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libzigc: dirname #35755

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chrboesch wants to merge 2 commits from chrboesch/libzigc-dirname into master AGit
pull from: chrboesch/libzigc-dirname
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recreate for agit-test, see #32096 and also Coordinate interdependence. Demo and promote free and open source projects. Package curation. Meetups. Learning about computers.
Contributes to: #30978

recreate for agit-test, see #32096 and also [Coordinate interdependence. Demo and promote free and open source projects. Package curation. Meetups. Learning about computers. ](https://zsf.zulipchat.com/#narrow/channel/454446-ecosystem/topic/improving.20the.20contributer.20to.20maintainer.20pipeline/near/601899686) Contributes to: #30978
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The musl version returns "." and "/" as pointers to constant memory. If you try to modify the result of dirname in that case, you get a segfault:

char const* path = "//";
char* dirc = strdup(path);
dname = dirname(NULL);
dname[0] = ':'; /* segfaults */

In your zig implementation the non const pointer returned by dirname actually points to mutable memory, but the contents of
dot_buf and slash_bufcan be silently overriden which changes the behaviour of future calls to dirname.

I'm not sure which behaviour is better, but I would tend to either matching the musl implementation or setting dot_buf = .{ '.', 0 }; and slash_buf = .{ '/', 0 }; before returing pointers to dot_buf/slash_buf

The musl version returns "." and "/" as pointers to constant memory. If you try to modify the result of dirname in that case, you get a segfault: ```c char const* path = "//"; char* dirc = strdup(path); dname = dirname(NULL); dname[0] = ':'; /* segfaults */ ``` In your zig implementation the non const pointer returned by dirname actually points to mutable memory, but the contents of `dot_buf` and `slash_buf`can be silently overriden which changes the behaviour of future calls to dirname. I'm not sure which behaviour is better, but I would tend to either matching the musl implementation or setting `dot_buf = .{ '.', 0 };` and `slash_buf = .{ '/', 0 };` before returing pointers to `dot_buf`/`slash_buf`
@ -0,0 +26,4 @@
while(true):(i-=1){
if(ptr[i]!='/')break;
if(i==0)returnslash;
}
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why not:

while(ptr[i]=='/'):(i-=1){if(i==0)returnslash;}
why not: ```zig while (ptr[i] == '/') : (i -= 1) { if (i == 0) return slash; } ```
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readability

readability
chrboesch marked this conversation as resolved
johan0A left a comment
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I'd argue using @constCast instead of pointers to mutable globals would be better here. it matches what musl does and if a caller writes through the returned pointer it just faults instead of silently corrupting buffer for later calls.

so it would look like:

constdot:[1:0]c_char=.{'.'};constslash:[1:0]c_char=.{'/'};// ...return@constCast(&dot);return@constCast(&slash);
I'd argue using @constCast instead of pointers to mutable globals would be better here. it matches what musl does and if a caller writes through the returned pointer it just faults instead of silently corrupting buffer for later calls. so it would look like: ```zig const dot: [1:0]c_char = .{'.'}; const slash: [1:0]c_char = .{'/'}; // ... return @constCast(&dot); return @constCast(&slash); ```
@ -0,0 +14,4 @@
fndirname(s:?[*:0]c_char)callconv(.c)[*:0]c_char{
constdot:[*:0]c_char=@ptrCast(&dot_buf);
constslash:[*:0]c_char=@ptrCast(&slash_buf);
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no need for a @ptrCast here, this works:

vardot_buf:[1:0]c_char=.{'.'};varslash_buf:[1:0]c_char=.{'/'};exportfndirname(s:?[*:0]c_char)callconv(.c)[*:0]c_char{constdot:[*:0]c_char=&dot_buf;constslash:[*:0]c_char=&slash_buf;
no need for a `@ptrCast` here, this works: ```zig var dot_buf: [1:0]c_char = .{'.'}; var slash_buf: [1:0]c_char = .{'/'}; export fn dirname(s: ?[*:0]c_char) callconv(.c) [*:0]c_char { const dot: [*:0]c_char = &dot_buf; const slash: [*:0]c_char = &slash_buf; ```
chrboesch marked this conversation as resolved
@ -0,0 +21,4 @@
constptr=s.?;
if(ptr[0]==0)returndot;
vari=std.mem.len(@as([*:0]u8,@ptrCast(ptr)))-1;
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why the cast to [*:0]u8?

strlen is implemented like so in compiler_rt:

fnstrlen(s:[*:0]constc_char)callconv(.c)usize{returnstd.mem.len(s);}
why the cast to `[*:0]u8`? strlen is implemented like so in compiler_rt: ```zig fn strlen(s: [*:0]const c_char) callconv(.c) usize { return std.mem.len(s); } ```
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strlen has a non-optional pointer.

strlen has a non-optional pointer.
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strlen has a non-optional pointer. But I will do this instead:

const ptr = s orelse return @constCast(&dot);
var i = std.mem.len(ptr) - 1;
strlen has a non-optional pointer. But I will do this instead: ``` const ptr = s orelse return @constCast(&dot); var i = std.mem.len(ptr) - 1; ```
chrboesch marked this conversation as resolved
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I'd argue using @constCast instead of pointers to mutable globals would be better here.

agree

> I'd argue using @constCast instead of pointers to mutable globals would be better here. agree
chrboesch force-pushed chrboesch/libzigc-dirname from 9b04e03d8c
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@PinieP @johan0A thanks!

@PinieP @johan0A thanks!
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