[nginx源碼分析]set process title

由於linux argv和environ環境變量內存地址連續，同時argv[0]代表進程name，所以就可以儘可能的用連續的內存空間(起始地址argv[0]）來設置進程name
test case如下：

 */
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

#define LOGDBG(fmt, args...) do {\
    printf("[%s:%d]%s   "fmt"\n", __FILE__, __LINE__, __FUNCTION__, ##args);\
}while(0);

extern char **environ;
int main(int argc, char** argv)
{
    uint32_t  i = 0;
    uint32_t  env_size = 0;
    for (; environ[i];i++)
    {
        env_size += strlen(environ[i])+1;//\0 char
        LOGDBG("environ[i]=%s", environ[i]);
    }
    LOGDBG("env_size:%u", env_size);

    char* env = (char*) malloc(env_size);
    char* temp = env;

    for(i = 0; environ[i]; i++)
    {
        snprintf(temp, strlen(environ[i])+1, "%s", environ[i]);
        environ[i] = temp;
        temp += strlen(environ[i])+1;
    }
    char* last = environ[i]-1;
    argv[1] = NULL;
    snprintf(argv[0], last - argv[0], "%s", "set title"); 


    for (i=0; environ[i];i++)
    {
        LOGDBG("environ[i]=%s", environ[i]);
    }
    sleep(10);
    return 0;
}

testcase 根據的是nginx設置進程爲master or worker process的做法，nginx的code 如下：

extern char **environ;

static char *ngx_os_argv_last;

ngx_int_t
ngx_init_setproctitle(ngx_log_t *log)
{
    u_char      *p;
    size_t       size;
    ngx_uint_t   i;

    size = 0;

    //首先計算出environ需要的內存空間
    for (i = 0; environ[i]; i++) {
        size += ngx_strlen(environ[i]) + 1;
    }

    //爲environ分配內存空間
    p = ngx_alloc(size, log);
    if (p == NULL) {
        return NGX_ERROR;
    }

    ngx_os_argv_last = ngx_os_argv[0];

    //check連續性
    for (i = 0; ngx_os_argv[i]; i++) {
        if (ngx_os_argv_last == ngx_os_argv[i]) {
            ngx_os_argv_last = ngx_os_argv[i] + ngx_strlen(ngx_os_argv[i]) + 1;
        }
    }

    //copy environ到新分配的內存
    for (i = 0; environ[i]; i++) {
        if (ngx_os_argv_last == environ[i]) {

            size = ngx_strlen(environ[i]) + 1;
            ngx_os_argv_last = environ[i] + size;

            ngx_cpystrn(p, (u_char *) environ[i], size);
            environ[i] = (char *) p;
            p += size;
        }
    }

    ngx_os_argv_last--;

    return NGX_OK;
}


void
ngx_setproctitle(char *title)
{
    u_char     *p;

#if (NGX_SOLARIS)

    ngx_int_t   i;
    size_t      size;

#endif

    //先把argv數組的第二個節點置爲空
    ngx_os_argv[1] = NULL;

    //copy
    p = ngx_cpystrn((u_char *) ngx_os_argv[0], (u_char *) "nginx: ",
                    ngx_os_argv_last - ngx_os_argv[0]);

    p = ngx_cpystrn(p, (u_char *) title, ngx_os_argv_last - (char *) p);

#if (NGX_SOLARIS)

    size = 0;

    for (i = 0; i < ngx_argc; i++) {
        size += ngx_strlen(ngx_argv[i]) + 1;
    }

    if (size > (size_t) ((char *) p - ngx_os_argv[0])) {

        /*
         * ngx_setproctitle() is too rare operation so we use
         * the non-optimized copies
         */

        p = ngx_cpystrn(p, (u_char *) " (", ngx_os_argv_last - (char *) p);

        for (i = 0; i < ngx_argc; i++) {
            p = ngx_cpystrn(p, (u_char *) ngx_argv[i],
                            ngx_os_argv_last - (char *) p);
            p = ngx_cpystrn(p, (u_char *) " ", ngx_os_argv_last - (char *) p);
        }

        if (*(p - 1) == ' ') {
            *(p - 1) = ')';
        }
    }

#endif

    if (ngx_os_argv_last - (char *) p) {
        ngx_memset(p, NGX_SETPROCTITLE_PAD, ngx_os_argv_last - (char *) p);
    }

    ngx_log_debug1(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
                   "setproctitle: \"%s\"", ngx_os_argv[0]);
}

#endif /* NGX_SETPROCTITLE_USES_ENV */

也是先copy environ到新的地址空間，用連續的內存空間來設置進程title