使用scanf接受用户输入的十六进制指令并将其保存在char中。 （C）

Question

So, I am building a program that should modify a specific number of bytes in a binary. You first input the binary, then the offset from which point you want the hex instructions to be replaced and then the bytes themselves.

Example: I load example.exe I specify offset 0 (beginning of the file) I specify hex instructions: 4D FF 33 FD FE The first 5 instructions of the file should be replaced with the ones I provided. I use fwrite to do the modifications and scanf to get the offset and the hex instructions, however, I can't find a way to actually store them as hex. The fwrite actually writes 4D FF 33 FD FE as text inside the binary file instead of hex. I assume I save them wrong in the char in the first place. I am new to C, so what I found online didn't really help.

Here's my code:

      scanf("%ld",&offset_ed);//get the offset from the user.
      fseek(f, offset_ed, SEEK_SET);
      printf("Specify HEX bytes to be written to the binary:  ");
      scanf("%s\n", &hexes);
      fwrite (hexes , sizeof(char), sizeof(hexes), f);
      fclose (f);

Where hexes is a char hexes;

Answer 1

General idea below.

Do not use scanf() , Use fgets() for reading a line of user input.

Error checking omitted (noted with // **)

 char buf[100];   // Use some reasonable upper bound of expected user input.

 puts("prompt for input");
 fgets(buf, sizeof buf, stdin);     // **
 offset_ed = strtol(buf, NULL, 10); // **
 fseek(f, offset_ed, SEEK_SET);     // **

 puts("prompt for input");
 fgets(buf, sizeof buf, stdin);  // **
 size_t byte_count = 0;
 unsigned char hexes[(sizeof buf)/2] = 0;  // place to save the "hex bytes"
 int n = 0;
 unsigned byte; 

Now parse the string looking for hex input.

 // Use "%n" to save the offset of the scan to later update `p`
 // Robust code would use `strtol()` for its better error handling
 char *p = buf;
 while (sscanf(p, "%x %n", &byte, &n) == 1) {
   if (byte > 0xFF) Handle_OutOfRange();
   hexes[byte_count] = byte;
   p += n;  
 }  

 if (byte_count == 0) Handle_EmptyUserInput();
 if (*p) Handle_ExtraJunkInUserInput();
 fwrite (hexes , byte_count, sizeof hexes[0], f); // **

---

OP had scanf("%s\\n",... . The "\\n" is a problem. scanf() will block until the user entered some non-white-space after the expected input. Avoid that.

The problem can be handled with scanf() , yet it is ugly. Effectively, code needs to look for the '\\n' somewhere after multiple "hex" bytes using scanf("%*1[^\\n]") or the like. scanf() is not the best tool for the overall coding goal here. scanf() is rarely the sharpest tool in the shed.

Answer 2

You have got the hexes as a string so you need to convert each of them to unsigned chars(a byte) before you can write them to a file.

#include <string.h>
#include <stdio.h>
#define ishexchar(c) (c>='A' && c<='F') || (c>='a' && c<='f')
#define toupper(c) c - 32
#define isupper(c) c>='A' && c<='F'

unsigned char hexconv(const char* str, int size){
    unsigned char ret = 0;
    for(int i=0; i<size; i++){
        if(ishexchar(str[i])){
            char c = (isupper(str[i])) ? str[i] : toupper(str[i]);
            ret = (ret*16) + (c - 'A') + 10;
        }else{
            ret = (ret*16) + (str[i] - '0');
        }
    }
    return ret;
}

int main(){
    printf("%x\n",hexconv("4b", 2));
}

The code above might need some modification for your needs but it compiles and works.