Convert Hexadecimal values of an array into String values of another array

Question

I am trying to convert the hexadecimal values of an array into characters of another one. Here is the only way, I could find but not working.

Thank you for your help !

char tmp[] = {0x81, 0x00, 0x00, 0x00, 0x12, 0x05};
char new[12];

for (int i = 0; i < 6; i++) {
    printf(" %x", tmp[i]);
    sprintf(new + i, "%x", tmp[i]);
}
for (int i = 0; i < 12; i++) {
        printf(" %c", new[i]);
}
printf("new: %s\n", new);

Here is the output :

 81 0 0 0 12 5
 8 0 0 0 1 5
new: 800015

So, it lacks some bytes ...

What do you mean "characters". The ASCII representation of hex values? But you only have 6 bytes so that won't fit. You are also using sprintf for writing into the same array as the target... — Lundin
– Lundin, Commented Feb 7, 2019 at 14:14
sprintf(tmp + i, "%x", tmp[i]); will lead to undefined behavior. You attempt to write a multi-character string into a single character. Not to mention that using tmp[i] as both an argument and as destination is also UB as per the C specification. — Some programmer dude
– Some programmer dude, Commented Feb 7, 2019 at 14:15
For binary to hex ASCII conversion, there are thousands of snippets already posted on the internet, such as: stackoverflow.com/a/54129120/584518 — Lundin
– Lundin, Commented Feb 7, 2019 at 14:16
Your edit doesn't change much: You're still writing a multi-character string into the single character new[i]. Also note that if char is signed (it may be signed or unsigned, it's an implementation (compiler) detail) then 0x81 would lead to sign extension as it's considered a negative value. Use uint8_t from <stdint.h> for generic unsigned bytes. And please learn about two's complement (which is the most common way to handle negative values). — Some programmer dude
– Some programmer dude, Commented Feb 7, 2019 at 14:21

bruno · Accepted Answer · 2019-02-07 14:28:57Z

3

probably

char tmp[] = {0x81, 0x00, 0x00, 0x00, 0x12, 0x05};
char new[6];

must be

int tmp[] = {0x81, 0x00, 0x00, 0x00, 0x12, 0x05};
char new[6*2+1];

and

sprintf(tmp + i, "%x", tmp[i]);

must be

sprintf(new + 2*i, "%02x", tmp[i]);

and

for (int i = 0; i < 6; i++) {
        printf(" %c", new[i]);
}

must be

for (int i = 0; i < 6*2; i++) {
        printf(" %c", new[i]);
}

Execution :

/tmp % ./a.out
 81 0 0 0 12 5 8 1 0 0 0 0 0 0 1 2 0 5new: 810000001205

Under valgrind :

/tmp % valgrind ./a.out
==15557== Memcheck, a memory error detector
==15557== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==15557== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==15557== Command: ./a.out
==15557== 
 81 0 0 0 12 5 8 1 0 0 0 0 0 0 1 2 0 5new: 810000001205
==15557== 
==15557== HEAP SUMMARY:
==15557==     in use at exit: 0 bytes in 0 blocks
==15557==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==15557== 
==15557== All heap blocks were freed -- no leaks are possible
==15557== 
==15557== For counts of detected and suppressed errors, rerun with: -v
==15557== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 6)

edited Feb 7, 2019 at 14:28

answered Feb 7, 2019 at 14:16

bruno

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6 Comments

bruno Over a year ago

@user694733 you are right, I see after, I edited my answer

Lundin Over a year ago

There's still IMO no reason to use such a cumbersome function as sprintf for something fairly trivial like this.

bruno Over a year ago

@Lundin lol I agree, I just corrected the errors from the original definition ^^

Gajuboy Over a year ago

@Lundin How you do without sprintf ?

Lundin Over a year ago

@GaelG I posted a link to a snippet as comment below your question.

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