Question

#include <unistd.h>
int main(int argc, char* argv[])
{
  char buf[500];
  read(0, buf, 5);
  return 0;
}

The above read 5 characters from stdin,but if I input more than 5:

12345morethan5
[root@ test]# morethan5
-bash: morethan5: command not found

The remaining characters will be executed as shell commands.

Is this kind of behavior defined by standard?

Answer 1

Sort of 🙂

Your program reads 5 characters, and that’s it. Not less, not more. The rest remain in the terminal buffer and get sent to your shell once your C program terminates.

Since you are using read(), which is a raw system call, instead of any of the C stdio buffering alternatives this behaviour is not just expected, but required.

From the POSIX standard on read():

The read() function shall attempt to
read nbyte bytes from the file
associated with the open file
descriptor, fildes, into the buffer
pointed to by buf.

…

Upon successful completion, where
nbyte is greater than 0, read() shall
mark for update the st_atime field of
the file, and shall return the number
of bytes read. This number shall never
be greater than nbyte.

…

Upon successful completion, read()
[XSI] [Option Start] and pread()
[Option End] shall return a
non-negative integer indicating the
number of bytes actually read.

I.e. read() should never read more bytes from the file descriptor than requested.

From the related part on terminals:

It is not, however, necessary to read
a whole line at once; any number of
bytes, even one, may be requested in a
read() without losing information.

…

The last process to close a terminal device file shall cause any output to be sent to the device and any input to be discarded.

Note: normally your shell will still have an open file descriptor for the terminal, until you end the session.