c - Can someone please explain how stdio buffering works? -

i don't understand buffer doing , how it's used. (also, if can explain buffer does) in particular, why need fflush in example?

int main(int argc, char **argv) {     int pid, status;     int newfd;  /* new file descriptor */      if (argc != 2) {         fprintf(stderr, "usage: %s output_file\n", argv[0]);         exit(1);     }     if ((newfd = open(argv[1], o_creat|o_trunc|o_wronly, 0644)) < 0) {         perror(argv[1]);    /* open failed */         exit(1);     }     printf("this goes standard output.\n");     printf("now standard output go \"%s\".\n", argv[1]);     fflush(stdout);      /* new file become standard output */     /* standard output file descriptor 1, use dup2 */     /* copy new file descriptor onto file descriptor 1 */     /* dup2 close current standard output */      dup2(newfd, 1);       printf("this goes standard output too.\n");     exit(0); } 

in unix system stdout buffering happens improve i/o performance. expensive i/o every time.

if don't want buffer there's options:

1. disable buffering calling setvbuf http://www.cplusplus.com/reference/cstdio/setvbuf/

2. call flush when want flush buffer

3. output stderr (that's unbuffered default)

here you've more details: http://www.turnkeylinux.org/blog/unix-buffering

i/o expensive operation, reduce number of i/o operations system store information in temporary memory location, , delay i/o operation moment when has amount of data.

this way you've smaller number of i/o operations, means, faster application.