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Fix/security optimization #6

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Merged
frapank merged 2 commits into from
Jul 3, 2026
Merged

Fix/security optimization #6

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9c90be0
fix: dangling s2 pointer in concat when s2 aliases s1's buffer
frapank Jul 3, 2026
829bb46
add: amortized capacity growth and memchr-based dstrfind
frapank Jul 3, 2026
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42 changes: 35 additions & 7 deletions dstr.c
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Original file line number Diff line number Diff line change
Expand Up @@ -152,6 +152,14 @@ static inline void* _dstr_get_hdr_and_type(dstr s, enum dstrhd_type* t_out)
}
}

static inline size_t _dstr_grow_cap(size_t old_cap, size_t needed)
{
size_t grown = old_cap < 16 ? 16 : old_cap;
if (grown <= SIZE_MAX - grown / 2)
grown += grown / 2;
return grown > needed ? grown : needed;
}

// strfind
ssize_t dstrfind(dstr s, const char* needle)
{
Expand All @@ -164,10 +172,17 @@ ssize_t dstrfind(dstr s, const char* needle)
if (nlen > len)
return -1;

for (size_t i = 0; i <= len - nlen; i++) {
if (memcmp(&s[i], needle, nlen) == 0) {
return i;
}
size_t remaining = len - nlen + 1;
const char* cursor = s;

while (remaining > 0) {
const char* hit = memchr(cursor, needle[0], remaining);
if (!hit)
return -1;
if (memcmp(hit, needle, nlen) == 0)
return hit - s;
remaining -= (size_t)(hit - cursor) + 1;
cursor = hit + 1;
}

return -1;
Expand Down Expand Up @@ -351,11 +366,21 @@ static dstr _dstr_concat_impl(dstr s1,
size_t new_len = s1_len + s2_len;
if (cap < new_len + 1)
cap = new_len + 1;

uintptr_t s1_addr = (uintptr_t)s1;
uintptr_t s2_addr = (uintptr_t)s2;
size_t s1_cap = dstrcap(s1);
bool s2_aliases_s1 = s2_addr >= s1_addr && s2_addr - s1_addr < s1_cap;
size_t s2_offset = s2_aliases_s1 ? (size_t)(s2_addr - s1_addr) : 0;

dstr tmp = dstrreserve(s1, cap);
if (!tmp)
return NULL;
s1 = tmp;

if (s2_aliases_s1)
s2 = s1 + s2_offset;

uint8_t hdr_type = DSTRGETTYPE(s1);
void* hd = NULL;
switch (hdr_type) {
Expand Down Expand Up @@ -387,7 +412,8 @@ dstr dstrcat_base(dstr s1, const char* s2)
return s1;
size_t s1_len = dstrlen(s1);
size_t s2_len = strlen(s2);
return _dstr_concat_impl(s1, s1_len, s2, s2_len, s1_len + s2_len + 1);
size_t cap = _dstr_grow_cap(dstrcap(s1), s1_len + s2_len + 1);
return _dstr_concat_impl(s1, s1_len, s2, s2_len, cap);
}

dstr dstrcat_custom(dstr s1, const char* s2, size_t cap)
Expand Down Expand Up @@ -427,7 +453,8 @@ dstr dstrpush_custom(dstr s1, const char c, size_t cap)

dstr dstrpush_base(dstr s1, const char c)
{
return dstrpush_custom(s1, c, dstrcap(s1) + 1);
size_t cap = dstrcap(s1);
return dstrpush_custom(s1, c, _dstr_grow_cap(cap, cap + 1));
}

// strappend
Expand All @@ -437,7 +464,8 @@ dstr dstrappend_base(dstr s1, const dstr s2)
return s1;
size_t s1_len = dstrlen(s1);
size_t s2_len = dstrlen(s2);
return _dstr_concat_impl(s1, s1_len, s2, s2_len, s1_len + s2_len + 1);
size_t cap = _dstr_grow_cap(dstrcap(s1), s1_len + s2_len + 1);
return _dstr_concat_impl(s1, s1_len, s2, s2_len, cap);
}

dstr dstrappend_custom(dstr s1, const dstr s2, size_t cap)
Expand Down
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