Files, I/O and File Systems

I/O

In C code, there are two ways to do I/O, via the C library functions, or directly via system calls (which the C library functions use). Here's an example of using the C library functions to create a file called hello.txt that contains the text Hello, World!:

#include <stdio.h>

int main() {
    FILE *fp = fopen("hello.txt", "w");
    if (fp == NULL) {
        perror("fopen");
        return 1;
    }

    fprintf(fp, "Hello, World!\n");

    fclose(fp);

    return 0;
}

Notice that fopen returns a FILE* pointer, which is a C library structure that represents a file. The fprintf function is used to write to the file, and fclose is used to close the file.

If we trace the system calls this code makes, we see

brk(NULL)                               = 0x56abe2786000
brk(0x56abe27a7000)                     = 0x56abe27a7000
openat(AT_FDCWD, "hello.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
fstat(3, {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
write(3, "Hello, World!\n", 14)         = 14
close(3)                                = 0
exit_group(0)                           = ?

We can write a version that uses the system calls directly:

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

int main() {
    int fd = open("hello.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    write(fd, "Hello, World!\n", 14);

    close(fd);

    return 0;
}

Notice that in this case, fd is an integer (a file descriptor) that represents the file. Data about what is going on in the file is not stored in a way directly accessible to us; that information is stored in the kernel. We use the write system call to write to the file, and the close system call to close the file.

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