Question

I have been able to write a program that can read any text files… except the ones found in /proc. Any file that I try to read from /proc shows up empty.

But whenever I type

cat /proc/cpuinfo

on terminal, I am presented with my CPU info.

I can also see the file when I open it with a text editor, such as gedit or leafpad.

So it seems that /proc files are indeed text files, but my C program is having a hard time reading them.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

char* readFileString( char* loc ) {
        char *fileDat;
        FILE * pFile;
        long lsize;

        pFile = fopen( loc, "r" );

        // Grab the file size.
        fseek(pFile, 0L, SEEK_END);
        lsize = ftell( pFile );
        fseek(pFile, 0L, SEEK_SET);

        fileDat = calloc( lsize + 1, sizeof(char) );

        fread( fileDat, 1, lsize, pFile );

        return fileDat;
}

int main( void ) {
        char *cpuInfo;

        cpuInfo = readFileString( "/proc/cpuinfo" );

        printf( "%s\n", cpuInfo );

        return 0;
}

Any idea why?

Answer 1

The files from /proc have a size of 0 byte because they are generated on the fly by the kernel.

See here for more information on proc filesystem:

http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html