Question

This function implements the API to change process title:

http://lxr.evanmiller.org/http/source/os/unix/ngx_setproctitle.c

59     for (i = 0; environ[i]; i++) {
 60         if (ngx_os_argv_last == environ[i]) {
 61 
 62             size = ngx_strlen(environ[i]) + 1;
 63             ngx_os_argv_last = environ[i] + size;
 64 
 65             ngx_cpystrn(p, (u_char *) environ[i], size);
 66             environ[i] = (char *) p;
 67             p += size;
 68         }
 69     }
 70 
 71     ngx_os_argv_last--;
 72 
 73     return NGX_OK;
 74 }

What I don’t understand is ,after copy the environment variables to block allocated by ngx_alloc(size, log);,how is that block connected with argv[] block?

I don’t see such logic there.

And there’s one line I don’t understand:

ngx_os_argv_last--;

What’s it for, is this line that connects the allocated block with argv[]?

Answer 1

The comment at the beginning of the module seems to explain a lot, have you read it?

http://lxr.evanmiller.org/http/source/os/unix/ngx_setproctitle.c#L14

It appears the ngx_init_setproctitle() function simply sets up the memory for setting the process title and does no real changes to the title. The comment at the beginning of the module states that it needs to setup the memory for setting the process title because argv[0] may not have the space for the new title (which is what needs to be set in order to change the title).

The ngx_os_argv_last variable simply points to the end of the contiguous chunk for argv[] and environ[]. It’s used later in the copy process in ngx_setproctitle(). The last ngx_os_argv_last-- is probably to account for the '\0' at the end of the string.

The ngx_init_setproctitle() create the space with ngx_alloc() then copies environ[] into the new space. The ngx_setproctitle() function simply copies over ngx_os_argv[0] with the new value title.