根据进程名获取进程pid

摘自：https://blog.csdn.net/shanguangy111/article/details/77136187

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

  1 #include <stdio.h>
  2 #include <string.h>
  3 #include <stdlib.h>
  4 #include <errno.h>
  5 #include <sys/types.h>
  6 #include <dirent.h>
  7 #include <sys/stat.h>
  8 #include <sys/mman.h>
  9 #include <fcntl.h>
 10 #include <ctype.h>
 11 #include <unistd.h>
 12 
 13 static void getProcessName(const int pid, char *pid_name, int pid_size);
 14 /*return:
 15  *      -1 not found
 16  *      not-negative is pid number
 17  *      writed by zhj 
 18  * */
 19 int getPidByName(const char *name)
 20 {
 21     DIR *dir_p = NULL;
 22     struct dirent entry, *res = NULL;
 23     int pid = -1, error_ret = 0;
 24     if(name == NULL)
 25     {
 26         return -1;
 27     }
 28 
 29     dir_p = opendir("/proc");
 30     if(NULL == dir_p)
 31     {
 32         printf("opendir error: %s\n", strerror(errno));
 33         goto ERR;
 34     }
 35     for(;;)
 36     {
 37         error_ret = readdir_r(dir_p, &entry, &res);
 38         if(error_ret != 0)
 39         {
 40             printf("readdir_r error: %s\n", strerror(error_ret));
 41             goto ERR;
 42         }
 43         if(res == NULL)
 44         {
 45             break;
 46         }
 47         if(isdigit(res->d_name[0]))
 48         {
 49             char pid_name[56] = "";
 50 
 51             pid = atoi(res->d_name);
 52 
 53             getProcessName(pid, pid_name, sizeof(pid_name));
 54             if(pid_name[0] != '\0' && strcmp(pid_name, name) == 0)
 55             {
 56                 break;
 57             }
 58         }
 59         pid = -1;
 60     }
 61 
 62 
 63     if(dir_p != NULL)
 64     {
 65         closedir(dir_p);
 66         dir_p = NULL;
 67     }
 68     return pid;
 69 ERR:
 70     if(dir_p != NULL)
 71     {
 72         closedir(dir_p);
 73         dir_p = NULL;
 74     }
 75     return -1;
 76 }
 77 
 78 
 79 static void getProcessName(const int pid, char *pid_name, int pid_size)
 80 {
 81     int fd = -1;
 82     char name[56] = "", *p = NULL, str[56] = "", buf[1024] = "";
 83     int size = 0;
 84     struct stat file_stat;
 85 
 86     pid_name[0] = '\0';
 87     size = snprintf(name, sizeof(name) - 1, "/proc/%d/status", pid);
 88     if(size <= 0 || size > sizeof(name) - 1)
 89     {
 90         printf("%s %d, snprintf error\n", __func__, __LINE__);
 91         goto ERR;
 92     }
 93 
 94     fd = open(name, O_RDONLY);  
 95     if(fd < 0)
 96     {
 97         printf("%s %d, open error: %s\n", __func__, __LINE__, strerror(errno));
 98         goto ERR;
 99     }
100     if(fstat(fd, &file_stat) < 0)
101     {
102         printf("fstat error: %s\n", strerror(errno));
103         goto ERR;
104     }
105     size = read(fd, buf, sizeof(buf) - 1);
106     if(size <= 0)
107     {
108         printf("read error: %s\n", strerror(errno));
109         goto ERR;
110     }
111     buf[size] = '\0';
112 
113     if(sscanf(buf, "%s %s", str, pid_name) != 2)    
114     {
115         goto ERR;
116     }
117 
118     if(fd != -1)
119     {
120         close(fd);
121         fd = -1;
122     }
123 
124     return ;
125 ERR:
126     if(fd !=  -1)
127     {
128         close(fd);
129         fd = -1;
130     }
131 
132     return ;
133 }