Explore why Linux processes go to sleep

The Linux process entering sleep refers to the process of transitioning the process from running state to sleep state. In Linux systems, there are many reasons why a process goes to sleep, including waiting for certain resources, waiting for I/O operations to complete, waiting for signals, etc. In this article, we will explore some common reasons why Linux processes go to sleep and illustrate them with concrete code examples.

Resource waiting

A process may enter sleep state because it needs certain resources, such as waiting for other processes to release a shared resource. In the following example, we create two child processes. One process obtains resources first, and the other process waits for the first process to release the resources before it can continue execution.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/types.h>

int main() {
    int fd[2];
    pipe(fd);

    pid_t pid1 = fork();
    if (pid1 == 0) {
        // Child process 1
        close(fd[0]); // Close the read port
        sleep(2); // Simulate the process of obtaining resources
        close(fd[1]); // Release resources
        exit(0);
    }

    pid_t pid2 = fork();
    if (pid2 == 0) {
        // Child process 2
        close(fd[1]); // Close the write port
        printf("Subprocess 2 is waiting for resources...
");
        char buf[10];
        read(fd[0], buf, sizeof(buf)); // Block waiting for resources
        printf("Subprocess 2 obtains resources and continues execution.
");
        exit(0);
    }

    // Wait for the child process to end
    wait(NULL);
    wait(NULL);

    return 0;
}

In the above code, child process 2 is blocked at the read() function and cannot continue execution until child process 1 releases resources.

I/O operations

The process may also enter sleep state because it needs to perform I/O operations. The following is a simple example showing a process waiting for user input.

#include <stdio.h>
#include <unistd.h>

int main() {
    char buf[10];
    printf("Please enter some content:
");
    fgets(buf, sizeof(buf), stdin); // Block waiting for user input
    printf("What you entered is: %s", buf);
    return 0;
}

In the above example, the fgets() function will always wait for user input.

Signal waiting

The process may also enter sleep state due to waiting for signals. The following example shows a process waiting for a signal.

#include <stdio.h>
#include <unistd.h>
#include <signal.h>

void signal_handler(int signal) {
    printf("Signal received: %d
", signal);
}

int main() {
    signal(SIGUSR1, signal_handler); // Register signal processing function

    printf("Waiting for signal...
");
    pause(); // The process keeps waiting for the signal

    return 0;
}

In the above example, the process waits for the arrival of the signal through the pause() function.

Through the above code examples, we can see that there are many reasons why a Linux process goes to sleep, including waiting for certain resources, waiting for I/O operations to complete, waiting for signals, etc. These are important aspects of process scheduling and operation in Linux systems. An in-depth understanding of these principles can help us better understand the operating mechanism of the process.

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