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XUtil: const-correctnes for strchrnul #1916

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Merged
BenBE merged 3 commits into from
Feb 24, 2026
Merged

XUtil: const-correctnes for strchrnul #1916

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898c9c9
Enforce const-correctness for String_strchrnul
BenBE Feb 24, 2026
6b212fb
Properly check snprintf return value
BenBE Feb 24, 2026
08f7356
Properly advance pointer for write location
BenBE Feb 24, 2026
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4 changes: 2 additions & 2 deletions XUtils.h
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Original file line number Diff line number Diff line change
Expand Up @@ -86,11 +86,11 @@ ATTR_NONNULL ATTR_MALLOC
char* String_readLine(FILE* fp);

ATTR_NONNULL ATTR_RETNONNULL
static inline char* String_strchrnul(const char* s, int c) {
static inline const char* String_strchrnul(const char* s, int c) {
#ifdef HAVE_STRCHRNUL
return strchrnul(s, c);
#else
char* result = strchr(s, c);
const char* result = strchr(s, c);
if (result)
return result;
return strchr(s, '\0');
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19 changes: 14 additions & 5 deletions linux/LinuxProcessTable.c
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Expand Up @@ -18,6 +18,7 @@ in the source distribution for its full text.
#include <limits.h>
#include <math.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
Expand Down Expand Up @@ -1040,31 +1041,39 @@ static void LinuxProcessTable_readCGroupFile(LinuxProcess* process, openat_arg_t
char output[PROC_LINE_LENGTH + 1];
output[0] = '\0';
char* at = output;
int left = PROC_LINE_LENGTH;
size_t left = PROC_LINE_LENGTH;
while (!feof(file) && left > 0) {
char buffer[PROC_LINE_LENGTH + 1];
const char* ok = fgets(buffer, PROC_LINE_LENGTH, file);
if (!ok)
break;

char* group = buffer;
const char* group = buffer;
for (size_t i = 0; i < 2; i++) {
group = String_strchrnul(group, ':');
if (!*group)
break;
group++;
}

char* eol = String_strchrnul(group, '\n');
*eol = '\0';
const char* eol = String_strchrnul(group, '\n');
char* eol_w = &buffer[eol - buffer];
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@Explorer09 Explorer09 Feb 24, 2026 •
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How does this &buffer[eol - buffer]; not trigger warnings at all?

Update: I have a better idea to the problem:

size_t eol_offset = (size_t)(eol - (const char*)buffer);
buffer[eol_offset] = '\0';

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@BenBE BenBE Feb 24, 2026 •
edited
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Based on my reading of the ISO C99 standard:

  • &p[i] returns a pointer to the ith element in p using the type of p
  • b - a returns uintptr_t in units of the pointed-to types for any a and b that point inside the same object (array) in memory, if the pointed-to types are compatible, and not incomplete; irrespective of cv-qualifiers.

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@Explorer09 Explorer09 Feb 25, 2026

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b - a should return a ptrdiff_t, but if they are in different types a compiler may label it as a warning (a strict aliasing violation, maybe). A compiler wasn't told that the pointer returned from strchrnul is derived from the first argument passed into it, and so it doesn't know that b - a would work unless it can examine the body of strchrnul function.

Note: I don't like the constness thing at all. Part of the PR #1915 that I was recently doing had been dealing with -Wcast-qual warnings from GCC that over-warns things. (Not strictly a false positive, but GCC wasn't taught that execv() is special that it has to make an exception and cast away the const qualifier.)

*eol_w = '\0';

if (at != output) {
*at = ';';
at++;
left--;
}

int wrote = snprintf(at, left, "%s", group);
left -= wrote;
if (wrote < 0 || (size_t)wrote >= left) {
// Output was truncated, we are done
break;
}

at += (size_t)wrote;
left -= (size_t)wrote;
}
fclose(file);

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