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.

Answer 1

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);