4.7 fork and exec Functions

Before describing how to write a concurrent server in the next section, we must describe the Unix fork function. This function (including the variants of it provided by some systems) is the only way in Unix to create a new process.

If you have never seen this function before, the hard part in understanding fork is that it is called once but it returns twice. It returns once in the calling process (called the parent) with a return value that is the process ID of the newly created process (the child). It also returns once in the child, with a return value of 0. Hence, the return value tells the process whether it is the parent or the child.

The reason fork returns 0 in the child, instead of the parent's process ID, is because a child has only one parent and it can always obtain the parent's process ID by calling getppid. A parent, on the other hand, can have any number of children, and there is no way to obtain the process IDs of its children. If a parent wants to keep track of the process IDs of all its children, it must record the return values from fork.

All descriptors open in the parent before the call to fork are shared with the child after fork returns. We will see this feature used by network servers: The parent calls accept and then calls fork. The connected socket is then shared between the parent and child. Normally, the child then reads and writes the connected socket and the parent closes the connected socket.

There are two typical uses of fork:

1. A process makes a copy of itself so that one copy can handle one operation while the other copy does another task. This is typical for network servers. We will see many examples of this later in the text.

2. A process wants to execute another program. Since the only way to create a new process is by calling fork, the process first calls fork to make a copy of itself, and then one of the copies (typically the child process) calls exec (described next) to replace itself with the new program. This is typical for programs such as shells.

The only way in which an executable program file on disk can be executed by Unix is for an existing process to call one of the six exec functions. (We will often refer generically to "the exec function" when it does not matter which of the six is called.) exec replaces the current process image with the new program file, and this new program normally starts at the main function. The process ID does not change. We refer to the process that calls exec as the calling process and the newly executed program as the new program.

Older manuals and books incorrectly refer to the new program as the new process, which is wrong, because a new process is not created.

The differences in the six exec functions are: (a) whether the program file to execute is specified by a filename or a pathname; (b) whether the arguments to the new program are listed one by one or referenced through an array of pointers; and (c) whether the environment of the calling process is passed to the new program or whether a new environment is specified.

These functions return to the caller only if an error occurs. Otherwise, control passes to the start of the new program, normally the main function.

The relationship among these six functions is shown in Figure 4.12. Normally, only execve is a system call within the kernel and the other five are library functions that call execve.

Note the following differences among these six functions:

1. The three functions in the top row specify each argument string as a separate argument to the exec function, with a null pointer terminating the variable number of arguments. The three functions in the second row have an argv array, containing pointers to the argument strings. This argv array must contain a null pointer to specify its end, since a count is not specified.

2. The two functions in the left column specify a filename argument. This is converted into a pathname using the current PATH environment variable. If the filename argument to execlp or execvp contains a slash (/) anywhere in the string, the PATH variable is not used. The four functions in the right two columns specify a fully qualified pathname argument.

3. The four functions in the left two columns do not specify an explicit environment pointer. Instead, the current value of the external variable environ is used for building an environment list that is passed to the new program. The two functions in the right column specify an explicit environment list. The envp array of pointers must be terminated by a null pointer.

Descriptors open in the process before calling exec normally remain open across the exec. We use the qualifier "normally" because this can be disabled using fcntl to set the FD_CLOEXEC descriptor flag. The inetd server uses this feature, as we will describe in Section 13.5.