Замена исполняемого файла в процессе (Exec)

Execute a file (a library function and/or a system call)
the computer operating system function

In computing, exec is a functionality of an operating system that runs an executable file in the context of an already existing process, replacing the previous executable. This act is also referred to as an overlay. It is especially important in Unix like systems, although it also exists elsewhere. As no new process is created, the process identifier (PID) does not change, but the machine code, data, heap, and stack of the process are replaced by those of the new program. The exec call is available for many programming languages including compilable languages and some scripting languages. In OS command interpreters, the built in command replaces the shell process with the specified program.

Interfaces to exec and its implementations vary. Depending on programming language it may be accessible via one or more functions, and depending on operating system it may be represented with one or more actual system calls. For this reason exec is sometimes described as a collection of functions. Standard names of such functions in C are , , , , , and (see below), but not "exec" itself. The Linux kernel has one corresponding system call named "execve", whereas all aforementioned functions are user space wrappers around it. Higher level languages usually provide one call named .

A file descriptor open when an exec call is made remains open in the new process image, unless was ed with FD CLOEXEC or opened with O CLOEXEC (the latter was introduced in POSIX.1 2001). This aspect is used to specify the standard streams (stdin, stdout and stderr) of the new program. A successful overlay destroys the previous memory address space of the process, and all its memory areas, that were not shared, are reclaimed by the operating system. Consequently, all its data that were not passed to the new program, or otherwise saved, become lost.

DOS is not a multitasking operating system, but replacing the previous executable image has a great merit there due to harsh primary memory limitations and lack of virtual memory. The same API is used for overlaying programs in DOS and it has effects similar to ones on POSIX systems. MS DOS exec functions always load the new program into memory as if the "maximum allocation" in the program's executable file header is set to default value 0xFFFF. The EXEHDR utility can be used to change the maximum allocation field of a program. However, if this is done and the program is invoked with one of the exec functions, the program might behave differently from a program invoked directly from the operating system command line or with one of the spawn functions (see below).

Many Unix shells also offer a builtin command that replaces the shell process with the specified program. The command can also perform a redirection. In some shells it is even possible to use the command for redirection only, without making an actual overlay.

The traditional Unix system does not have the functionality to create a new process running a new executable program in one step, which explains the importance of exec for Unix programming. Other systems may use spawn as the main tool for running executables. Its result is equivalent to the fork–exec sequence of Unix like systems. POSIX supports the posix spawn routines as an optional extension that usually is implemented using vfork.

OS/360 and successors include a system call XCTL (transfer control) that performs a similar function to exec.