Handle (computing)

In computer programming, a handle is an abstract reference to a resource that is used when application software references blocks of memory or objects that are managed by another system like a database or an operating system. A resource handle can be an opaque identifier, in which case it is often an integer number (often an array index in an array or "table" that is used to manage that type of resource), or it can be a pointer that allows access to further information. Common resource handles include file descriptors, network sockets, database connections, process identifiers (PIDs), and job IDs. PIDs and job IDs are explicitly visible integers; while file descriptors and sockets (which are often implemented as a form of file descriptor) are represented as integers, they are typically considered opaque. In traditional implementations, file descriptors are indices into a (per-process) file descriptor table, thence a (system-wide) file table.

In computer programming, a handle is an abstract reference to a resource that is used when application software references blocks of memory or objects that are managed by another system like a database or an operating system. A resource handle can be an opaque identifier, in which case it is often an integer number (often an array index in an array or "table" that is used to manage that type of resource), or it can be a pointer that allows access to further information. Common resource handles include file descriptors, network sockets, database connections, process identifiers (PIDs), and job IDs. PIDs and job IDs are explicitly visible integers; while file descriptors and sockets (which are often implemented as a form of file descriptor) are represented as integers, they are typically considered opaque. In traditional implementations, file descriptors are indices into a (per-process) file descriptor table, thence a (system-wide) file table. int fd = open("/etc/passwd", O_RDWR); This call asks the operating system to open the specified file with the specified access rights. If the OS allows this, then it opens the file (creates an entry in the per-process file descriptor table) and returns a handle (file descriptor, index into this table) to the user: the actual access is controlled by the OS, and the handle is a token of that. Conversely, the OS may deny access, and thus neither open the file nor return a handle. In a capability-based system, handles can be passed between processes, with associated access rights. Note that in these cases the handle must be something other than a systemwide-unique small integer, otherwise it is forgeable. Such an integer may nevertheless be used to identify a capability inside a process; e.g., file descriptor in Linux is unforgeable because its numerical value alone is meaningless, and only in the process context may refer to anything. Transferring such a handle requires special care though, as its value often has to be different in the sending and receiving processes. In non-capability-based systems, on the other hand, each process must acquire its own separate handle, by specifying the identity of the resource and the desired access rights (e.g., each process must open a file itself, by giving the filename and access mode). Such usage is more common even in modern systems that do support passing handles, but it is subject to vulnerabilities like the confused deputy problem.

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