lower case string in c

Question

i had a few line here :

#include <stdio.h>

char *tolower(char *data)
{
    char *p = data;
    while(*p)
    {
        printf("nilai p : %c\n",*p);
            if(*p >= 'A' && *p <= 'Z')
            {
                *p += ('a' - 'A');
            }
        p++;
    }
    return p;

}

int main()
{

    char *a = "HajAR BleH";
    char *b = tolower(a);

    printf("nilai b : %s\n",b);
    printf("nilai a - A : %d\n",'a' - 'A');
return 0;
}

next, compiled, running on gdb, and segmentation traced

xxx@aaa:/tmp$ gcc -o aa aa.c --debug
xxx@aaa:/tmp$ gdb -q aa
Reading symbols from /tmp/aa...done.
(gdb) r
Starting program: /tmp/aa 
nilai p : H

Program received signal SIGSEGV, Segmentation fault.
0x0804841e in tolower (data=0x804855e "HajAR BleH") at aa.c:11
11                  *p += ('a' - 'A');
(gdb) 

question

1. i think *p += ('a' - 'A'); will equal as 'H' += ('a' - 'A') and equal with 72 += 32 but, accidentally segmentation fault, how come it could be ?

2. why need to add ('a' - 'A') to make char / byte get lower ?

thats all for now, thank in advance

Answer 1

You are trying to modify a string literal. Change:

char *a = "HajAR BleH";

to:

char a[] = "HajAR BleH";

Also, there already is a standard library function called tolower(), which you should in fact be using in your code. Call your own function something else.

Answer 2

The most likely cause of the SEGV is that the compiler has placed a in read-only memory, as it is allowed to do with string literals.

Try changing a 's definition to:

char a[] = "HajAR BleH";

As a matter of style, there already exists a standard function called tolower , which does something different (convert one character to lowercase). With this in mind, I'd suggest:

1. giving your function a different name to avoid confusion;
2. using tolower in your implementation to convert one character to lowercase.

Answer 3

The reason for the seg-fault is you are trying to modify a string literal which is resides in read only memory leading to undefined behavior.

Try changing

char *a = "HajAR BleH";

to

char a[] = "HajAR BleH";

Answer 4

As well as the problem with the read-only memory, you need to return data rather than p . By the time your loop has finished, p points to the null-terminator.

Answer 5

1: Essentially your code is correct, however you're trying to modify a constant string ( char *a should reda const char *a ). You'll have to create another character array you'll be able to modify.

2: ('a' - 'A') calculates the "difference" in the ascii values between lower case characters and upper case characters (32).