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createWindowsEnvBlock: Reduce NUL terminator count to only what's required

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This code previously added 4 NUL code units, but that was likely due to a misinterpretation of this part of the CreateProcess documentation:

> A Unicode environment block is terminated by four zero bytes: two for the last string, two more to terminate the block.

(four zero *bytes* means *two* zero code units)

Additionally, the second zero code unit is only actually needed when the environment is empty due to a quirk of the CreateProcess implementation. In the case of a non-empty environment, there always ends up being two trailing NUL code units since one will come after the last environment variable in the block.
This commit is contained in:
Ryan Liptak 2025-03-17 17:53:12 -07:00
parent 2a4e06bcb3
commit 423761bb6d
1 changed files with 10 additions and 7 deletions
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17
lib/std/process.zig
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@ -2036,7 +2036,8 @@ test createNullDelimitedEnvMap {
pub fn createWindowsEnvBlock(allocator: mem.Allocator, env_map: *const EnvMap) ![]u16 {
// count bytes needed
const max_chars_needed = x: {
var max_chars_needed: usize = 4; // 4 for the final 4 null bytes
// Only need 2 trailing NUL code units for an empty environment
var max_chars_needed: usize = if (env_map.count() == 0) 2 else 1;
var it = env_map.iterator();
while (it.next()) |pair| {
// +1 for '='
@ -2060,12 +2061,14 @@ pub fn createWindowsEnvBlock(allocator: mem.Allocator, env_map: *const EnvMap) !
}
result[i] = 0;
i += 1;
result[i] = 0;
i += 1;
result[i] = 0;
i += 1;
result[i] = 0;
i += 1;
// An empty environment is a special case that requires a redundant
// NUL terminator. CreateProcess will read the second code unit even
// though theoretically the first should be enough to recognize that the
// environment is empty (see https://nullprogram.com/blog/2023/08/23/)
if (env_map.count() == 0) {
result[i] = 0;
i += 1;
}
return try allocator.realloc(result, i);
}