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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.

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4 Answers 4

Reset to default
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);
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4 Comments

Suppose if OP adds \n in first printf("Today......\n") then he should get uniform behavior, because \n force to fflush. Am I correct?
@GrijeshChauhan it does not work on my system. but all other suggestions work
@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.
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.
3 
printf(" Todays date is ..........:");

==>

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

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

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6 Comments

\n always fflush in C ? , because this post says it doesn't
I believe it flushes if stdout is connected to a line-buffered terminal, but it is implementation- (and system-) dependent.
@GrijeshChauhan In line buffer \n flush the buffer. Terminal is line buffer by default
@LidongGuo that I also observe in past. But is it well defined behavior in C?
@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).
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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");
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1 
  #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

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