How does nginx write files in C

I searched for access_log in the entire project and stumbled upon the ngx_http_log_module.c file which also has the following code:

static void
ngx_http_log_write(ngx_http_request_t *r, ngx_http_log_t *log, u_char *buf,
    size_t len)
{
    ....

        } else {
            n = ngx_write_fd(log->file->fd, buf, len);
        }

    ....

The func ngx_write_fd is defined in src/os/unix/ngx_files.h.

/*
 * we use inlined function instead of simple #define
 * because glibc 2.3 sets warn_unused_result attribute for write()
 * and in this case gcc 4.3 ignores (void) cast
 */
static ngx_inline ssize_t
ngx_write_fd(ngx_fd_t fd, void *buf, size_t n)
{
  return write(fd, buf, n);
}

This function is platform dependant, and is also defined in src/os/win32/ngx_files.c, but I'll ignore that as I'm not using a Windows system. fd stands for file descriptor.

Now I need to find where these file descriptors are being initialzed. The same files also contain the code to open files.

#define ngx_open_file(name, mode, create, access)                            \
    open((const char *) name, mode|create, access)

And it is referenced in ngx_cycle.c:

        file[i].fd = ngx_open_file(file[i].name.data,
                                   NGX_FILE_APPEND,
                                   NGX_FILE_CREATE_OR_OPEN,
                                   NGX_FILE_DEFAULT_ACCESS);

Remember the goal is to find some code that will help us write files. I have skipped over how nginx brings all of this together. I'll get to that when I get to that.
For now, I can stitch together what I've found:

Copy the functions into a new file.
Search and copy definitions of all undefined typedefs and constants.
Write a main function that calls the copied functions.

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

#ifndef ngx_inline
#define ngx_inline inline
#endif

typedef int ngx_fd_t;

#define NGX_FILE_RDWR O_RDWR
#define NGX_FILE_TRUNCATE (O_CREAT | O_TRUNC)
#define NGX_FILE_DEFAULT_ACCESS 0644

static ngx_inline ssize_t ngx_write_fd(ngx_fd_t fd, void *buf, size_t n) {
  return write(fd, buf, n);
}

#define ngx_open_file(name, mode, create, access)                              \
  open((const char *)name, mode | create, access)

int main() {
  ngx_fd_t fd = ngx_open_file("hello.txt", NGX_FILE_RDWR, NGX_FILE_TRUNCATE,
                     NGX_FILE_DEFAULT_ACCESS);


  char buf[] = "Hello, World!";
  ssize_t buf_sz = sizeof(buf) / sizeof(buf[0]);

  ngx_write_fd(fd, buf, buf_sz-1);

  return 0;
}

Run this:

bash

$ gcc main.c
$ chmod +x a.out
$ ./a.out
$ cat hello.txt

There is a lot of interesting stuff happening in the snippet above.

Finding the length of an array

  char buf[] = "Hello, World!";
  ssize_t buf_sz = sizeof(buf) / sizeof(buf[0]);
  // char arrays end with \0 which we do not want to write to file
  // which is why we're writing buf_sz-1 bytes only
  ngx_write_fd(fd, buf, buf_sz-1);

The ssize_t type
Just read What is size_t for? and What is ssize_t in C?

Besides writing to log files, nginx reads files as well, important configuration files. And that's my next challenge. How is the config file parsed?

How does nginx write files in C ​

How does nginx write files in C