如何在 Boehm GC 中使用 asprintf？

Question

据我所知，asprintf 调用 malloc。如果我用 Boehm GC 替换 malloc，对 asprintf 的调用仍会调用传统的 malloc - 至少 valgrind 是这样告诉我的：

这是 malloc 宏：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>

#include <gc.h>
#define malloc(n)    GC_MALLOC(n)
#define calloc(m,n)  GC_MALLOC((m)*(n))
#define realloc(p,n) GC_REALLOC((p),(n))

typedef char * string;

这是 valgrind 报告：

hopcroft:didactic_scheme(flexible_strings) scotttaylor$ valgrind --suppressions=./boehm-gc.suppressions --leak-check=full bin/escheme -e 1
==16130== Memcheck, a memory error detector
==16130== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==16130== Using Valgrind-3.6.0.SVN and LibVEX; rerun with -h for copyright info
==16130== Command: bin/escheme -e 1
==16130== 
--16130-- bin/escheme:
--16130-- dSYM directory is missing; consider using --dsymutil=yes
1==16130== 
==16130== HEAP SUMMARY:
==16130==     in use at exit: 4,312 bytes in 3 blocks
==16130==   total heap usage: 3 allocs, 0 frees, 4,312 bytes allocated
==16130== 
==16130== 128 bytes in 1 blocks are definitely lost in loss record 2 of 3
==16130==    at 0x100012D75: malloc (vg_replace_malloc.c:236)
==16130==    by 0x1000918EC: asprintf (in /usr/lib/libSystem.B.dylib)
==16130==    by 0x1000013FA: printInt (in bin/escheme)
==16130==    by 0x100001D38: print (in bin/escheme)
==16130==    by 0x100001DC5: main (in bin/escheme)
==16130== 
==16130== LEAK SUMMARY:
==16130==    definitely lost: 128 bytes in 1 blocks
==16130==    indirectly lost: 0 bytes in 0 blocks
==16130==      possibly lost: 0 bytes in 0 blocks
==16130==    still reachable: 4,184 bytes in 2 blocks
==16130==         suppressed: 0 bytes in 0 blocks
==16130== Reachable blocks (those to which a pointer was found) are not shown.
==16130== To see them, rerun with: --leak-check=full --show-reachable=yes
==16130== 
==16130== For counts of detected and suppressed errors, rerun with: -v
==16130== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 66 from 13)

以下是 malloc 调用的来源代码：

static string printInt(Object self) {
  string str;
  asprintf(&str, "%lu", getValueInt(self));
  return str;
}

一种解决方法可能是使用 asprintf，然后使用 malloc 复制它 以便使用 malloc 宏而不是原始函数：

static string printInt(Object self) {
  string tmp;
  string str;

  asprintf(&tmp, "%lu", getValueInt(self));
  str = calloc(sizeof(string), strlen(tmp) + 1);

  strcpy(str, tmp);
  free(tmp);

  return str;
}

这看起来很愚蠢——它涉及到一堆不必要的复制，而且恰好是一个代码眼痛的 IHMO。那么有没有一种安全的方法来使用 asprintf 和其他可能在仍然使用 Boehm GC 的同时调用本机 malloc 的系统库？我应该使用 asprintf 的替代品吗？

Answer 1

snprintf 返回如果提供的缓冲区足够大，将写入的字符数。您可以调用此方法两次（一次获得正确的缓冲区大小，然后再次使用足够大的缓冲区来获取输出），但这可能比将asprintf 的输出复制到可收集缓冲区效率低.这是一个包含代码的示例，该代码分配了一个足够大的缓冲区以包含 32 位系统的 unsigned long 的最大值。在没有足够空间的系统上，缓冲区会被重新分配并重新格式化。

#include <limits.h>

...

unsigned long intValue = getValueInt(self);
size_t maxLength = 11; // heuristic
char *buf = malloc(maxLength);

int result = snprintf(buf, maxLength, "%lu", intValue);
if (result > maxLength)
{
    // shouldn't ever get here, but just in case the buffer is too small
    // we reallocate it to the correct size and try again.
    buf = malloc(result);
    snprintf(buf, result, "%lu", intValue);
}

return buf;

Answer 2

根据this page，可以用

编译libgc（boehm-gc库）

-DREDIRECT_MALLOC=GC_malloc -DIGNORE_FREE

应该 intercept the call 到 asprintf 中的 malloc。注意：没试过。