File descriptor

 

When using ILE C stream I/O functions as defined by the American National Standards Institute (ANSI) to perform operations on a file, you identify the file through the use of pointers. When using the integrated file system C functions, you identify the file by specifying a file descriptor. A file descriptor is a positive integer that must be unique in each job.

The job uses a file descriptor to identify an open file when performing operations on the file. The file descriptor is represented by the variable fildes in C functions that operate on the integrated file system and by the variable descriptor in C functions that operate on sockets.

Each file descriptor refers to an open file description, which contains information such as a file offset, status of the file, and access modes for the file. The same open file description can be referred to by more than one file descriptor, but a file descriptor can refer to only one open file description. Figure 1. File descriptor and open file description

If an ILE C stream I/O function is used with the integrated file system, the ILE C runtime support converts the file pointer to a file descriptor.

When using the "root" (/), QOpenSys, or user-defined file systems, you can pass access to an open file description from one job to another, thus allowing the job to access the file. You do this by using the givedescriptor() or takedescriptor() function to pass the file descriptor between jobs.

 

Parent topic:

Performing operations using APIs

 

Related information

givedescriptor()--Pass Descriptor Access to Another Job API
takedescriptor()--Receive Socket Access from Another Job API
Sockets programming
Sockets APIs