scan_asn1dertag man page
scan_asn1dertag — decode an unsigned integer from ASN.1 DER length encoding
size_t scan_asn1dertag(const char *src,size_t len,unsigned long long *dest);
scan_asn1dertag decodes an unsigned integer in ASN.1 DER tag 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_asn1dertag never reads more than len bytes from src. If the sequence is longer than that, or the memory area contains an invalid sequence, scan_asn1dertag returns 0 and does not touch dest.
The length of the longest ASN.1 DER length sequence is 128 bytes. In practice the largest sequence is sizeof(*dest)+1.