/* Converts the string to uppercase */
char* toUpperCase(char* s){

        char* q = makeCopy(s);
        int i;
        for(i = 0; i < len(q); i++){
                *(q + i) = toupper(*(q + i));
        }

        return q;

}

Now you can do:

printf("I expect capitalized version of edgar, I got %s\n", toUpperCase("edgar"));

Now I understand that you don't want to touch any of the stuff in string.h , but it is useful, for without the functions there you may need to implement much of the stuff on your own.

For example, the function toupper() comes from string.h . This converts a character to its uppercase equivalent. If you insist on reinventing the wheel you can hack a tiny function that handles it.

char getUpperCaseChar(char lower){
    if(lower < 91)
        return lower;
    if(lower > 96)
        return (lower - 32);
}