fileno(stdin) versus STDIN_FILENO and friends

[Eric Norum]

The document at http://pubs.opengroup.org/onlinepubs/009695399/basedefs/unistd.h.html implies that they really are supposed to be constants:

The following symbolic constants shall be defined for file streams:
STDERR_FILENO
File number of stderr; 2.
STDIN_FILENO
File number of stdin; 0.
STDOUT_FILENO
File number of stdout; 1.

but I agree that that is rather counterintuitive.

On Mar 11, 2011, at 12:21 PM, Peter Dufault wrote:

> Shouldn't STDOUT_FILENO etc always be valid, regardless of thread?
> 
> Consider this code snippet installed in the RTEMS shell as "filenos":
> ===
> static int
> filenos_cmd(int argc, char **argv)
> {
>    int stdin_fileno = fileno(stdin),
>        stdout_fileno = fileno(stdout),
>        stderr_fileno = fileno(stderr);
> 
>    fprintf(stderr,
>        "fileno(stdin),  STDIN_FILENO:  %d %d\n"
>        "fileno(stdout), STDOUT_FILENO: %d %d\n"
>        "fileno(stderr), STDERR_FILENO: %d %d\n",
>        stdin_fileno,  STDIN_FILENO,
>        stdout_fileno, STDOUT_FILENO,
>        stderr_fileno, STDERR_FILENO);
> 
>    fprintf(stderr, "- %s\n",
>        (stdin_fileno == STDIN_FILENO &&
>        stdout_fileno == STDOUT_FILENO &&
>        stderr_fileno == STDERR_FILENO) ? "OK" : "BAD");
> 
>    return 0;
> }
> ====
> 
> Called from the console:
> 
> [/] # filenos                                                                                                                                                                                                                             
> fileno(stdin),  STDIN_FILENO:  0 0                                                                                                                                                                                                         
> fileno(stdout), STDOUT_FILENO: 1 1                                                                                                                                                                                                         
> fileno(stderr), STDERR_FILENO: 2 2                                                                                                                                                                                                         
> - OK                                                                                                                                                                                                                                         
> [/] #                                                                                                                                                                 
> 
> Called via telnet:
> 
> [/] # filenos
> fileno(stdin),  STDIN_FILENO:  7 0
> fileno(stdout), STDOUT_FILENO: 8 1
> fileno(stderr), STDERR_FILENO: 9 2
> - BAD
> [/] # 
> 
> Shouldn't the macros reference something that changes with the thread context to appropriate file descriptors without forcing reference to <stdio.h>?
> 
> Peter
> -----------------
> Peter Dufault
> HD Associates, Inc.      Software and System Engineering
> 
> 
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-- 
Eric Norum
wenorum at lbl.gov

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