E Maximum Number of Open Files

You can enable extended (64K) file descriptors in DIGITAL UNIX Version 4.0D, as described in Section 1.10, by following these steps:

Set the SSI_FD_NEWMAX option of the setsysinfo() system call to 1. When SSI_FD_NEWMAX is set to 1, the process utask bit is turned on and the process's hard file limit is raised to 64K. This setting is inherited by child processes.
Set the process's file descriptor soft limit to be greater than 4K by using the setrlimit() call:
```
#include <sys/resource.h>

 

struct rlimit *rlp;

 

rlp->rlim_cur = 6000;
rlp->rlim_max = 6000;
setrlimit(RLIMIT_NOFILE, rlp);
```
This setting is inherited by child processes. See the setrlimit() reference page for details.
This step is required only for applications that use select(), fd_set, FD_CLR, FD_ISSET, FD_SET, or FD_ZERO. Choose one of the following:
- Override the default value (4k) for FD_SETSIZE in <sys/select.h> by specifying the new maximum (65536). You must do this before including the <sys/time.h> header file, which includes <sys/select.h>:
```
#define FD_SETSIZE 65536
#include <sys/time.h>
```
  This setting is not inherited by child processes; therefore, FD_SETSIZE must be set explicitly in the code of a child process that requires 64k file descriptor support.
- Instead of using statically-defined fd_set structures, use fd_set pointers in conjunction with a malloc function to provide forward compatibility with any future changes to the maximum file descriptor limit:
```
fd_set *fdp;

 

fdp = (fd_set *) malloc(
(fds_howmany(max_fds,FD_NFDBITS))*sizeof(fd_mask));
```
  The max_fds would be the number of file descriptors to be manipulated. The soft file descriptor limit is a good choice. All other keywords are defined in the select.h header file:
```
#include <sys/time.h>
#include <sys/resource.h>

 


 

my_program()
{
fd_set *fdp;
struct rlimit rlim;
int max_fds;

 


 

getrlimit(RLIMIT_NOFILE, &rlim);
max_fds = rlim.rlim_cur;

 

fdp = (fd_set *) malloc(
(fds_howmany(max_fds,FD_NFDBITS))*sizeof(fd_mask));

 

FD_SET(2, fdp);

 

for (;;) {
switch(select(max_fds, (fd_set *)0, fdp, (fd_set
*)0,
struct timeval *)0)) {
...
}
```

System administrators should note that max-vnodes must be set high enough for the needs of any application that requires a high number of descriptors. The max-vnodes is the number of in-memory structures for files, and is usually set by default to 5% of system memory. A rough guide is 1 vnode per descriptor. For more information about this setting, see the System Administration and System Configuration and Tuning guides.

To disable support for up to 64K file descriptors once the previous procedure has been carried out, set the SSI_FD_NEWMAX option of the setsysinfo() system call to 0. When SSI_FD_NEWMAX is set to 0, the process utask bit is turned off and the process's hard file limit returns to the default maximum of 4K, unless the process is using more than 4K file descriptors. In that case, the setsysinfo() call returns an error, EINVAL. Additionally, if a calling process's file descriptor hard or soft limit exceeds 4K, it is set to 4K. This setting can be inherited by child processes.