creating a variable from constant

[Robert]

Hi, I am kinda new to c and i've encountered a problem with
program because i need to change a constant. I ofcourse don't
want to change a constant so is there any way to convert a
constant to a variable? I've tried something with malloc but
it didn't work for me. Here's what i tried:

char *url;
char *filename;
char *tmpstring;
url = strtok(header, " ");
url = strtok(NULL, " ");
puts (url);
//tmpstring = (char *)malloc(sizeof(SERVER_ROOT)*strlen(SERVER_ROOT));
tmpstring = (char *)malloc(1000);
tmpstring = SERVER_ROOT;
//strcat (url, SERVER_ROOT);
filename = strcat(tmpstring, url);
puts (url);

as you can see i am trieing to combine SERVER_ROOT and url to
get something like /var/www/html/news.html where SERVER_ROOT
is /var/www/html and url = /news.html

Can anyone please help me?
Thanks in advance,

Robert

[Ben Pfaff]

Robert <R.****@hetnet.nl> writes:

Hi, I am kinda new to c and i've encountered a problem with
program because i need to change a constant. I ofcourse don't
want to change a constant so is there any way to convert a
constant to a variable? I've tried something with malloc but
it didn't work for me. Here's what i tried:

char *url;
char *filename;
char *tmpstring;
url = strtok(header, " ");
url = strtok(NULL, " ");
strtok() has at least these problems:

* It merges adjacent delimiters. If you use a comma as
your delimiter, then "a,,b,c" is three tokens, not
four. This is often the wrong thing to do. In fact,
it is only the right thing to do, in my experience,
when the delimiter set is limited to white space.

* The identity of the delimiter is lost, because it is
changed to a null terminator.

* It modifies the string that it tokenizes. This is bad
because it forces you to make a copy of the string if
you want to use it later. It also means that you can't
tokenize a string literal with it; this is not
necessarily something you'd want to do all the time but
it is surprising.

* It can only be used once at a time. If a sequence of
strtok() calls is ongoing and another one is started,
the state of the first one is lost. This isn't a
problem for small programs but it is easy to lose track
of such things in hierarchies of nested functions in
large programs. In other words, strtok() breaks
encapsulation.
puts (url);
//tmpstring = (char *)malloc(sizeof(SERVER_ROOT)*strlen(SERVER_ROOT));
When calling malloc(), I recommend using the sizeof operator on
the object you are allocating, not on the type. For instance,
*don't* write this:

int *x = malloc (sizeof (int) * 128); /* Don't do this! */

Instead, write it this way:

int *x = malloc (sizeof *x * 128);

There's a few reasons to do it this way:

* If you ever change the type that `x' points to, it's not
necessary to change the malloc() call as well.

This is more of a problem in a large program, but it's still
convenient in a small one.

* Taking the size of an object makes writing the statement
less error-prone. You can verify that the sizeof syntax is
correct without having to look at the declaration.
I don't recommend casting the return value of malloc():

* The cast is not required in ANSI C.

* Casting its return value can mask a failure to #include
<stdlib.h>, which leads to undefined behavior.

* If you cast to the wrong type by accident, odd failures can
result.

Also, I don't understand why you'd multiply those two values.
What is SERVER_ROOT?
tmpstring = (char *)malloc(1000);
tmpstring = SERVER_ROOT;
Here's a real problem. You malloc() 1000 bytes of memory, and
then you just go ahead and throw it away. You probably want to
use strcpy() instead of assignment in the second statement there.
//strcat (url, SERVER_ROOT);
strcat() only works on strings, and before you put any data into
malloc()'d memory, it's not a string. You could use strcat() if
you first assigned '\0' to the first byte malloc()'d,
e.g. tmpstring[0] = '\0';
filename = strcat(tmpstring, url);
puts (url);

as you can see i am trieing to combine SERVER_ROOT and url to
get something like /var/www/html/news.html where SERVER_ROOT
is /var/www/html and url = /news.html

You can do that in a single function call with sprintf():
sprintf(tmpstring, "%s%s", SERVER_ROOT, url);
--
"I don't have C&V for that handy, but I've got Dan Pop."
--E. Gibbons

[Robert]

Ben Pfaff wrote:

Robert <R.****@hetnet.nl> writes:

Hi, I am kinda new to c and i've encountered a problem with
program because i need to change a constant. I ofcourse don't
want to change a constant so is there any way to convert a
constant to a variable? I've tried something with malloc but
it didn't work for me. Here's what i tried:

char *url;
char *filename;
char *tmpstring;
url = strtok(header, " ");
url = strtok(NULL, " ");

strtok() has at least these problems:

* It merges adjacent delimiters. If you use a comma as
your delimiter, then "a,,b,c" is three tokens, not
four. This is often the wrong thing to do. In fact,
it is only the right thing to do, in my experience,
when the delimiter set is limited to white space.

* The identity of the delimiter is lost, because it is
changed to a null terminator.

* It modifies the string that it tokenizes. This is bad
because it forces you to make a copy of the string if
you want to use it later. It also means that you can't
tokenize a string literal with it; this is not
necessarily something you'd want to do all the time but
it is surprising.

* It can only be used once at a time. If a sequence of
strtok() calls is ongoing and another one is started,
the state of the first one is lost. This isn't a
problem for small programs but it is easy to lose track
of such things in hierarchies of nested functions in
large programs. In other words, strtok() breaks
encapsulation.

puts (url);
//tmpstring = (char *)malloc(sizeof(SERVER_ROOT)*strlen(SERVER_ROOT));

When calling malloc(), I recommend using the sizeof operator on
the object you are allocating, not on the type. For instance,
*don't* write this:

int *x = malloc (sizeof (int) * 128); /* Don't do this! */

Instead, write it this way:

int *x = malloc (sizeof *x * 128);

There's a few reasons to do it this way:

* If you ever change the type that `x' points to, it's not
necessary to change the malloc() call as well.

This is more of a problem in a large program, but it's still
convenient in a small one.

* Taking the size of an object makes writing the statement
less error-prone. You can verify that the sizeof syntax is
correct without having to look at the declaration.
I don't recommend casting the return value of malloc():

* The cast is not required in ANSI C.

* Casting its return value can mask a failure to #include
<stdlib.h>, which leads to undefined behavior.

* If you cast to the wrong type by accident, odd failures can
result.

Also, I don't understand why you'd multiply those two values.
What is SERVER_ROOT?

tmpstring = (char *)malloc(1000);
tmpstring = SERVER_ROOT;

Here's a real problem. You malloc() 1000 bytes of memory, and
then you just go ahead and throw it away. You probably want to
use strcpy() instead of assignment in the second statement there.

//strcat (url, SERVER_ROOT);

strcat() only works on strings, and before you put any data into
malloc()'d memory, it's not a string. You could use strcat() if
you first assigned '\0' to the first byte malloc()'d,
e.g. tmpstring[0] = '\0';

filename = strcat(tmpstring, url);
puts (url);

as you can see i am trieing to combine SERVER_ROOT and url to
get something like /var/www/html/news.html where SERVER_ROOT
is /var/www/html and url = /news.html

You can do that in a single function call with sprintf():
sprintf(tmpstring, "%s%s", SERVER_ROOT, url);

Thanks, i got it working now after a few experiments.
It's now something like:

char *url;
char *filename;
char *tmpstring;
url = strtok(header, " ");
url = strtok(NULL, " ");
puts (url);
//tmpstring = (char *)malloc(sizeof(SERVER_ROOT)*strlen(SERVER_ROOT));
tmpstring = (char *)malloc(1000);
tmpstring[0] = "\0";
strcpy (tmpstring, SERVER_ROOT);
strcat (tmpstring, url);
//filename = strcat(tmpstring, url);

//sprintf(tmpstring,"%s%u", SERVER_ROOT, url);

puts (tmpstring);

btw, as you can see i was to lazy to change the malloc really
before i posted this, i am going to change that ofcourse.

Once again thanks.
Robert

[Ben Pfaff]

Robert <R.****@hetnet.nl> writes:

tmpstring = (char *)malloc(1000);
tmpstring[0] = "\0";
strcpy (tmpstring, SERVER_ROOT);
strcat (tmpstring, url);

If you use strcpy(), you don't need to initialize tmpstring[0].
strcpy() doesn't assume that its destination is a string.
--
"I don't have C&V for that handy, but I've got Dan Pop."
--E. Gibbons