CODEWITHSUNDEEP
c
Sam
Sam

Reputation: 407

Convert string decimal to hex decimal

How can I convert a string decimal to hex decimal and add them to char pointer? Why does the memcpy(value + 2, &value_value, value_length); not work, the result is 0x02 0x01 and not 0x02 0x01 0x4f.

#include <string.h> /* strlen */
#include <stdio.h> /* printf */
#include <stdint.h> /* int32_t */
#include <stdlib.h> /* strtol */

int main()
{
    char value_type = 0x02;
    char *value_value = "79"; /* Must convert to 4f */

    char value_length;
    int32_t num = strtol(value_value, NULL, 10);
    if (num < 255)
        value_length = 0x01;
    else if (num < 65536)
        value_length = 0x02;
    else if (num < 16777216)
        value_length = 0x03;
    else if (num < 4294967295)
        value_length = 0x04;

    char *value = malloc(2 + value_length);

    memcpy(value, &value_type, 1);
    memcpy(value + 1, &value_length, 1);
    memcpy(value + 2, &value_value, value_length);

    /* expectation: 0x02 0x01 0x4f */
    for (int i = 0; i < strlen(value); i++)
        printf("%02x\n", value[i]);
    return 0;
}

Upvotes: 0

Views: 279

Answers (2)

bukkojot
bukkojot

Reputation: 1530

You are using wrong variable:

memcpy(value + 2, &num, value_length);

Also, you must not trait your value as string, as far as it uses binary value. Change your code to:

    /* expectation: 0x02 0x01 0x4f */
    int dumplen=len+2;
    for (int i = 0; i < dumplen; i++)
        printf("%02x\n", value[i]);
    return 0;

Upvotes: 0

zerkms
zerkms

Reputation: 254926

memcpy(value + 2, &value_value, value_length);

this expression copies value_length bytes from &value_value.

Given that it's declared as

char *value_value

&value_value is read as a pointer-to-a-pointer-to-a-char. So effectively you're reading the pointer value.

How to solve:

memcpy(value + 2, &num, value_length);

Another problem with your code:

you're using strlen(value) while value is not a null-terminated C-string. It's just an array of bytes that you fill with your data.

How to fix:

don't use strlen in this case, you know the size of the array: 2 + value_length. For clarity you may put it to a separate variable.

Upvotes: 1

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