Geek asks and answers

Is it possible to find out how many bytes were printed in the file stream, for example, standard output?

Is it possible for the calling program in C to know how many bytes it printed in the file stream, for example stdout, without actually counting or adding return values printf?

I am trying to implement a control over the amount of output from a C program that uses libraries for printing, but the libraries do not report the amount of data they printed.

I am interested in either a general solution or a Unix-specific one.

+3
source share
4 answers

POSIX-specific: stdout , , st_size ( ls).

. , . POSIX head . , .

+1

, , ftell stdout:

long int start = ftell(stdout);
printf("abcdef\n");
printf("%ld\n", ftell(stdout) - start); // >> 7

Ubuntu Precise: , , , .

$ ./a.out 
abcdef
0
$ ./a.out >tt
$ cat tt
abcdef
7
$ echo `./a.out`
abcdef 0
$ echo `cat tt`
abcdef 7
0

, :

  • , pipe()
  • : stdout ( stdout)
  • : stdout ( stdout) -
  • stdout ( )
  • IPC .

, , .

IPC - , ( / ) , ( , ..) , .

- , .. , , , , , ( , ). , - - , , IPC ; , stdout , eof.

If you need more frequent readings that need to be accurate, then you need something more than copmlex, but this can be achieved by developing a kind of protocol using sockets, pipes or even converters / semaphores / etc in shared memory.

0
source

printf returns the number of bytes written.

Add them.

0
source

More articles:


All Articles