Question

Is there any standard C function that converts from hexadecimal string to byte array?
I do not want to write my own function.

Answer 1

As far as I know, there’s no standard function to do so, but it’s simple to achieve in the following manner:

#include <stdio.h>

int main(int argc, char **argv) {
    const char hexstring[] = "DEadbeef10203040b00b1e50", *pos = hexstring;
    unsigned char val[12];

     /* WARNING: no sanitization or error-checking whatsoever */
    for (size_t count = 0; count < sizeof val/sizeof *val; count++) {
        sscanf(pos, "%2hhx", &val[count]);
        pos += 2;
    }

    printf("0x");
    for(size_t count = 0; count < sizeof val/sizeof *val; count++)
        printf("%02x", val[count]);
    printf("\n");

    return 0;
}

---

Edit

As Al pointed out, in case of an odd number of hex digits in the string, you have to make sure you prefix it with a starting 0. For example, the string "f00f5" will be evaluated as {0xf0, 0x0f, 0x05} erroneously by the above example, instead of the proper {0x0f, 0x00, 0xf5}.

Amended the example a little bit to address the comment from @MassimoCallegari