写一个用mmap在内存中可以替换长度不相等的字符串的函数

下面是一个使用`mmap`在内存中替换长度不相等的字符串的函数：

#include <iostream>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <cstring>

void replace_string_with_mmap(const char* file_path, const char* old_str, const char* new_str) {
    int fd = open(file_path, O_RDWR);
    if (fd == -1) {
        std::cerr << "Failed to open file.\n";
        return;
    }

    struct stat sb;
    if (fstat(fd, &sb) == -1) {
        std::cerr << "Failed to get file size.\n";
        close(fd);
        return;
    }

    char* file_contents = static_cast<char*>(mmap(NULL, sb.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0));
    if (file_contents == MAP_FAILED) {
        std::cerr << "Failed to map file into memory.\n";
        close(fd);
        return;
    }

    size_t old_str_len = strlen(old_str);
    size_t new_str_len = strlen(new_str);

    char* match = strstr(file_contents, old_str);
    while (match != nullptr) {
        if (new_str_len != old_str_len) {
            memmove(match + new_str_len, match + old_str_len, strlen(match + old_str_len) + 1);
        }

        memcpy(match, new_str, new_str_len);
        match = strstr(match + new_str_len, old_str);
    }

    if (munmap(file_contents, sb.st_size) == -1) {
        std::cerr << "Failed to unmap file from memory.\n";
    }

    close(fd);
}

int main() {
    replace_string_with_mmap("test.txt", "hello", "world");
    return 0;
}

该函数通过`mmap`将文件映射到内存中，使用`strstr`在内存中查找要替换的字符串，并使用`memcpy`实现替换。如果替换的字符串长度不相等，则使用`memmove`移动后面的字符以保证内存不被破坏。最后，通过`munmap`将内存映射释放掉。