read from text file and write in a binary or hex file

Question

Would please somebody tell me why i constantly get coredump segmentation? when i choose n=1 work correctly but the problem is n=2 and n=3 but i have no idea why.I want to read a text file and return a text, binary or hex file.

I edited the code and i compile it and i get just one warning which is referring to %x but i do not care about it cause first i have to solve n==2. Now I do not have coredump error. it seems there is no error but my second file is empty.

int main(int argc, char *argv[])
{    
    if (argc != 3){
        printf("Use this scheme: ./executabel read_file write_file -option(-b,-t,-x)\n");
        exit(EXIT_FAILURE);
    }
    
    int fd1=open(argv[1], O_RDONLY);
        int fd2=open(argv[2], O_WRONLY | O_CREAT);
        char buffer[1024];
        int rd_count = 0;
        int wr_count = 0;
        int n;
        FILE * fpw;
        fpw = fdopen (fd2, "w");
        
        if (fd1 == -1){
        printf("Failed to open the read-file.\n");
        exit(EXIT_FAILURE);
    }
    
    if (fd2 == -1){
        printf("Failed to open the write-file.\n");
        exit(EXIT_FAILURE);
    }
    
    printf("Please choose one:\n");
    printf("1. –t forces the output file to be a text one.\n");
    printf("2. –b forces the output file to be binary.\n");
    printf("3. –x forces the output file to be a hexadecimal representation.\n");
    scanf("%d", &n);
    
    
    while ((rd_count = read(fd1, (void*) buffer, 1024)) > 0){
        int bytes_wr = 0;
        while (bytes_wr != rd_count){
            if (n==1){
                    wr_count = write(fd2, (void*) (buffer+bytes_wr), rd_count-bytes_wr);
                }else if (n==2){
                    wr_count = fwrite( (void*) (buffer+bytes_wr), sizeof(float), rd_count-bytes_wr, fpw);
                }else{
                    fprintf(fpw,"%x",(void*) (buffer+bytes_wr));
                }
            if (wr_count < 0) {
                perror("Failed to write to file");
                exit(EXIT_FAILURE);
            } else {
                bytes_wr += wr_count;
            }
        }
        }

        if (rd_count < 0) {
            perror("Failed to read from file");
            exit(EXIT_FAILURE);
        }
    

    close(fd1);
        close(fd2);
        chmod(argv[2],777);
    
        exit(EXIT_SUCCESS);
        
        
}

By the way this is the warning

mahsa@mahsa-G53SW:~$ gcc -o ex4.o ex4.c -Wall -Wextra
ex4.c: In function ‘main’:
ex4.c:55:23: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 3 has type ‘void *’ [-Wformat=]
   55 |         fprintf(fpw,"%x",(void*) (buffer+bytes_wr));
      |                      ~^  ~~~~~~~~~~~~~~~~~~~~~~~~~
      |                       |  |
      |                       |  void *
      |                       unsigned int
      |                      %p

I change this line

 fprintf(fpw,"%x",(void*) (buffer+bytes_wr));

to this

fprintf(fpw,"%02x",buffer[bytes_wr]);

now i do not have even warning. it seems everything works correctly but when i open the second file is empty.

Answer 1

You have multiple issues in your code:

1. The signature for main has to be int main(void) or int main(int argc, char *argv[]).

2. You access your buffer out of bounds.

wr_count = fwrite( (void*) (buffer+bytes_wr), sizeof(float), rd_count-bytes_wr, fd2);

When you start with bytes_wr being 0 and you have a full buffer (rd_count holds 1024) you will write 1024 * sizeof(float) bytes while your buffer is only 1024 bytes large. Accessing any memory location after these 1024 bytes causes undefined behaviour.

3. You have incorrect types for your files: Initially pointed out by Andreas Wenzel.

    int fd2=open(argv[2], O_WRONLY | O_CREAT);
...
    wr_count = fwrite( (void*) (buffer+bytes_wr), sizeof(float), rd_count-bytes_wr, fd2);
...
    fprintf(fd2,"%x",(void*) (buffer+bytes_wr));

Functions fwrite and fprintf require a parameter of type FILE* to identify the file. You pass a file descriptor of type int. When the called function will try to dereference the expected pointer you cause undefined behaviour.

There are two sets of I/O functions:

• fopen, fprintf, fscanf, fflush, fwrite, fclose etc.
• open, write, close etc. One set takes FILE* while the other takes int. Don't mix that.

Your compiler should warn you about that. If that didn't happen you need to turn up your warnings. For GCC you can use -Wall -Wextra

4. You don't print hex dump correctly:

    fprintf(fd2,"%x",(void*) (buffer+bytes_wr));

You pass an address which is not correct here. You probably want to pass a single byte to fprintf. This would be the way to do it:

    fprintf(fd2,"%02x", buffer[bytes_wr]);

Also note the format string containing 02 to specify the width of the field. Without this you could end up with 1 digit and would not be able to see bye boundaries in the output. You should also change buffer to unsigned char to prevent sign extension.