calling system() from c

Question

I was trying to execute system calls from c. When the following code is executed, the date is printed first followed by " Todays date is ..........:" on a new line. When I replaced printf by puts, it executed as I intended.(the objdump showed puts@plt in place of the second printf). Can anybody tell me why it is so?

  #include <stdlib.h>

    int main() { printf(" Todays date is ..........:");

    system("/bin/date");
    printf("\n This is your exclusive shell\n");  
    system("/bin/sh");
    return 0; 
    }

Thanks in advance.

score 4 · Accepted Answer · edited Aug 20 '13 at 11:43

4

The printf() put your string in a buffer, and once you go down a line it write it to the screen. that's why when you do

printf(" Todays date is ..........:");

system("/bin/date");

You might get the date printed first.

The stdout stream is buffered, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:

Print to stderr instead using fprintf:

fprintf(stderr, "I will be printed immediately");

Flush stdout whenever you need it to using fflush:

printf("Buffered, will be flushed");
fflush(stdout); // Will now print everything in the stdout buffer

or you can also disable buffering on stdout by using setbuf:
```
setbuf(stdout, NULL);
```

edited Aug 20 '13 at 11:43

Grijesh Chauhan

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answered Aug 20 '13 at 11:14

No Idea For Name

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Suppose if OP adds `\n` in first `printf("Today......\n")` then he should get uniform behavior, because `\n` force to `fflush`. Am I correct? – Grijesh Chauhan Aug 20 '13 at 11:47
@GrijeshChauhan it does not work on my system. but all other suggestions work – user146297 Aug 20 '13 at 12:10
@user146297 Thanks for informing. Though NoIdeaForName's answer perfectly clears what happened and how to resolve. I think `\n` fflush is implementation defined if I get any link then I will share with you. – Grijesh Chauhan Aug 20 '13 at 12:18
It depends if the stream is fully buffered (_IOFBF) or line buffered (_IOLBF). The standard does not state anything about how stdout should behave. But usually it is defaultly set to line buffered (or even unbuffered) in interactives modes. – NoWiS Aug 20 '13 at 12:53

Lidong Guo · Answer 2 · 2013-08-20T11:18:03.040

3

printf(" Todays date is ..........:");

==>

printf(" Todays date is ..........:\n");

Or add a fflush(stdout); after the printf line;

edited Aug 20 '13 at 11:18

answered Aug 20 '13 at 11:12

Lidong Guo

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`\n` always fflush in C **?** , because [this post](http://stackoverflow.com/questions/213907/c-stdendl-vs-n) says it doesn't – Grijesh Chauhan Aug 20 '13 at 11:50
I believe it flushes if stdout is connected to a line-buffered terminal, but it is implementation- (and system-) dependent. – Vatine Aug 20 '13 at 11:52
@GrijeshChauhan In line buffer `\n` flush the buffer. Terminal is line buffer by default – Lidong Guo Aug 20 '13 at 12:24
@LidongGuo that I also observe in past. But is it well defined behavior in C? – Grijesh Chauhan Aug 20 '13 at 12:25
@GrijeshChauhan No , just some kind if easy way. – Lidong Guo Aug 20 '13 at 12:48
1

@GrijeshChauhan No, it's not specified in the C standard. It may be defined in POSIX, though. This is one of those cases where it crucially depends on what stdout actuall means ("stdout" is, I believe, well-defined as SOMETHING in a C program compiled in a hosted environment, but it could be a regular file, a terminal or even network I/O and if the newline `printf("foo\n")` flushes or not is outside the scope of the C standard (and it probably does different things under Unix, Windows or VMS anyway). – Vatine Aug 22 '13 at 10:33

score 2 · Answer 3 · answered Aug 20 '13 at 11:17

2

printf use a buffer.
If you want to print the text immediately you have to call fflush

printf(" Todays date is ..........:");
fflush(stdout);
system("/bin/date");

answered Aug 20 '13 at 11:17

Michael M.

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score 1 · Answer 4 · answered Aug 20 '13 at 11:38

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

    int main() { printf(" Todays date is ..........:\n"); //add \n at the end. this is same behavior as puts. now date will print after this

    system("/bin/date");
    printf("\n This is your exclusive shell\n");  
    system("/bin/sh");
    return 0; 
    }

or else you can use fflush(stdout); after printf("Todays date is ....:"); statement

calling system() from c

4 Answers4