根据进程名获取进程id

原理：
linux操作系统中有一个名为/proc的虚拟文件系统，其中记录着进程和用户的相关信息，其中/proc/N (注：N表示数字)目录表示进程ip号为N的进程信息，就是这里找到我们要找的进程信息，其中/proc/N/status记录了进程状态信息，包含进程名等，比如：
root@proc # cat /proc/3544/status
Name: lighttpd
State: S (sleeping)
Tgid: 3544
Pid: 3544
PPid: 1
TracerPid: 0
Uid: 500 500 500 500
Gid: 500 500 500 500
Utrace: 0
FDSize: 32
Groups: 500
VmPeak: 2980 kB
VmSize: 2976 kB
。。。。。

注：对于存在多个同名的进程，只返回第一个进程ip，而不返回所有的进程ip

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <ctype.h>
#include <unistd.h>

static void getProcessName(const int pid, char *pid_name, int pid_size);
/*return:
 *      -1 not found
 *      not-negative is pid number
 *      writed by zhj 
 * */
int getPidByName(const char *name)
{
    DIR *dir_p = NULL;
    struct dirent entry, *res = NULL;
    int pid = -1, error_ret = 0;
    if(name == NULL)
    {
        return -1;
    }

    dir_p = opendir("/proc");
    if(NULL == dir_p)
    {
        printf("opendir error: %s\n", strerror(errno));
        goto ERR;
    }
    for(;;)
    {
        error_ret = readdir_r(dir_p, &entry, &res);
        if(error_ret != 0)
        {
            printf("readdir_r error: %s\n", strerror(error_ret));
            goto ERR;
        }
        if(res == NULL)
        {
            break;
        }
        if(isdigit(res->d_name[0]))
        {
            char pid_name[56] = "";

            pid = atoi(res->d_name);

            getProcessName(pid, pid_name, sizeof(pid_name));
            if(pid_name[0] != '\0' && strcmp(pid_name, name) == 0)
            {
                break;
            }
        }
        pid = -1;
    }


    if(dir_p != NULL)
    {
        closedir(dir_p);
        dir_p = NULL;
    }
    return pid;
ERR:
    if(dir_p != NULL)
    {
        closedir(dir_p);
        dir_p = NULL;
    }
    return -1;
}


static void getProcessName(const int pid, char *pid_name, int pid_size)
{
    int fd = -1;
    char name[56] = "", *p = NULL, str[56] = "", buf[1024] = "";
    int size = 0;
    struct stat file_stat;

    pid_name[0] = '\0';
    size = snprintf(name, sizeof(name) - 1, "/proc/%d/status", pid);
    if(size <= 0 || size > sizeof(name) - 1)
    {
        printf("%s %d, snprintf error\n", __func__, __LINE__);
        goto ERR;
    }

    fd = open(name, O_RDONLY);  
    if(fd < 0)
    {
        printf("%s %d, open error: %s\n", __func__, __LINE__, strerror(errno));
        goto ERR;
    }
    if(fstat(fd, &file_stat) < 0)
    {
        printf("fstat error: %s\n", strerror(errno));
        goto ERR;
    }
    size = read(fd, buf, sizeof(buf) - 1);
    if(size <= 0)
    {
        printf("read error: %s\n", strerror(errno));
        goto ERR;
    }
    buf[size] = '\0';

    if(sscanf(buf, "%s %s", str, pid_name) != 2)    
    {
        goto ERR;
    }

    if(fd != -1)
    {
        close(fd);
        fd = -1;
    }

    return ;
ERR:
    if(fd !=  -1)
    {
        close(fd);
        fd = -1;
    }

    return ;
}