Learn Tracing System Calls with strace | System Call Fundamentals

Understanding how programs interact with the Linux kernel is essential for effective debugging and development. The strace tool allows you to observe the system calls a program makes as it runs. By tracing these calls, you can see how your program requests services from the operating system, such as opening files, reading or writing data, or creating processes.

You should use strace when you need to debug issues related to:

File access;
Permissions;
Resource usage;
Unexpected program behavior that might be related to system-level operations.

Its importance lies in providing visibility into the low-level interactions between your code and the kernel, making it easier to diagnose problems that are otherwise hidden from view.

main.c


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

int main() {
    int fd = open("output.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    const char *msg = "Hello, strace!\n";
    ssize_t written = write(fd, msg, 15);
    if (written == -1) {
        perror("write");
        close(fd);
        return 1;
    }

    if (close(fd) == -1) {
        perror("close");
        return 1;
    }

    return 0;
}

To use strace with the provided program, compile it and then run it under strace by executing strace ./a.out. The output will show each system call as it is made, along with its arguments and return values. For this example, you will see calls such as open, write, and close.

The open call attempts to create or truncate output.txt and open it for writing, showing the file path, flags, and permissions;
The return value is a file descriptor, or -1 if it fails;
The write call shows the file descriptor, the buffer being written, and the number of bytes, followed by the number of bytes actually written;
Finally, the close call releases the file descriptor, with its return value indicating success or failure.

Each line in the strace output corresponds to a system call, helping you understand exactly how your program interacts with the operating system and where errors may occur.

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Section 1. Chapter 5

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