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Can't open folders from /proc location in C linux

I'm going to read "comm" file in each directory of /proc . My program can see list of directory, but when I try to open folder in this directory I get an error. My system is Debian Linux hardware Beaglebone Black. I'm learning programming under linux so I have a lot of questions (stupid sometimes).

list of code:

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <dirent.h>

#include <string.h>
#include <fcntl.h>

int main()
{  
    char path[50] = {0};
    register struct dirent *dirbuf;
    char* dirname = "/proc";
    DIR *fdir;

    fdir = opendir(dirname);
    if (NULL == fdir)
    {  
        printf("Can't open %s\n", dirname);
        return;
    }  

    while(( dirbuf = readdir(fdir)) != NULL)
    {  
        if ((strcmp(dirbuf->d_name, ".") == 0)||(strcmp(dirbuf->d_name, "..") == 0))
        {  
            continue;
        }  

        printf("folder name: %s\n", dirbuf->d_name);
        strcat(path, dirbuf->d_name);
        strcat(path, "/comm");
        printf("path: %s\n", path);

        int fd = open(path, O_RDONLY);
        if ( -1  == fd)
        {  
            printf("Can't open file %s\n", path);
        }  
        else
        {  
            //read file
            close(fd);
        }  
        memset(path, 0, strlen(path) + 1); //clear path buffer

    }  
    closedir(fdir);
    return 0; 
}

Log from linux console:

enter image description here

Solution

  • You need to use the full path to open the file. ie: /proc/PID/comm instead of PID/comm.

    You can use something like this to format the path:

    char* path[PATH_MAX];
snprintf(path, PATH_MAX, "/proc/%s/comm", dirbuf->d_name);

    To format the path appropriately.