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C语言怎么开辟超大内存？（malloc）_malloc申请大内存
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用malloc在堆中开辟，栈里顶多只能开几兆
char* save_video_box_buffer = (char*)malloc(1920*1080*3/2);
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20230816
C语言中的大内存管理
在C语言中，malloc()函数是用于分配动态内存的一种常见方法。但是，如果要开辟超大内存，就需要了解和处理更多细节。
1. 基本内存管理
1.1 malloc函数简介
在C语言中，malloc()函数是在堆区开辟内存的基础方式。其原型如下：
void* malloc (size_t size);
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当成功分配内存时，malloc()返回指向该块内存的指针。如果分配失败（例如，由于内存不足），则返回NULL。
int *p = NULL;
p = (int *)malloc(10 * sizeof(int));
if (p == NULL) {
printf("Failed to allocate memory.\n");
return -1;
}
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1.2 内存释放
在使用完动态分配的内存后，应通过free()函数将其释放回系统，以避免内存泄漏。
free(p);
p = NULL; // 防止悬挂指针
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2. 大内存的问题
尽管malloc()可以满足大多数场景的内存需求，但在面临超大内存分配需求时，可能会出现问题。例如，当请求的内存量超过可用内存时，malloc()会失败并返回NULL。
3. 超大内存的解决方案
为了解决这个问题，有几种可能的策略。
3.1 使用64位操作系统
32位操作系统只能管理4GB的内存空间，而64位操作系统理论上可以管理16EB（Exabytes）的内存空间。因此，如果需要开辟超大内存，首先确保使用的是64位操作系统。
3.2 持续检查和处理错误
在任何时候调用malloc()时，都应检查其返回值以确认是否成功分配了内存。
int *p = NULL;
p = (int *)malloc(10 * sizeof(int));
if (p == NULL) {
fprintf(stderr, "Failed to allocate memory.\n");
exit(EXIT_FAILURE);
}
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3.3 分段申请内存
如果单次申请大块内存失败，可以考虑分段申请内存。每次申请较小的内存块，然后逐步构建所需的大型数据结构。
#define CHUNK_SIZE 1024
#define TOTAL_SIZE 1000000
char* big_memory = NULL;
for (size_t i = 0; i < TOTAL_SIZE; i += CHUNK_SIZE) {
char* chunk = malloc(CHUNK_SIZE);
if (chunk == NULL) {
fprintf(stderr, "Failed to allocate chunk at %zu\n", i);
exit(EXIT_FAILURE);
}
big_memory[i] = chunk;
}
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以上代码试图分配TOTAL_SIZE字节的内存，但一次只分配CHUNK_SIZE字节。注意，还需要管理这些小块内存，并在适当的时候释放它们。
3.4 使用内存映射
另一个可能的解决方案是使用内存映射（memory mapping）。在UNIX和类UNIX系统（包括Linux）中，可以使用mmap()函数将文件或设备映射到内存。这样，即使物理内存有限，也可以利用硬盘空间作为虚拟内存。
以下是一个简单的例子：
#define _GNU_SOURCE #include <sys/mman.h> #include <fcntl.h> #include <unistd.h> #define FILEPATH "/tmp/mmapped.bin" #define NUMINTS (1000) #define FILESIZE (NUMINTS * sizeof(int)) int main() { int fd; int result; int *map; fd = open(FILEPATH, O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600); if (fd == -1) { perror("Error opening file for writing"); exit(EXIT_FAILURE); } result = lseek(fd, FILESIZE-1, SEEK_SET); if (result == -1) { close(fd); perror("Error calling lseek() to 'stretch' the file"); exit(EXIT_FAILURE); } result = write(fd, "", 1); if (result != 1) { close(fd); perror("Error writing last byte of the file"); exit(EXIT_FAILURE); } map = mmap(0, FILESIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if (map == MAP_FAILED) { close(fd); perror("Error mmapping the file"); exit(EXIT_FAILURE); } /* Now write int's to the file as if it were memory (an array of ints) */ for (off_t i = 1; i <=NUMINTS; ++i) { map[i] = 2 * i; } if (munmap(map, FILESIZE) == -1) { perror("Error un-mmapping the file"); } close(fd); return 0; }
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此代码创建了一个大小为FILESIZE的文件，并将其映射到内存。然后，它将整数写入映射的内存，就像写入数组一样。最后，它解除映射并关闭文件。
参考文献
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