How to remove line breaks after "ctime" in C program?

Question

Please bear with me since I'm still really new to C programming. When I run this code:

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

int main(void)
{
    while (1) {
        time_t mytime;
        mytime = time(NULL);
        printf("%s Hello world\n", ctime(&mytime));
        sleep(1);
    }
    return 0;
}

The output always looks like this:

Wed Jan 18 02:32:32 2017
 Hello world
Wed Jan 18 02:32:33 2017
 Hello world
Wed Jan 18 02:32:34 2017
 Hello world

What I want is like this:

Wed Jan 18 02:32:32 2017 Hello world
Wed Jan 18 02:32:33 2017 Hello world
Wed Jan 18 02:32:34 2017 Hello world

How can I do that ?

Note: If I remove \n from printf("%s Hello world\n", ctime(&mytime)); it'll result like this:

Wed Jan 18 02:38:29 2017
 Hello worldWed Jan 18 02:38:30 2017
 Hello worldWed Jan 18 02:38:31 2017
 Hello worldWed Jan 18 02:38:32 2017
 Hello worldWed Jan 18 02:38:33 2017

OP doesn't want a line break *before* `Hello World`; desired output includes the one after it. — Scott Hunter, Jan 17 '17 at 19:39
You will have to copy the output of `ctime` to some string, and replace the last character with `\0`. — Eugene Sh., Jan 17 '17 at 19:40
@EugeneSh.: I see. The format of `ctime` is well defined; just remove the trailing newline. (That's indeed a stupid addition of `asctime`; linebreaks should be left to the user) — too honest for this site, Jan 17 '17 at 19:41

dbush · Accepted Answer · 2017-01-17T19:43:59.557

10

The ctime function will return a pointer to a string which ends in a newline.

From the man page:

The call ctime(t) is equivalent to asctime(localtime(t)). It converts the calendar time t into a null-terminated string of the form "Wed Jun 30 21:49:08 1993\n"

If you don't want the newline, you need to save the pointer and remove the newline before printing.

char *t = ctime(&mytime);
if (t[strlen(t)-1] == '\n') t[strlen(t)-1] = '\0';
printf("%s Hello world\n", t);

edited Jan 17 '17 at 19:43

answered Jan 17 '17 at 19:40

dbush

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But is it guaranteed to be a `\n` and never a `\r\n` across different systems? – Havenard Jan 17 '17 at 19:42
1

@Havenard Yes, the C standard also states the above equivalence between `ctime` and `asctime` and output format for `asctime`. – dbush Jan 17 '17 at 19:46
Reasonable. Yet why the test for `t[strlen(t)-1] == '\n'`? If code is not confident that the `'\n` ends the buffer, then it should not be confident that `'\n'` even exist and the bad string could begin with `'\0'` which leads to UB with `t[strlen(t)-1]`. – chux - Reinstate Monica Jan 17 '17 at 20:08
Simpler solution, works in all cases: `t[strcspn(t, "\r\n")] = '\0';` – chqrlie Mar 15 '20 at 14:22

score 4 · Answer 2 · answered Apr 09 '21 at 09:23

4

I faced the same problem so the workaround I figured was using strtok() in the string.h library. This will omit '\n' and store the whole token before it:

char *p = strtok(ctime(&mytime), '\n');

answered Apr 09 '21 at 09:23

Muhid

91
5

3

'\n' should be "\n" – Connor Clark Oct 04 '22 at 00:56

Kerrek SB · Answer 3 · 2017-01-17T20:45:41.450

3

Use strftime to format your own string:

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

int main(void)
{
    char buf[100];
    strftime(buf, 100, "%a %b %d %T %Y", localtime(&(time_t){time(NULL)}));
    printf("%s Hello world\n", buf);
}

For simple formatting tasks like the one in the question, you can spare yourself the printf and let strftime do all the work:

    strftime(buf, 100, "%a %b %d %T %Y Hello world\n",
             localtime(&(time_t){time(NULL)}));
    fputs(buf, stdout);

You should also check the return value of strftime, zero may indicate failure.

edited Jan 17 '17 at 20:45

answered Jan 17 '17 at 19:46

Kerrek SB

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It does look like a more appropriate solution than messing around with `ctime()`'s static buffer. – Havenard Jan 17 '17 at 20:00
@Havenard `"%a %b %d %T %Y"` is locale dependent, `ctime()` is not. – chux - Reinstate Monica Jan 17 '17 at 20:02
@chux: Specifically, `%a` and `%b` are locale-dependent; the other parts are not. – Kerrek SB Jan 17 '17 at 20:07
@KerrekSB Nice use of the C99 compound literal `(time_t){time(NULL)}` to provide an address to the returned `time_t` from `time()`. – chux - Reinstate Monica Jan 17 '17 at 20:52

chqrlie · Answer 4 · 2021-03-02T20:01:29.453

Here are 2 simple solutions:

store the pointer returned by ctime() and overwrite the trailing newline with:

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

int main(void) {
    while (1) {
        time_t mytime;
        mytime = time(NULL);
        char *p = ctime(&mytime);
        p[strcspn(p, "\n")] = '\0';
        printf("%s Hello world\n", p);
        sleep(1);
    }
    return 0;
}

even simpler: just print the first 24 characters of the string returned by ctime():

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

int main(void) {
    while (1) {
        time_t mytime;
        mytime = time(NULL);
        printf("%.24s Hello world\n", ctime(&mytime));
        sleep(1);
    }
    return 0;
}

The date format produced by ctime is "Www Mmm dd hh:mm:ss yyyy" so you'll want to use `%.24s` to strip the new-line and null-character at the end. — Matt K, Mar 02 '21 at 18:30

score 1 · Answer 5 · edited May 23 '17 at 12:33

Lop off the '\n' in the popular fashion as suggested here after fgets(). It does not even have a problem if '\n' is missing.

char * ct = ctime(&mytime));
ct[strcspn(ct, "\n")] = '\0';
printf("%s Hello world\n", ct);

ctime() nominally returns a pointer to static char some_name[26]; in the following example form

Sun Sep 16 01:03:52 1973\n\0

The C spec has a qualifier about struct tm fields values being in their normal ranges, else the results are UB. Going on the idea that the year could be outside the range 1000-9999, this renders the fixed placement of '\n' problematic.

How to remove line breaks after "ctime" in C program?

5 Answers5

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