Problem with following function

Materialised wrote on 05/05/05 :

char *currenttime(){
char *curtime;
time_t t;
char *p, *e;
t = time(NULL);
p = ctime(&t);
if (p && ((e = strchr(p, '\n')))) *e = '\0';
curtime = malloc(strlen(p)+2);
if(curtime == NULL) {
printf("out of memory\n");
exit(1);
}
snprintf(curtime, sizeof(curtime+2), "%c%s%c", '[', p, ']');
printf("%s\n", curtime);
return curtime;
}

You should learn more about the meaning of sizeof. BTW, you don't need
snprintf() here, because you have calculated the size (kinda belt +
suspenser strategy, isn't it ?)

Try that

#include <stdio.h>
#include <time.h>
#include <string.h>
#include <stdlib.h>

char *currenttime (void)
{
char *curtime = NULL;
time_t t = time (NULL);
char *p = ctime (&t);

if (p != NULL)
{
{
char *e = strchr (p, '\n');

if (e != NULL)
{
*e = '\0';
}
}

curtime = malloc (strlen (p) + 2);

if (curtime != NULL)
{
sprintf (curtime, "%c%s%c", '[', p, ']');
}
}
return curtime;
}

int main (void)
{
char *s = currenttime ();

if (s != NULL)
{
printf ("currenttime = %s\n", s);
free (s), s = NULL;
}

return 0;
}

currenttime = [Thu May 05 19:26:10 2005]

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

I once asked an expert COBOL programmer, how to
declare local variables in COBOL, the reply was:
"what is a local variable?"

Nov 14 '05 #6

Je***********@physik.fu-berlin.de wrote on 05/05/05 :

Here is my function as it stands:
char *currenttime(){
char *curtime;
time_t t;
char *p, *e;
t = time(NULL);
p = ctime(&t);
if (p && ((e = strchr(p, '\n')))) *e = '\0';

First problem: 'e' is an uninitialized pointer,

Ok.
so you can't store
anything where it's currently pointing to. But since 'e' is never
going to be used you can simply remove the whole line.

Huh ? What about 'e = strchr(p, '\n')' <...> *e = '\0'; ?

Ok, the way it was written was not the clearest way, but the OP wanted
to remove the trailing '\n' given by ctime(), and he has done it
correctly.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

I once asked an expert COBOL programmer, how to
declare local variables in COBOL, the reply was:
"what is a local variable?"

Nov 14 '05 #7

(supersedes <mn***********************@YOURBRAnoos.fr>)

Materialised wrote on 05/05/05 :

char *currenttime(){
char *curtime;
time_t t;
char *p, *e;
t = time(NULL);
p = ctime(&t);
if (p && ((e = strchr(p, '\n')))) *e = '\0';
curtime = malloc(strlen(p)+2);
if(curtime == NULL) {
printf("out of memory\n");
exit(1);
}
snprintf(curtime, sizeof(curtime+2), "%c%s%c", '[', p, ']');
printf("%s\n", curtime);
return curtime;
}

You should learn more about the meaning of sizeof. BTW, you don't need
snprintf() here, because you have calculated the size (kinda belt +
suspenser strategy, isn't it ?)

Try that

#include <stdio.h>
#include <time.h>
#include <string.h>
#include <stdlib.h>

char *currenttime (void)
{
char *curtime = NULL;
time_t t = time (NULL);
char *p = ctime (&t);

if (p != NULL)
{
{
char *e = strchr (p, '\n');

if (e != NULL)
{
*e = '\0';
}
}

curtime = malloc (strlen (p) + 3);

if (curtime != NULL)
{
sprintf (curtime, "%c%s%c", '[', p, ']');
}
}
return curtime;
}

int main (void)
{
char *s = currenttime ();

if (s != NULL)
{
printf ("currenttime = %s\n", s);
free (s), s = NULL;
}

return 0;
}

currenttime = [Thu May 05 19:26:10 2005]

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

..sig under repair

Nov 14 '05 #8

Default User

Materialised wrote:

I am having a problem with the following functions, I've been racking my brains trying to figure out where I am going wrong. What I need to do is return a formatted string with the current date and time in the

format of:
How are we supposed to divine what you consider to be the "problem" you
are having? Next time, tell us.

Brian

Nov 14 '05 #9

David Resnick

Jens.Toerr...@physik.fu-berlin.de wrote:

In comp.lang.c Materialised <Ma**********@privacy.net> wrote:
I am having a problem with the following functions, I've been racking my brains trying to figure out where I am going wrong. What I need to do is return a formatted string with the current date and time in the
format of:
[Thu May 5 17:06:27 2005]

Here is my function as it stands:

char *currenttime(){
char *curtime;
time_t t;
char *p, *e;
t = time(NULL);
p = ctime(&t);
if (p && ((e = strchr(p, '\n')))) *e = '\0';

First problem: 'e' is an uninitialized pointer, so you can't store
anything where it's currently pointing to. But since 'e' is never
going to be used you can simply remove the whole line.

That code looks OK. He is trimming off a newline. In the case
where e is dereferenced it is always initialized. Given that he needs
to figure out the lenght of p anyway this is perhaps an inefficient way
to do that but...

-David

Nov 14 '05 #10

Martin Ambuhl

Materialised wrote:

I am having a problem with the following functions, I've been racking my
brains trying to figure out where I am going wrong. What I need to do is
return a formatted string with the current date and time in the format of:

[Thu May 5 17:06:27 2005]

#include <time.h>
#include <stdio.h>

#define BIGENUF 256

int main(void)
{
char nowstr[BIGENUF];
time_t t = time(0);
struct tm now;
now = *localtime(&t);
strftime(nowstr, BIGENUF, "[ %c ]", &now);
printf("%s\n", nowstr);
return 0;
}

Nov 14 '05 #11

Chris Torek

>Materialised wrote:

char *currenttime(){
char *curtime;
time_t t;
char *p, *e;
t = time(NULL);
p = ctime(&t);
if (p && ((e = strchr(p, '\n')))) *e = '\0';
curtime = malloc(strlen(p)+2);
if(curtime == NULL) {
printf("out of memory\n");
exit(1);
}
snprintf(curtime, sizeof(curtime+2), "%c%s%c", '[', p, ']');
printf("%s\n", curtime);
return curtime;
}

In article <1n*****************@newssvr19.news.prodigy.com>
Thomas Matthews <Th*************************@sbcglobal.net> wrote:Your problem: sizeof(curtime + 2)
This is the most obvious problem, to be sure, but not the only one.

Suppose p == NULL, so that:

if (p && ...)

does not do anything. In that case, what does:

strlen(p)

do?

Luckily, ctime() is forbidden from returning NULL, so the "p &&"
is unnecessary in the first place. We could just use:

if ((e = strchr(p, '\n')) != NULL) *e = '\0';
curtime = malloc( ... );

Then:
You want (strlen(p) + 2) instead.

This gets into the second problem, which someone else noted in a
different followup: strlen(p) + 2 lacks room for the terminating
'\0'. The malloc() argument needs to be strlen(p) + 3. This is
also the number you should pass to snprintf(). (That is, you supply
the actual number of bytes, and snprintf subtracts 1 to account
for the '\0'. Note that this is not the case for scanf field
widths, where you need to pass a number 1 byte smaller than the
buffer. The set of C functions that take or return "actual buffer
sizes", versus those that take or return "length of string not
including terminating '\0'", gets confusing. There is little if
any pattern; you must simply memorize, or look up, each function.)

In this case, of course, there is no real need to use snprintf()
at all, since curtime was presumably malloc()ed to the correct
size in the first place (except, well, "it wasn't" :-) ).

There is a slight possibility that time() will return (time_t)-1,
meaning "the system has no idea what time it is". I have no idea
what the OP would like to happen in this case.

Finally, there are a number of ways to simplify, or at least shorten,
the code, by using various features of the Standard C Library.
Probably the best alternative is to use strftime(), but I will
illustrate a little-used feature of printf:

char *currenttime(void) {
char *curtime;
time_t t;
char *p;
size_t len;

t = time(NULL);
if (t == (time_t)-1)
... do something ...
p = ctime(&t);
len = strlen(p); /* actually, always exactly 25 */

/*
* Explanation of constants: len+1 is the number of bytes
* required for the string in p. We add 2 for the two
* brackets [], and subtract 1 for the newline we will
* remove.
*/
curtime = malloc(len + 1 + 2 - 1);
if (curtime == NULL) {
printf("out of memory\n");
exit(1);
}

/*
* We use %.*s to trim off the newline.
*/
sprintf(curtime, "[%.*s]", (int)(len - 1), p);
printf("%s\n", curtime);
return curtime;
}

The ".*" modifier in "%.*s" means "fetch an int, and use it
to limit the number of characters printed". In this case,
we truncate the last character of the string in p, which is
the unwanted newline. The cost here is relatively low since
we have already computed the length for the malloc() call.

Of course, with the comments added, the code has gotten
longer, but then, comments tend to do that. :-)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

Nov 14 '05 #12

Randy Howard

In article <d5*********@news4.newsguy.com>, no****@torek.net says...

[bulk of another excellent post by Chris Torek snipped]

Of course, with the comments added, the code has gotten
longer, but then, comments tend to do that. :-)

Chris, when are you going to write your own programming book?
Your examples are incredibly well written and thorough.

--
Randy Howard (2reply remove FOOBAR)
"If the evidence doesn't seem to fit a particular conspiracy theory,
just create a bigger conspiracy theory." --Robert D. Hicks

Nov 14 '05 #13

problem with explicit template instantiation in Visual C++ 6.0 .

Randy Howard wrote on 05/05/05 :

In article <d5*********@news4.newsguy.com>, no****@torek.net says...

[bulk of another excellent post by Chris Torek snipped]
Of course, with the comments added, the code has gotten
longer, but then, comments tend to do that. :-)

Chris, when are you going to write your own programming book?
Your examples are incredibly well written and thorough.

Chris's site is fine if you can't wait:

http://web.torek.net/torek/c/

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

"C is a sharp tool"

Nov 14 '05 #14

James Dennett

Emmanuel Delahaye wrote:

Materialised wrote on 05/05/05 :
char *currenttime(){
char *curtime;
time_t t;
char *p, *e;
t = time(NULL);
p = ctime(&t);
if (p && ((e = strchr(p, '\n')))) *e = '\0';
curtime = malloc(strlen(p)+2);
if(curtime == NULL) {
printf("out of memory\n");
exit(1);
}
snprintf(curtime, sizeof(curtime+2), "%c%s%c", '[', p, ']');
printf("%s\n", curtime);
return curtime;
}

You should learn more about the meaning of sizeof. BTW, you don't need
snprintf() here, because you have calculated the size (kinda belt +
suspenser strategy, isn't it ?)

Avoiding use of sprintf altogether isn't a bad idea; sometimes it
might be infinitesimally superior to snprintf, just as on occasion
it could be argued that a goto would actually make for good code
(I might disagree with such an argument, but that's a different
point...) but the correctness/security issues that come from using
sprintf are horrible enough to want to ban it from most uses.

(That said, for portability reasons in the real world I'd still
use a wrapper over snprintf to ensure NUL termination and to allow
for workarounds on systems lacking a native snprintf.)

-- James

Nov 14 '05 #15

Jens.Toerring

In comp.lang.c Emmanuel Delahaye <em***@yourbranoos.fr> wrote:

Je***********@physik.fu-berlin.de wrote on 05/05/05 :
Here is my function as it stands:
char *currenttime(){
char *curtime;
time_t t;
char *p, *e;
t = time(NULL);
p = ctime(&t);
if (p && ((e = strchr(p, '\n')))) *e = '\0';

First problem: 'e' is an uninitialized pointer, Ok. so you can't store
anything where it's currently pointing to. But since 'e' is never
going to be used you can simply remove the whole line.

Huh ? What about 'e = strchr(p, '\n')' <...> *e = '\0'; ? Ok, the way it was written was not the clearest way, but the OP wanted
to remove the trailing '\n' given by ctime(), and he has done it
correctly.

Yes, sorry, I somehow managed to overlook the assignment to 'e' in
the if-clause. Perhaps I should go and have my glasses checked;-)

Regards, Jens
--
\ Jens Thoms Toerring ___ Je***********@physik.fu-berlin.de
\__________________________ http://www.toerring.de

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