共享使用mmap结构的数组 [英] Sharing an array of structs using mmap

问题描述

我想创建一个父子进程间共享结构的数组。试图访问数组数据时，我得到一个分段错误。

我觉得肯定的是，问题已经是与我使用指针的方式，因为这是一个方面我不是很舒服。

请注意，我编辑了大部分的code，它似乎没有相关的。

  / *登记表*结构/
结构registrationTable {
    INT口;
    焦炭名[MAXNAME]
    INT req_no;
};主（）{    / *的registrationTable是要与每个孩子的共享存储器空间
    处理能够访问和更新。没有并发控制是
     实现。父进程负责清理后
     小子。
 * /
结构registrationTable base_table [REG_TABLE_SIZE]
的for（int i = 0; I＆LT; REG_TABLE_SIZE;我++）{
    base_table [I] .req_no = 0;
    memset的（base_table [I] .name和'\\ 0'，MAXNAME）;
    base_table [I] .port = 0;
}的printf（\\ nMapping共享内存\\ n）;//设置共享的内存空间
//无效* MMAP（void *的地址，为size_t长度，PROT INT，INT标志，
    // INT FD，off_t偏移）;
//地址= NONE，PROT = PROT_NONE，旗帜= MAP_SHARED
结构registrationTable *表= MMAP（base_table，sizeof的（base_table）
                        PROT_READ | PROT_WRITE，
                        MAP_SHARED | MAP_ANONYMOUS，
                        -1，0）;而（1）{
    将为pid_t孩子= fork（）的;    如果（孩子== 0）{//是孩子        的for（int i = 0; I＆LT; REG_TABLE_SIZE;我++）{            的printf（\\ nExamining表％s的端口寻找客户数：％d \\ n，
                    packet_reg.data，clientAddr.sin_port）;            的printf（\\ ntable [1] .req_no数：％d，TA [I] .req_no）;                            //对下一行分段错误
            如果（STRCMP（表[I] .name和packet_reg.data）== 0
                    ＆功放;＆安培;表[I] .port == clientAddr.sin_port）{
                表[I] .req_no ++;
}
 

解决方案

您还没有初始化后，它被MMAP分配的表的内容。所以它包含垃圾。所以 STRCMP（表[I] .name和packet_reg.data）有很大的机会去了分配的缓冲区和访问例如不分配的内存。

1. 初始化表是否正常;


2. 使用STRNCMP的比较有。

I am trying to create an array of structs that is shared between a parent and child processes. I am getting a segmentation fault when trying to access the array data.

I feel certain that the problem has something to do with the way I'm using pointers, as this is an area I'm not very comfortable with.

Please note that I edited out most of the code that didn't seem relevant.

/* structure of Registration Table */
struct registrationTable{
    int port;
    char name[MAXNAME];
    int req_no;
};

main() {

    /* the registrationTable is to be a shared memory space with each child
    process able to access and update. No concurrency controls are 
     implemented. The parent process is responsible for cleaning up after
     the kiddies.
 */
struct registrationTable base_table[REG_TABLE_SIZE];
for (int i = 0; i < REG_TABLE_SIZE; i++) {
    base_table[i].req_no = 0;
    memset(base_table[i].name, '\0', MAXNAME);
    base_table[i].port = 0;
}

printf("\nMapping Shared Memory\n");

//set up shared memory space
//void *mmap(void *addr, size_t length, int prot, int flags,
    //              int fd, off_t offset);
//      addr = NONE, prot = PROT_NONE, flags = MAP_SHARED 
struct registrationTable *table = mmap(base_table, sizeof(base_table),
                        PROT_READ | PROT_WRITE, 
                        MAP_SHARED | MAP_ANONYMOUS,
                        -1, 0);

while(1){
    pid_t child = fork();

    if (child == 0) {//is child

        for(int i = 0; i < REG_TABLE_SIZE; i++) {

            printf("\nExamining table looking for client at %s port: %d\n", 
                    packet_reg.data, clientAddr.sin_port);

            printf("\ntable[1].req_no: %d", ta[i].req_no);

                            //segmentation fault on next line
            if (strcmp(table[i].name, packet_reg.data) == 0 
                    && table[i].port == clientAddr.sin_port) {
                table[i].req_no++;
}

解决方案

You haven't initialized content of the table after it was allocated by mmap. So it contains garbage. And so strcmp(table[i].name, packet_reg.data) has a great chance to go over allocated buffers and access e.g. non-allocated memory.

1. initialize table properly;
2. use strncmp for the comparison there.

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