CODEWITHSUNDEEP
cmemory
user8081549
user8081549

Reputation:

Not able to remove specific bytes from buffer using memcpy

I m not able to truncate the buffer from the size what I want. I'm bit perplexed why I get wrong results? Even though my usage of memcpy() is right. I read the man page also. I'm doing something wrong here?

What I want to achieve:

Buff: 00 83 00 00 16 0a 44 00 00 00 00 bd 0e 8a 0c 61 01 13 51 24 ad 9a 0b 1c 0e ff ff 00

Copy the bytes which start from bd, that means 12th bytes till the end-

Desired Output should be:

bd 0e 8a 0c 61 01 13 51 24 ad 9a 0b 1c 0e ff ff 00

This is my code: I'm receiving the response from serial device and I need chop some bytes.

void rx_dev(transport, int addr, const void *buf, unsigned count) {
    uint8_t s[1024];
    uint8_t *p;

    memset (s, 0, sizeof(s));
    // This below function does only the hex parsing.
    hex_parse(s, sizeof(s), buf, count);
    printf("* %02x %s\n", addr, s);

    printf("Count: %zu\n", count);
    p = s;

    printf("p* %s\n", p);

    // I'm doing this check to avoid something greater than 14.
    if (count > 14) {
        memcpy(p, s+11, count-11);
        printf("*Trim:%s\n", p);
    }
}

EDIT: Added the more details

int hex_parse(char *out, unsigned size, const void *buf, unsigned count)
{
    const uint8_t *p = buf;
    unsigned i;
    int n = 0;

    if (count)
    {
        if (n + 2 < size)
        {
            out[n+0] = hexchars[(p[0] >> 4) & 15];
            out[n+1] = hexchars[p[0] & 15];
        }
        n += 2;
    }

    for (i = 1; i < count; i++) {
        if (n + 3 < size)
        {
            out[n+0] = ' ';
            out[n+1] = hexchars[(p[i] >> 4) & 15];
            out[n+2] = hexchars[p[i] & 15];
        }
        n += 3;
    }

    if (n < size)
        out[n] = '\0';
    else if (size)
        out[size-1] = '\0';

    return n;
}

My ouutput:

* 01 00 83 00 00 16 0a 44 00 00 00 00 bd 0e 8a 0c 61 01 13 51 24 ad 9a 0b 1c 0e ff ff 00

p* 00 83 00 00 16 0a 44 00 00 00 00 bd 0e 8a 0c 61 01 13 51 24 ad 9a 0b 1c 0e ff ff 00

Here I don't get the correct output, why is it prinitng 28bytes, this is not my desired result I wanted?

Trim: 16 0a 44 00 00 0 44 00 00 00 00 bd 0e 8a 0c 61 01 13 51 24 ad 9a 0b 1c 0e ff ff 00

Upvotes: 0

Views: 421

Answers (3)

Art
Art

Reputation: 20392

The source and destination in your memcpy call overlap. From the standard:

If copying takes place between objects that overlap, the behavior is undefined.

The standard function memmove is like memcpy but defined for overlapping objects, use that instead.

Upvotes: 2

alinsoar
alinsoar

Reputation: 15803

You can use memmove , as in memcpy the target and the source must not overlap each other, while in memmove it does not matter. So you can do

 int offset = 11,size = 28;
 memmove(buf, buf+off, size - off);

Upvotes: 2

MSalters
MSalters

Reputation: 179927

When you're calling memcpy, you're not copying the bytes from the buffer. You're copying the "hex_parsed" string. As that format uses 3 characters per byte (2 digits and a space), chopping 11 characters is chopping about 4 bytes.

Upvotes: 0

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