55 lines
1.9 KiB
Python
55 lines
1.9 KiB
Python
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"""Encode valid C string literals from Python strings.
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If a character is not allowed in C string literals, it is either emitted
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as a simple escape sequence (e.g. '\\n'), or an octal escape sequence
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with exactly three digits ('\\oXXX'). Question marks are escaped to
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prevent trigraphs in the string literal from being interpreted. Note
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that '\\?' is an invalid escape sequence in Python.
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Consider the string literal "AB\\xCDEF". As one would expect, Python
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parses it as ['A', 'B', 0xCD, 'E', 'F']. However, the C standard
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specifies that all hexadecimal digits immediately following '\\x' will
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be interpreted as part of the escape sequence. Therefore, it is
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unexpectedly parsed as ['A', 'B', 0xCDEF].
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Emitting ("AB\\xCD" "EF") would avoid this behaviour. However, we opt
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for simplicity and use octal escape sequences instead. They do not
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suffer from the same issue as they are defined to parse at most three
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octal digits.
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"""
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import string
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from typing_extensions import Final
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CHAR_MAP: Final = [f"\\{i:03o}" for i in range(256)]
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# It is safe to use string.printable as it always uses the C locale.
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for c in string.printable:
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CHAR_MAP[ord(c)] = c
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# These assignments must come last because we prioritize simple escape
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# sequences over any other representation.
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for c in ('\'', '"', '\\', 'a', 'b', 'f', 'n', 'r', 't', 'v'):
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escaped = f'\\{c}'
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decoded = escaped.encode('ascii').decode('unicode_escape')
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CHAR_MAP[ord(decoded)] = escaped
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# This escape sequence is invalid in Python.
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CHAR_MAP[ord('?')] = r'\?'
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def encode_bytes_as_c_string(b: bytes) -> str:
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"""Produce contents of a C string literal for a byte string, without quotes."""
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escaped = ''.join([CHAR_MAP[i] for i in b])
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return escaped
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def c_string_initializer(value: bytes) -> str:
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"""Create initializer for a C char[]/ char * variable from a string.
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For example, if value if b'foo', the result would be '"foo"'.
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"""
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return '"' + encode_bytes_as_c_string(value) + '"'
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