Trying to copy contens of a file multiple times into another file in c

Question

I am trying to write a program in c that copies the contents of a file into another one multiple times but something is off. Some weird characters appear, and it only copies once.

c code

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

char* laod_with_correct_size(char* file_location, int size) {

    char* buffer=NULL;
    FILE* file;
    size=0;
    int len;

    file=fopen(file_location,"rb");
    if(file==NULL) {
        fclose(file);
        return NULL;
    }

    fseek(file,0,SEEK_END);

    len=ftell(file);

    if(len<1) {
        fclose(file);
        return NULL;
    }
    
    rewind(file);

    buffer=(char*) malloc(len);
    if(buffer==NULL) {
        fclose(file);
        return NULL;
    }

    if(fread(buffer,1,len,file)!=(size_t)len) {
        free(buffer);
        fclose(file);
        return NULL;
    }
    fclose(file);
    size=len;
    printf("Size in fucntion is %d\n",len);    

    return buffer;
}

int get_size(char* filepath) {
    FILE* f;
    int len;
    f=fopen(filepath,"rb");
    fseek(f,0,SEEK_END);
    len=ftell(f);
    fclose(f);
    printf("TOTAL SIZE THAT SHOULD BE IN THE FUCNTION IS %d",len);
}

 int write_correctly(char* file,char* buffer,int len) {
     // printf("len size is %d\n",len);
     int file_descriptor = open(file,O_APPEND || O_CREAT);
     int len_to_use=get_size(file);
     int size=write(file_descriptor,buffer,len_to_use);
     printf("Second size in fucntion is %d\n",size);    
     close(file_descriptor);

     printf("SIZE OF SIZE+1 IS %d and SIZE OF LEN IS %d\n",size+1,len);
     if(size!=len) {
         return-1;

     } else {
         return size;

     }
 }



int main(int argc,char** argv) { 
    int size=0;
    size=get_size(argv[1]);
    char* buffer=laod_with_correct_size(argv[1],size);
    printf("Size of %s is %d\n",argv[1],size);
    // if(write_correctly(argv[2],buffer,size)<0) {
    //     printf("Couldn't write\n");
    // }
    write_correctly(argv[2],buffer,size);
    write_correctly(argv[2],buffer,size);
    write_correctly(argv[2],buffer,size);
    write_correctly(argv[2],buffer,size);
    

    return 0;
}

first file(to copy from)

salut
buna
alo

second file(to copy to)

salut
buna
alo
\00\00\00\00\00\00\00\00\00Q\F7\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00

My pointers are rusty but I hope I didn't make any huge mistakes.

Minor point: you don't need to call ```fclose()``` when ```fopen()``` fails. Calling ```fclose()``` with an invalid stream pointer is undefined behavior. — sj95126, Oct 09 '21 at 14:44

Andreas Wenzel · Answer 1 · 2021-10-09T15:09:55.047

The function get_size is declared to return a value of type int, but never returns a value.

By using these non-existant return values in the functions main and write_correctly, your program is invoking undefined behavior.

Also, it does not make sense that in the function write_correctly, you are attempting to write an amount of data equal to the size of the already existing output file. Instead, you should always write the size of the buffer, which is supposed to be the size of the input file.

You may find it easier to understand what is going on in your program if you run it line by line in a debugger, while monitoring the values of all variables.

Trying to copy contens of a file multiple times into another file in c

1 Answers1