[C/C++] Help with fseek, fwrite

Discussion in 'Mixed Languages' started by Mazzif, Dec 12, 2013.

  1. Mazzif

    Mazzif Elitebook Pwner

    Oct 18, 2013
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    #1 Mazzif, Dec 12, 2013
    Last edited by a moderator: Apr 20, 2017
    hex1.png
    (Corrupted on top, original on bottom)

    I need a little help with this. I'm going outside my comfort zone (I have never worked with C/C++) here and working with a DOS 16Bit Compiler to produce an exe that will run in native DOS. I need to be able to open another exe file for editing as binary and write data to a couple specific locations and save the edited file. I can do all this, but what is happening is other parts of the file are becoming corrupted. I accurately write my data, and its of the correct length, but the file corrupts. This project requires DOS, and cannot be run from within Windows.

    Here is my CPP

    Code:
    #include <stdio.h>
    #include <string.h>
    
    
    int main(void)
    
    
    {
        FILE *key;
        key=fopen ("Testfile.exe","r+b");
    
    
        char test1[100];
        char test2[100];
        printf("Test data to input:");
        fgets(test1, sizeof test1, stdin);
        //printf("Your input: %s", test1);
        printf("Second test data to input:");
        fgets(test2, sizeof test2, stdin);
        //printf("Your input: %s", test2);
        
        //Test1 24524  Test2 24583 and 24890
    
    
    fseek (key,24523,SEEK_SET);  //file offset location to begin write
    fwrite (test1,1,sizeof(test1),key);
    
    
    fseek (key,24582,SEEK_SET); //file offset location to begin write
    fwrite (test2,1,sizeof(test2),key);
    
    
    fseek (key,24889,SEEK_SET); //file offset location to begin write
    fwrite (test2,1,sizeof(test2),key);
    
    
    fclose(key);
    printf ("Finished");
    return(0);
    }
    
    Any suggestions? Does anything jump out and shout "Hey, hes doing this all wrong!"?

    Im using Open Watcom Compiler with DOS 16 Bit Target. My exe runs just fine, but again, Testfile.exe gets corrupted.
     
  2. Mazzif

    Mazzif Elitebook Pwner

    Oct 18, 2013
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    #2 Mazzif, Dec 13, 2013
    Last edited by a moderator: Apr 20, 2017
    (OP)
    For others looking to do the same, here is how I worked around it

    Code:
    #include<stdio.h>
    #include <string.h>
    
        int main(void)
    
        {
        FILE *key;
        key=fopen ("test.exe","r+b");
        char test1[10];
        char test2[32];
        printf("Input Test1 data:");
        scanf ("%10s",test1);  //only read 10 Chars
        printf("Input test2 data:");
        scanf ("%32s",test2); //only read 32 Chars
        fseek (key,24523,SEEK_SET); //file offset location to begin write
        fputs (test1,key);
        fseek (key,24582,SEEK_SET); //file offset location to begin write
        fputs (test2,key);
        fseek (key,24889,SEEK_SET); //file offset location to begin write
        fputs (test2,key);
        fclose(key);
        printf ("Finished");
        return(0); }
     
  3. vanelle

    vanelle MDL Addicted

    Sep 22, 2014
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    #3 vanelle, Feb 15, 2016
    Last edited by a moderator: Apr 20, 2017
    sorry for the old thread, but mybe somebody is interested in the solution

    sizeof() return the size of the allocated memory (in your case 100)!!
    but you need the size of the string ;)

    you must use strlen()
    note:
    in case your compiler allocate memory and write Null to the buffer -> all is ok
    in case your compiler allocate only memory you must fill the memory with Null !!!!

    char test1[100];
    char test2[100];
    memset (test1,0,sizeof(test1));
    memset (test2,0,sizeof(test2));

    Code:
    #include <stdio.h>
    #include <string.h>
    
    
    int main(void)
    
    
    {
        FILE *key;
        key=fopen ("Testfile.exe","r+b");
    
    
        char test1[100];
        char test2[100];
    
    memset (test1,0,sizeof(test1));  // more robust (depend on compiler)
    memset (test2,0,sizeof(test2));  // more robust (depend on compiler)
    
        printf("Test data to input:");
        fgets(test1, sizeof test1, stdin);
        //printf("Your input: %s", test1);
        printf("Second test data to input:");
        fgets(test2, sizeof test2, stdin);
        //printf("Your input: %s", test2);
        
        //Test1 24524  Test2 24583 and 24890
    
    
    fseek (key,24523,SEEK_SET);  //file offset location to begin write
    //fwrite (test1,1,sizeof(test1),key);
    fwrite (test1,1,strlen(test1),key);
    
    fseek (key,24582,SEEK_SET); //file offset location to begin write
    //fwrite (test2,1,sizeof(test2),key);
    fwrite (test2,1,strlen(test2),key);
    
    fseek (key,24889,SEEK_SET); //file offset location to begin write
    //fwrite (test2,1,sizeof(test2),key);
    fwrite (test2,1,strlen(test2),key);
    
    fclose(key);
    printf ("Finished");
    return(0);
    }
    
     
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