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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openpty(3) Library Functions Manual openpty(3)
NAME
openpty, forkpty - Open and fork pseudoterminals
LIBRARY
Standard C Library (libc.a)
SYNOPSIS
#include <sys/termios.h>
<sys/ioctl.h>
int openpty( int *master, int *slave, char *name, struct termios *termp, struct winsize *winp);
pid_t forkpty( int *master, char *name, struct termios *termp, struct winsize *winp);
PARAMETERS
Points to the returned file descriptor for the master pseudoterminal (pty). Points to the returned file descriptor for the slave pty.
Points to the pathname of the slave pty. This parameter is optional. Specifies the termios structure containing the terminal attributes
for the opened slave pty. This parameter is optional. Specifies the winsize structure containing the window attributes for the opened
slave pty. This parameter is optional.
DESCRIPTION
The openpty() function opens the pty master/slave pair and sets the terminal attributes of the slave pseudoterminal according to the speci-
fications in the termp and winp parameters. The forkpty() function creates a child process and establishes the slave pty as the child
process' controlling terminal. The openpty() function will first attempt to open an SVR4 slave device, such as /dev/pts/11. If unsuccess-
ful, it will attempt a BSD device, such as /dev/ttyp1.
Note
If a signal handler for SIGCHLD exists and the openpty() or forkpty() function is invoked without superuser privileges, the signal
handler must be able to dismiss an unexpected SIGCHLD signal.
RETURN VALUES
Upon successful completion, the openpty() function returns a value of 0 (zero). Otherwise, it returns a value of -1.
On success, the forkpty() function returns a value of 0 (zero) to the child process and returns the process ID of the child process to the
parent process. On error, the forkpty() function returns a value of -1 to the parent process and does not create a child process.
ERRORS
If any of the following conditions occurs, the openpty() function sets errno to the corresponding value: The slave pty special files have
been exhausted. No more ptys can be opened. The configured number of ptys has been reached. The system limit for open file descriptors
per process has already reached OPEN_MAX. The system limit for open file descriptors has been reached.
[Tru64 UNIX] Either the OPEN_MAX value or the per-process soft descriptor limit is checked. The system file table is full. The
system was unable to allocate kernel memory for more file descriptors/processes. The system-imposed limit on the total number of
processes executing for a single user has been exceeded. This limit can be exceeded by a process with superuser privilege.
RELATED INFORMATION
Functions: fork(2) delim off
openpty(3)