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

'a' - 'A' doesn't make sense unless you were familiar with the ASCII table. The numeric difference between 'a' and 'A' is what you have to add to an uppercase letter in order to have the equivalent letter in lowercase. — Neil
– Neil, Commented May 16, 2011 at 10:33

user2100815 · Accepted Answer · 2011-05-16 10:30:25Z

7

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.

answered May 16, 2011 at 10:30

user2100815

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Comments

NPE · Accepted Answer · 2011-05-16 10:30:56Z

4

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:

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

answered May 16, 2011 at 10:30

NPE

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1 Comment

capede Over a year ago

even i dont really care whether there is or isnt any function tolower() exist , thank for remind me though

codaddict · Accepted Answer · 2011-05-16 10:44:23Z

2

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";

edited May 16, 2011 at 10:44

answered May 16, 2011 at 10:30

codaddict

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1 Comment

codaddict Over a year ago

Actually it is s/char \[\]a/char a\[\]/ ;) [] are char class.

David Heffernan · Accepted Answer · 2011-05-16 10:32:58Z

0

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.

answered May 16, 2011 at 10:32

David Heffernan

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Comments

Mario · Accepted Answer · 2011-05-16 10:36:23Z

0

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).

answered May 16, 2011 at 10:36

Mario

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