C. bad file descriptor error by working on files. This is an assignment for college. I need to compress file txt and then to uncompress it. The compress method working fine and it compresses by the idea that the last binary digit (Msb) is zero always in any ASCII char. Does anybody know why it happens? The problem happens when I do fgetc(input_file_8to7) in the uncompress method The mainly problem is that i get -1 from the 2 fgetc(input_file_8to7) in uncompresd method

#include <stdio.h> 
#include <string.h> 
#include <malloc.h> 
#include <stdlib.h> 
#include <stdint.h> 

the input for the uncompress is the output of the compress , the input is txt file that conatain the next 2 lines: A hex dump is a hexadecimal view of computer data, from memory or from a computer file.

***compres method***

    void compress8to7(FILE *input,FILE* output8to7)
    {
        // 0x00 87 00 87 00 87 00 87 ll
        uint64_t magic=0x87008700870087ll;
        uint64_t inputsize=0;
    
        fwrite(&magic,sizeof (magic),1,output8to7);
        fwrite(&inputsize,sizeof(inputsize),1,output8to7);
        unsigned char  st;
        unsigned char st2;
        char check;
        char check2;
        char shift=7;
        char shift_st=0;
        unsigned char inbfile;

// will contain the resullt of the asked new binary lines comprees

        int breakflag=0;
        int breakflag2=0;
        int cnt=-1;

//this parameter will help to know when we dont need to move 1 back in the pointer of input file

        while(1) {
    
            cnt  ;
            if (ftell(input)>1 && cnt%7!=0) //
            {
                fseek(input,-1 ,SEEK_CUR) ;
            }
    
            check = fgetc(input);
            st=check;
            if(check2==EOF){breakflag2=1;}
    
            check2 = fgetc(input); 

> //if the length  is odd number check2 will get the eof

            st2=check2;
            if(check==EOF){breakflag=1;}
    
            st2=st2<<shift; 

> //move the digit to the right position

bit manipulation

            st=st>>shift_st;
            shift_st  ;
            if(shift_st==7)
            {shift_st=0;}
    
            shift=shift-1;
            if(shift==0)
                shift=7;
    
            if(breakflag2!=1)
                {inbfile=st2|st;
                }else{ inbfile=st; }
            fwrite(&inbfile, sizeof(inbfile),1,output8to7);

write to the file

            if(feof(input))
            {
                inputsize= ftell(input);
           
                fseek(output8to7,8,SEEK_SET);
                fwrite(&inputsize,sizeof (inputsize),1,output8to7);
               // if(breakflag==1)
                break;}
    
        }
    
    }
*** uncompress method***

the problem is in this method

void uncompress8to7 (FILE *input_file_8to7  ,FILE *output_file_txt){
     char st;
     char st2;
    char check;
    char check2;
    char shift2 = 7;
    char shift_st = 0;
    char shift_helper=7;
    char shift_helper2=6;
    char sthelper;
    char sthelper2;
    char inbfile; // will contain the resullt of the asked new  binary lines comprees
    int breakflag = 0;
    int breakflag2 = 0;
    int cnt = -1;//this parameter will help to know when we dont need to move 1 back in the pointer of input file
    rewind(input_file_8to7);

    printf("%d",ftell(input_file_8to7));
    fseek(input_file_8to7,16,SEEK_SET);
    printf("\n%d",ftell(input_file_8to7));
    int a=0;
    while(1) {
    cnt  ;

    if(cnt>1) //
        {fseek(input_file_8to7,-1 ,SEEK_CUR);}
    printf("\n%d",ftell(input_file_8to7));

from that fgetc i get the bad file descriptor erorr

    check =  fgetc(input_file_8to7);


    if(ferror(input_file_8to7)){
        perror("eror by perror");
        printf("file erorr");}

  //  printf("\n%d",ftell(input_file_8to7));

    st = check;
    check2 = fgetc(input_file_8to7);
    st2 = check2;

    if(cnt<2)
        fseek(input_file_8to7,0,SEEK_SET);

    if(check2==EOF){
    breakflag2 = 1;
    }

    sthelper2=st2;
    sthelper2=sthelper2>>shift_helper2;

    st2=st2<<shift2;
    st2=st2>>shift2;

    sthelper=st;
    sthelper=sthelper>>shift_helper;
    sthelper=shift_helper<<shift_helper-1;
    st=st<<shift_st;// to make all zero after the msb
    st=st>>shift_st;// to make all zero after the msb


    shift_helper2--;
    if(shift_helper==-1)
        {shift_helper2=6;}

    shift_helper--;
    if(shift_helper==-1){
        shift_helper=7;
    }

    shift_st  ;
    if(shift_st==7)
        {shift_st=0;}

    shift2=shift2-1;
    if(shift2==0)
        shift2=7;

    if(breakflag2==1)
        {break;}
    if(cnt%7==0){
        inbfile=st;
    }else{
        inbfile=sthelper|st2;
    }

writing to the file

    fwrite(&inbfile,sizeof(inbfile),1,output_file_txt);

break the loop when we got to the end of file

    if(feof(input_file_8to7))
         { break;}

    }
}


***main***

int main(int argc, char **argv) {
    char* input=NULL;
    char* output8to7=NULL;
    input=argv[1];
    output8to7=argv[2];

open files

    FILE* inputfile = fopen(input, "r");
    if(inputfile==NULL)
    {
        printf("couldnt open input file ");
        exit(-1);
    }

    FILE* file8to7=fopen(output8to7, "wb");
    if(file8to7==NULL)
    {
        printf("couldnt open output file");
        printf(output8to7);
        exit(-1);
    }

compress

    compress8to7(inputfile,file8to7);

    FILE* file8to7input=fopen("exampleout.bin", "ab");

    FILE* output_file=fopen("UNoutput_file2.txt", "wb");
    if(output_file==NULL)
    {printf("couldnt open output file");
        exit(-1);
    }

    uncompress8to7(file8to7input,output_file);
   fclose(output_file);
    fclose(file8to7input);
    fclose(inputfile);
    fclose(file8to7);
    return 0;
}

CodePudding user response：

This is the code to open the file:

    FILE* file8to7input=fopen("exampleout.bin", "ab");

This opens it as an output file in append mode. You're trying to read from it in the uncompress8to7() function. You need to open it as in input file in read mode. Change that line to:

    FILE* file8to7input=fopen("exampleout.bin", "rb");