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Full Discussion: file descriptors
Top Forums UNIX for Dummies Questions & Answers file descriptors Post 46836 by fpmurphy on Tuesday 27th of January 2004 06:46:14 PM
Old 01-27-2004
Note that processes do not have a limit of 20 file descriptors
as suggested in your mail - rather the limit is OPEN_MAX.

Here is the formal POSIX definition of a file descriptor:

A per-process unique, non-negative integer used to identify an open file for the purpose of file access. The value of a file descriptor is from zero to OPEN_MAX. A process can have no more than OPEN_MAX file descriptors open simultaneously.

Note also the difference between file DESCRIPTOR and file DESCRIPTION. From SUSv3 ....

(Open) File Description

A record of how a process or group of processes is accessing a file. Each file descriptor refers to exactly one open file description, but an open file description can be referred to by more than one file descriptor. The file offset, file status, and file access modes are attributes of an open file description.

- Finnbarr
 

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getdtablesize(2)						System Calls Manual						  getdtablesize(2)

NAME
getdtablesize - Gets the descriptor table size SYNOPSIS
#include <unistd.h> int getdtablesize ( void ); STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: getdtablesize(): XPG4-UNIX Refer to the standards(5) reference page for more information about industry standards and associated tags. DESCRIPTION
The getdtablesize() function returns the total number of file descriptors in a process' descriptor table. Each process has a fixed size descriptor table that is guaranteed to have at least 64 slots. The entries in the descriptor table are numbered with small integers start- ing at 0 (zero). The getdtablesize() function returns the total number of file descriptors that a process can have open simultaneously. Each process is limited to a fixed number of open file descriptors. This limit is at least 64. The system defined limit is configurable. If it is greater than 64, the system will allocate more file descriptors for a process up to the defined maximum. RETURN VALUES
The getdtablesize() function returns the size of the descriptor table, and is always successful. RELATED INFORMATION
Functions: close(2), open(2), select(2) Standards: standards(5) delim off getdtablesize(2)
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