How to read virtual files on Linux using a C program?

[livid oracle]

Hey, Im writing a simple program, where every process from /proc is printed out on the screen and I also want to hace the names of each process, so I tried file /proc/88/status => and it says its empty but when I try to read it by cat status it spits out the needed information, so my question is how and if I can even read it?

thank you very much

[raw trellis]

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[green belfry]

depending on the mount options you might not be able to read it

Users may not access files and subdirectories inside
any /proc/pid directories but their own (the /proc/pid
directories themselves remain visible). Sensitive
files such as /proc/pid/cmdline and /proc/pid/status
are now protected against other users. This makes it
impossible to learn whether any user is running a
specific program (so long as the program doesn't
otherwise reveal itself by its behavior).

[livid oracle]

i tried also the comm file but no luck, is there a way to "sudo" read it?

[green belfry]

https://www.man7.org/linux/man-pages/man5/proc.5.html

you basically change the mount options on boot

not sure if systemd allows that

[livid oracle]

so i suppose i have option 1, right?

so i should change it to 0 and then try?

[green belfry]

yes, tho depending on what youre trying to do its a bad idea

its not locked down "for fun" by default

[livid oracle]

lol, thank you, you saved me a lot of time

[green belfry]

can you cat /proc/1/status ?

[livid oracle]

yes i can

i can read every file on there but not with a script

[green belfry]

how do you read it

[livid oracle]

ill show u the code

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <ctype.h>
#include <string.h>

int is_all_digits(struct dirent *entry)
{
    char *pToName = entry->d_name;
    size_t lenOfName = strlen(pToName); // ahoj = 4 => 0-3

    if(strcmp(entry->d_name, ".") != 0 && strcmp(entry->d_name,"..") != 0)
    {
        for(size_t i = 0; i < lenOfName; i++)
        {
            if(isdigit(pToName[i]))
            {
                return 0;
            }
            else
            {
                return 1;
            }
        }
    }
    else
    {
        printf("\nROVNA SE TO\n");
        return 1;
    }

    return 0;
}


char nameOfPID(DIR *directory, const char *newPathToOpenDir) // /proc/PID/comm
{
    struct dirent *newEntry;
    
    if((directory = opendir(newPathToOpenDir)) == NULL)
    {
        printf("chyba pro otevirani slozky");
        return 1;
    }

    //FILE *file_pointer

    

}

and main:

int main()
{
    printf("---procesy z /proc a jejich nazvy---\n");

    char newPathToOpenDir[20]; // /proc/maxDelkaPID4 => 20 pro jistotu
    char newPathToFile[20]; // /proc/maxDelkaPID4/comm => pro jistotu
    unsigned char data[30];

    DIR *directory; // neco jako FILE *file_pointer, ale pro slozky
    struct dirent *entry; // ukazatel na strukturu dirent, ktera obsahuje ruzne informace current slozce/souboru => entry

    if((directory = opendir("/proc")) == 0)
    {
        printf("chyba u otevirani slozky /proc");
        return 1;
    }

    size_t i = 1;
    while((entry = readdir(directory)) != NULL)
    {
        // offset v /proc se udela sam interne
        if (is_all_digits(entry) == 0)
        {
            printf("%d: %s\n", i, entry->d_name);
            
            snprintf(newPathToOpenDir, 12, "/proc/%s", entry->d_name);
            snprintf(newPathToFile, 16, "/proc/%s/comm", entry->d_name);

            FILE *filePointer = fopen(newPathToFile, "rb");

            fseek(filePointer, 0, SEEK_SET);
            fseek(filePointer, 0, SEEK_END);
            size_t delkaSouboru = ftell(filePointer);
            fseek(filePointer, 0, SEEK_SET);

            if (fread(data, sizeof(unsigned char), delkaSouboru, filePointer) != delkaSouboru)
            {
                printf("nejak se to nerovna, error");
                fclose(filePointer);
                return 1;
            }
            data[delkaSouboru] = '\0';

            printf("\ntady to je: %s\n", data);
            fclose(filePointer);
            
            i++;
        }
        else
        {
            //printf("\nNE\n");
        }
    }
    if(closedir(directory) != 0)
    {
        return 1;
    }

    return 0;
}

[green belfry]

works on my machine

[livid oracle]

what is the assert.h for?

ill try it on a whole new project

[green belfry]

if fp == NULL it will kill the program

[livid oracle]

got you

oh yeah it works lol

wow, thank you for the solution, ill do it in the main project, really appreaecite it

[raw trellis]

@livid oracle Has your question been resolved? If so, type !solved :)

[livid oracle]

!solved