I am trying to learn how to write modern programs. I want to store binary data in strings and then output it. Why? For example I want to read a whole file to a string and then output it to stdout.
How can I do it? Printf and puts stop outputting data after the first \0
And what should I use to store filenames so I can work with international characters? wchar_t?
Let's say you have a binary file. Let's say you want to read 8 bytes from the file.
FILE *fp = fopen("a", "rb");
if(fp == NULL)
return;
unsigned char bytes[8];
if(fread(bytes, sizeof bytes / sizeof *bytes, 1, fp) != 1)
{
fprintf(stderr, "could not read 8 bytes\n");
return;
}
// printing the bytes:
for(size_t i = 0; i < sizeof bytes / sizeof *bytes; ++i)
printf("%02X ", bytes[i]);
puts("");
fclose(fp);
I run this code reading the compiled binary (filename is a) of this same code, the first 8
bytes are:
7F 45 4C 46 02 01 01 00
As you can see there are bytes that have no representation and if I would print that as a string, this would be the result:
which is less helpful, here I was lucky that there were at least printable integers.
And if you want to have also the printable characters printed as as well (like hex editors do):
size_t n;
// yes I swapped the number of items and the number of bytes here
// for reading the whole file
while((n = fread(bytes, 1, sizeof bytes / sizeof *bytes, fp)))
{
size_t i;
for(i = 0; i < n; ++i)
printf("%02X ", bytes[i]);
// if less than array size bytes read, to align output
printf("%*s", (int) ((sizeof bytes / sizeof *bytes) - i)*3, "");
printf("%10s |", "");
for(i = 0; i < n; ++i)
{
if(isprint(bytes[i]))
printf("%c", bytes[i]);
else
printf(".");
}
// same here as above
printf("%*s", (int) ((sizeof bytes / sizeof *bytes) - i), "");
puts("|");
}
This outputs:
7F 45 4C 46 02 01 01 00 |.ELF....|
00 00 00 00 00 00 00 00 |........|
03 00 3E 00 01 00 00 00 |..>.....|
A0 07 00 00 00 00 00 00 |........|
40 00 00 00 00 00 00 00 |@.......|
F0 23 00 00 00 00 00 00 |.#......|
00 00 00 00 40 00 38 00 |[email protected].|
09 00 40 00 23 00 22 00 |..@.#.".|
...
00 6C 69 62 63 2E 73 6F |.libc.so|
2E 36 00 66 6F 70 65 6E |.6.fopen|
00 70 75 74 73 00 5F 5F |.puts.__|
73 74 61 63 6B 5F 63 68 |stack_ch|
6B 5F 66 61 69 6C 00 70 |k_fail.p|
75 74 63 68 61 72 00 70 |utchar.p|
72 69 6E 74 66 00 66 63 |rintf.fc|
6C 6F 73 65 00 5F 5F 63 |lose.__c|
74 79 70 65 5F 62 5F 6C |type_b_l|
...
39 01 00 00 00 00 00 00 |9.......|
00 00 00 00 00 00 00 00 |........|
01 00 00 00 00 00 00 00 |........|
00 00 00 00 00 00 00 00 |........|
fread(bytes, sizeof bytes / sizeof *bytes, 1, fp) != 1 may be better as fread(bytes, sizeof bytes, 1, fp) != 1.
sizeof bytes because sizeof *bytes is 1 anyway, but I wanted to have the correct way of having the number of elements of an array regardless of the type. If you think that the answer is more clear with having only sizeof bytes, let me know so that I can update the answer.
I want to store binary data in strings and then output it.
Once you have your binary data in a storage (in our case it is char bin[])
you can make char ascii string out of it using sprintf :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(){
int i;
char bin[] = { 0, 1 , '1', '2', '3' , 0 , 7 };
char str[256];
char *buf2 = str;
size_t size = sizeof(bin)*2;
for(i= 0; i < size; i++ )
{
buf2 += sprintf(buf2, "%02X", bin[i]);
}
str[i] = 0; // terminate your string
printf("STRING: %s", str);
return 0;
}
Output:
STRING: 00013132330007
uint8_tarrays orunsigned chararrays, but do not treat them as strings. Also when you read a binary file, many bytes are just not printable and if you print them, you will get "garbage" on the screen. It's better to print the hex representation of the bytes.printf("%.*s", str_len, str_ptr);fwriteto write arbitrary data to stdout as long as you remember how much data you have (because, again, it's not a string...there's noarbitrarydatalenfunction, and there can't be).printffor that:printf("%02x\n", val);NUL, binary data does not have to fit that format at all.