malloc/calloc显然使用交换空间来满足超出可用空闲内存的请求。随着磁盘使用指示灯持续亮起，这几乎使系统挂起。在我遇到这种情况之后，我还不确定为什么，我编写了以下5行测试程序来检查这是否确实是系统挂起的原因，
/* --- test how many bytes can be malloc'ed successfully --- */
#include <stdio.h>
#include <stdlib.h>
int main ( int argc, char *argv[] ) {
unsigned int nmalloc = (argc>1? atoi(argv[1]) : 10000000 ),
size = (argc>2? atoi(argv[2]) : (0) );
unsigned char *pmalloc = (size>0? calloc(nmalloc,size):malloc(nmalloc));
fprintf( stdout," %s malloc'ed %d elements of %d bytes each.\n",
(pmalloc==NULL? "UNsuccessfully" : "Successfully"),
nmalloc, (size>0?size:1) );
if ( pmalloc != NULL ) free(pmalloc);
} /* --- end-of-function main() --- */
如果您的两个命令行参数的乘积超过物理内存，则确实会挂断系统。最简单的解决方案是通过某种方式自动使malloc/calloc失败。更加困难且不可移植的是编写一个小的包装器，该包装器使用popen()的一个免费命令，解析输出，并且仅在可用的“空闲”内存可以满足该请求的情况下才调用malloc/calloc，也许会有一点安全性内置的。
有没有更简单，更便携的方法来实现这一目标？ (显然类似于can calloc or malloc be used to allocate ONLY physical memory in OSX?这个问题，但我希望得到某种"is"的答案。)
埃迪特--------------
决定遵循汤姆的/proc/meminfo建议。也就是说，不是popen()“免费”，而是直接解析现有且易于解析的/proc/meminfo文件。然后是表格的单行宏
完成工作。如下所示，memfree()并不像我想要的那样可移植，但是如果/当有需要时，可以轻松，透明地将其替换为更好的解决方案，而现在并非如此。
#include <stdio.h>
#include <stdlib.h>
#define _GNU_SOURCE /* for strcasestr() in string.h */
#include <string.h>
char *strcasestr(); /* non-standard extension */
/* ==========================================================================
* Function: memfree ( memtype )
* Purpose: return number of Kbytes of available memory
* (as reported in /proc/meminfo)
* --------------------------------------------------------------------------
* Arguments: memtype (I) (char *) to null-terminated, case-insensitive
* (sub)string matching first field in
* /proc/meminfo (NULL uses MemFree)
* --------------------------------------------------------------------------
* Returns: ( int ) #Kbytes of memory, or -1 for any error
* --------------------------------------------------------------------------
* Notes: o
* ======================================================================= */
/* --- entry point --- */
int memfree ( char *memtype ) {
/* ---
* allocations and declarations
* ------------------------------- */
static char memfile[99] = "/proc/meminfo"; /* linux standard */
static char deftype[99] = "MemFree"; /* default if caller passes null */
FILE *fp = fopen(memfile,"r"); /* open memfile for read */
char memline[999]; /* read memfile line-by-line */
int nkbytes = (-1); /* #Kbytes, init for error */
/* ---
* read memfile until line with desired memtype found
* ----------------------------------------------------- */
if ( memtype == NULL ) memtype = deftype; /* caller wants default */
if ( fp == NULL ) goto end_of_job; /* but we can't get it */
while ( fgets(memline,512,fp) /* read next line */
!= NULL ) { /* quit at eof (or error) */
if ( strcasestr(memline,memtype) /* look for memtype in line */
!= NULL ) { /* found line with memtype */
char *delim = strchr(memline,':'); /* colon following MemType */
if ( delim != NULL ) /* NULL if file format error? */
nkbytes = atoi(delim+1); /* num after colon is #Kbytes */
break; } /* no need to read further */
} /* --- end-of-while(fgets()!=NULL) --- */
end_of_job: /* back to caller with nkbytes */
if ( fp != NULL ) fclose(fp); /* close /proc/meminfo file */
return ( nkbytes ); /* and return nkbytes to caller */
} /* --- end-of-function memfree() --- */
#if defined(MEMFREETEST)
int main ( int argc, char *argv[] ) {
char *memtype = ( argc>1? argv[1] : NULL );
int memfree();
printf ( " memfree(\"%s\") = %d Kbytes\n Have a nice day.\n",
(memtype==NULL?" ":memtype), memfree(memtype) );
} /* --- end-of-function main() --- */
#endif
最佳答案
扩展我对原始问题的评论:
如果要禁用交换，请使用swapoff命令(sudo swapoff -a)。我通常以这种方式运行我的机器，以避免在Firefox执行不应执行的操作时冻结。您可以使用setrlimit()(或ulimit命令)来设置最大VM大小，但这不能适当补偿突然决定是内存消耗的其他某些过程(请参见上文)。
即使您选择上述选项之一，也应阅读本答案的其余部分，以了解如何避免在第一次调用calloc()时进行不必要的初始化。
至于精确的测试工具，事实证明您正在触发GNU calloc()的优化的不幸异常。
这是我对另一个答案所作的评论(现已删除)，事实证明这并不是严格意义上的准确性:
事实证明，我错过了calloc优化的一个异常(exception)。由于GNU malloc实现初始化malloc系统的方式，对calloc的首次调用始终使用memset()将新分配的存储设置为0。对calloc()的所有其他调用都将通过整个calloc逻辑，从而避免了在存储位置上调用memset()的情况。已经新鲜出炉了。
因此，对测试程序的以下修改显示了截然不同的行为:
#include <stdio.h>
#include <stdlib.h>
int main ( int argc, char *argv[] ) {
/* These three lines were added */
void* tmp = calloc(1000, 1); /* force initialization */
printf("Allocated 1000 bytes at %p\n", tmp);
free(tmp);
/* The rest is unchanged */
unsigned int nmalloc = (argc>1? atoi(argv[1]) : 10000000 ),
size = (argc>2? atoi(argv[2]) : (0) );
unsigned char *pmalloc = (size>0? calloc(nmalloc,size):malloc(nmalloc));
fprintf( stdout," %s malloc'ed %d elements of %d bytes each.\n",
(pmalloc==NULL? "UNsuccessfully" : "Successfully"),
nmalloc, (size>0?size:1) );
if ( pmalloc != NULL ) free(pmalloc);
}
请注意，如果将
MALLOC_PERTURB_设置为非零值，那么它将用于初始化malloc()的块，并强制将calloc()的块初始化为0。在下面的测试中将使用它。下面，我使用
/usr/bin/time来显示执行期间的页面错误数。请注意次要错误的数量，这是操作系统在匿名mmap'd区域中零初始化先前未引用的页面的结果(以及其他一些情况，例如映射Linux页面高速缓存中已经存在的页面)。另外还要查看常驻集大小，以及执行时间。$ gcc -Og -ggdb -Wall -o mall mall.c
$ # A simple malloc completes instantly without page faults
$ /usr/bin/time ./mall 4000000000
Allocated 1000 bytes at 0x55b94ff56260
Successfully malloc'ed -294967296 elements of 1 bytes each.
0.00user 0.00system 0:00.00elapsed 100%CPU (0avgtext+0avgdata 1600maxresident)k
0inputs+0outputs (0major+61minor)pagefaults 0swaps
$ # Unless we tell malloc to initialise memory
$ MALLOC_PERTURB_=35 /usr/bin/time ./mall 4000000000
Allocated 1000 bytes at 0x5648c2436260
Successfully malloc'ed -294967296 elements of 1 bytes each.
0.19user 1.23system 0:01.43elapsed 99%CPU (0avgtext+0avgdata 3907584maxresident)k
0inputs+0outputs (0major+976623minor)pagefaults 0swaps
# Same, with calloc. No page faults, instant completion.
$ /usr/bin/time ./mall 1000000000 4
Allocated 1000 bytes at 0x55e8257bb260
Successfully malloc'ed 1000000000 elements of 4 bytes each.
0.00user 0.00system 0:00.00elapsed 100%CPU (0avgtext+0avgdata 1656maxresident)k
0inputs+0outputs (0major+62minor)pagefaults 0swaps
$ # Again, setting the magic malloc config variable changes everything
$ MALLOC_PERMUTE_=35 /usr/bin/time ./mall 1000000000 4
Allocated 1000 bytes at 0x5646f391e260
Successfully malloc'ed 1000000000 elements of 4 bytes each.
0.00user 0.00system 0:00.00elapsed 100%CPU (0avgtext+0avgdata 1656maxresident)k
0inputs+0outputs (0major+62minor)pagefaults 0swaps
关于c - 如果请求超出可用物理内存，如何使malloc/calloc失败(即，不要使用swap)，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/55625390/