size_t scan_asn1derlengthvalue(const char *src,size_t len,unsigned long long *dest);
scan_asn1derlengthvalue decodes an unsigned integer in ASN.1 DER length encoding from a memory area holding binary data. It writes the decode value in dest and returns the number of bytes it read from src.
scan_asn1derlength never reads more than len bytes from src. If the sequence is longer than that, or the memory area contains an invalid sequence, scan_asn1derlength returns 0 and does not touch dest.
The length of the longest spec-compliant ASN.1 DER length is 128 bytes, but this implementation will return an error if the value does not fit into the target integer type. In practice the largest sequence is sizeof(*dest)+1.
This implementation will reject values that are not encoded in the minimum amount of bytes.
If you need to decode the length value so you can parse actual ASN.1 tag/length/value structures, please consider using scan_asn1derlength instead, as it will do additional checking for you.