Possible values:

• a - NUL-padded string
• A - SPACE-padded string
• h - Hex string, low nibble first
• H - Hex string, high nibble first
• c - signed char
• C - unsigned char
• s - signed short (always 16 bit, machine byte order)
• S - unsigned short (always 16 bit, machine byte order)
• n - unsigned short (always 16 bit, big endian byte order)
• v - unsigned short (always 16 bit, little endian byte order)
• i - signed integer (machine dependent size and byte order)
• I - unsigned integer (machine dependent size and byte order)
• l - signed long (always 32 bit, machine byte order)
• L - unsigned long (always 32 bit, machine byte order)
• N - unsigned long (always 32 bit, big endian byte order)
• V - unsigned long (always 32 bit, little endian byte order)
• q - signed long long (always 64 bit, machine byte order)
• Q - unsigned long long (always 64 bit, machine byte order)
• J - unsigned long long (always 64 bit, big endian byte order)
• P - unsigned long long (always 64 bit, little endian byte order)
• f - float (machine dependent size and representation)
• g - float (machine dependent size, little endian byte order)
• G - float (machine dependent size, big endian byte order)
• d - double (machine dependent size and representation)
• e - double (machine dependent size, little endian byte order)
• E - double (machine dependent size, big endian byte order)
• x - NUL byte
• X - Back up one byte
• Z - NUL-padded string
• @ - NUL-fill to absolute position