Hex to char array in C

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You can’t fit 5 bytes worth of data into a 4 byte array; that leads to buffer overflows.

If you have the hex digits in a string, you can use sscanf() and a loop:

#include <stdio.h>
#include <ctype.h>

int main()
{
    const char *src = "https://stackoverflow.com/questions/1557400/0011223344";
    char buffer[5];
    char *dst = buffer;
    char *end = buffer + sizeof(buffer);
    unsigned int u;

    while (dst < end && sscanf(src, "%2x", &u) == 1)
    {
        *dst++ = u;
        src += 2;
    }

    for (dst = buffer; dst < end; dst++)
        printf("%d: %c (%d, 0x%02x)\n", dst - buffer,
               (isprint(*dst) ? *dst : '.'), *dst, *dst);

    return(0);
}

Note that printing the string starting with a zero-byte requires care; most operations terminate on the first null byte. Note that this code did not null-terminate the buffer; it is not clear whether null-termination is desirable, and there isn’t enough space in the buffer I declared to add a terminal null (but that is readily fixed). There’s a decent chance that if the code was packaged as a subroutine, it would need to return the length of the converted string (though you could also argue it is the length of the source string divided by two).

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