fileno - map a stream pointer to a file descriptor
The fileno() function shall return the integer file descriptor associated with the stream pointed to by stream.
Upon successful completion, fileno() shall return the integer value of the file descriptor associated with stream. Otherwise, the value -1 shall be returned and errno set to indicate the error.
The fileno() function shall fail if:
- The stream is not associated with a file.
The fileno() function may fail if:
- The file descriptor underlying stream is not a valid file descriptor.
Without some specification of which file descriptors are associated with these streams, it is impossible for an application to set up the streams for another application it starts with fork() and exec. In particular, it would not be possible to write a portable version of the sh command interpreter (although there may be other constraints that would prevent that portability).
Interaction of File Descriptors and Standard I/O Streams, dirfd, fdopen, fopen, stdin
First released in Issue 1. Derived from Issue 1 of the SVID.
The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:
The [EBADF] optional error condition is added.
SD5-XBD-ERN-99 is applied, changing the definition of the [EBADF] error.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0115  is applied.
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