48 lines
2.3 KiB
C
48 lines
2.3 KiB
C
#include <stdio.h>
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#include <sys/wait.h>
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#include <unistd.h>
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int main() {
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// The program that will get run. It is similar to the command-line:
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// echo "Hello, world!"
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char *const cmdline[] = { "echo", "Hello, world!", NULL };
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// Create a new process that's a clone of this one. The original process is
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// known as the parent, and the new process is known as the child. The child
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// process activates in the `fork` call, meaning both the parent and the child
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// will return from fork and assign a value to the variable `pid`.
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pid_t pid = fork();
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// When the child process returns from `fork`, the value assigned to `pid`
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// will be 0. In the parent process, `pid` will be the process ID (a non-0
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// number) of the child process. It is important to note that the process ID
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// of the child isn't actually 0. The call to `fork` only returns 0 to
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// allow code to determine if it is the child.
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if(pid == 0) {
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// Since this block only executes if `pid` is 0, the call to `execvp` will
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// only be run by the child process. This is done because the `exec` family
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// of functions replace the process' instructions with the instructions
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// found in `cmdline[0]` (in this case, `echo`). The `v` in `execvp` means
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// vector, which in this case refers to the array `cmdline`. The `p` means
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// path, indicating that it will search all of the directories found in the
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// environment vaiable `PATH` for the program (in this case, `execvp` is
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// searching the directories for `echo`).
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execvp(cmdline[0], cmdline);
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} else {
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// In the parent process, pause until the child process exits. The `wait`
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// function takes a pointer to an integer that contains information about
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// how the child process exited. If the exit information isn't necessary,
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// `NULL` can be passed instead. Several macros are available to obtain
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// further information about how the process exited.
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int status = 0;
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wait(&status);
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printf("Process %d exited with status %d.\n", pid, status);
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}
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// The parent process is now finished, and will return the number `0` to
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// indicate it was successful. The child process will never reach this point
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// in this program, since its instructions were replaced with `echo`s
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// instructions earlier.
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return 0;
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}
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