Here is a bit of C code that is part of something much bigger.
#include <stdio.h>
#include <unistd.h>
#define GIT_VERSION "1.23-dev"
#define GIT_COUNT "73"
#define GIT_ID "daa3ab8"
#define VERSION "1.23"
main()
{
printf("\nThingy version %s (%s-%s-%s) (built %s, %s)\n\n", VERSION,
GIT_VERSION, GIT_COUNT, GIT_ID, __TIME__, __DATE__);
// fflush(stdout);
_exit(0);
}
If one compiles it (cc vtest.c -o vtest) and runs it at a shell prompt,
one gets:
jim@mike2:~/thingy$ vtest
Thingy version 1.23 (1.23-dev-73-daa3ab8) (built 16:21:49, Jun 30 2011)
jim@mike2:~/thingy$
All is well and seemingly as expected. However now do this:
jim@mike2:~/thingy$ vtest | grep version
jim@mike2:~/thingy$
Stdout appears not to have been have been flushed. Which, after reading
the man page for _exit is fair enough as it says: "The function _exit()
terminates the calling process "immediately". Any open file descriptors
belonging to the process are closed". This is, in fact, what is wanted
in the larger program. This behaviour has only come to light, after
several years mind, because some (other) poor fool wanted to grab the
version no for some shell script or other, instead of just checking what
a user had installed via a shell prompt.
The fix is simply to fflush(stdout), before _exit().
But how come I get output at a shell prompt, and not down a pipe (or a
redirection either)? What special magic is occurring here?
Dirk
