用於緩衝和附加一個用C

Question

我傳遞緩衝器（字符*）到功能中C.

在函數內部傳遞一個緩衝器（字符*）於函數時，我分配存儲器字符串（來自虛擬服務器的響應）。當在函數內部打印時，字符串正在從服務器發送顯示。

當試圖從功能外讀取緩衝區，我沒有得到結果，我需要：

一個。 // char * server_message; //不工作，錯誤::分段錯誤（核心轉儲）

b。 // char * server_message = calloc（（BUFFER_SIZE）+ 1，sizeof（char））; //不起作用，沒有產生錯誤，下面的server_message的printf打印出一個空值：==> server_message :: || ||

c。 char * server_message = calloc（（1000 * BUFFER_SIZE）+ 1，sizeof（char））; //分配超過需要的內存工作:: ==> server_message :: || 2016-03-08 12:20:13 hello there ||

我不喜歡選項c，因爲我分配的內存比我需要的多。 出於測試目的，BUFFER_SIZE當前設置爲10。

這裏是主叫部分::

write_to_server(client_socket, message, server_message); 
free(message); 
printf("\n\n==> server_message:: || %s ||\n\n", server_message); 

if (server_message != NULL) 
    free(server_message); 

這裏是write_to_server功能代碼::

void write_to_server(int file_descriptor, char* message, char* server_message) 
{ 

    char* msg = strcat(message, "\n"); 

int n_bytes = write(file_descriptor, msg, strlen(msg) + 1); 

if (n_bytes <= 0) 
{ 
    perror("write"); 
    exit(EXIT_FAILURE); 
} 

if (DEBUG_MODE) 
    printf("\nsuccessfully written %d bytes to server:: %s", n_bytes, msg); 

// char* message_back = calloc(BUFFER_SIZE + 1, sizeof(char)); 
if (server_message == NULL) 
{ 
    server_message = calloc(BUFFER_SIZE + 1, sizeof(char)); 
} 

if (server_message == NULL) 
{ 
    perror("Could not allocate memory for server_message"); 
    exit(EXIT_FAILURE); 
} 

char* tmp_message_back = calloc(BUFFER_SIZE + 1, sizeof(char)); 

if (tmp_message_back == NULL) 
{ 
    if (DEBUG_MODE) 
     perror("Could not allocate memory for tmp_message_back"); 

    exit(EXIT_FAILURE); 
} 

int n_read = 0; 
int n_total_read = 0; 

while ((n_read = read(file_descriptor, tmp_message_back, BUFFER_SIZE)) > 0) 
{ 
    n_total_read += n_read; 
    tmp_message_back[n_read] = '\0'; 

    /* 
    char* strcat(char* destination, const char* source); 

    Appends a copy of the source string to the destination string. 
    The terminating null character in destination is overwritten by the first character of source, 
    and a null-character is included at the end of the new string formed by the concatenation of both in destination. 
    */ 
    server_message = strcat(server_message, tmp_message_back); 

    // void* realloc(void* ptr, size_t size); 
    /* 
    char* new_message_back = realloc(server_message, n_total_read + (BUFFER_SIZE * sizeof(char))); 

    if (new_message_back == NULL) 
    { 
     perror("Could not allocate memory for server_message while receiving bytes from server"); 

     free(tmp_message_back); 

     return; 
    } 

    server_message = new_message_back; 
    */ 
    server_message = realloc(server_message, n_total_read + (BUFFER_SIZE * sizeof(char))); 
    server_message[n_total_read] = '\0'; 

    if (DEBUG_MODE) 
     printf("\nread %d (%d in total), size:: %zu ===> | %s |\t| %s |\n", n_read, n_total_read, strlen(server_message), server_message, tmp_message_back); 
} 

server_message = realloc(server_message, n_total_read); 

if (DEBUG_MODE) 
{ 
    printf("\n\n\n\n-- read %d in total, size:: %zu\n", n_total_read, strlen(server_message)); 
    printf("\n\nserver_message:: %s\n\n", server_message); 
} 

free(tmp_message_back); 
// free(message_back); 
} 

Answer 1

在C參數通過值傳遞。 基本上你這樣做是希望輸出爲
Hello world!：

void Test(char *ptr) 
{ 
    ptr = malloc(100); // this modifies ptr but not 
         // somepointer in the main function 
    strcpy(ptr, "Hello world!"); 
} 

int main() 
{ 
    char *somepointer; // somepointer is not initialized and contains 
         // a random value 
    Test(somepointer); 
         // here the value of somepointer has not changed 

    printf("%s", somepointer); // here you try to printf a char pointer 
           // that points to a rando location 
    return 0; 
} 

這是不確定的behavioir並很可能會打印亂碼或崩潰。

你需要的是這樣的：

void Test(char **ptr) 
{ 
    *ptr = malloc(100); 
    strcpy(*ptr, "Hello world!"); 
} 

int main() 
{ 
    char *somepointer; 
    Test(&somepointer); // pas the pointer to sompointer to Test 
    printf("%s", somepointer); // now somepointer points to the memory 
           // mallocd in Test 
    return 0; 
} 

Answer 2

我不知道，但我覺得你的問題是，你希望你的函數改變參數「Server_message」。 Server_Message是一個char數組（== char *） - 但是當讓一個函數改變你的參數時，你必須傳遞一個指向你的參數的指針。

所以不是這樣的： write_to_server（client_socket，message，server_message）; 試試這個： write_to_server（client_socket，message，& server_message）;

函數參數應該是char ** Server_message - 並且代碼中的一些更改也需要編譯。

也許，這有幫助。

Answer 3

好了，現在我已經改變了功能write_to_server接受的char ** p_server_message並把它稱爲使用：

write_to_server(client_socket, message, &server_message); 

我提到我的困難面對這條線：

（* p_server_message）[n_total_read] ='\ 0';

我不得不加括號，否則產生::分割故障（核心轉儲）

現在的問題只持續一個初始化的緩衝（情況下，在我的問題）::

char* server_message; 

write_to_server(client_socket, message, &server_message); 
free(message); 
printf("\n\n==> server_message:: || %s ||\n\n", server_message); 

該函數的新代碼是::

void write_to_server(int file_descriptor, char* message, char** p_server_message) 
{ 

char* msg = strcat(message, "\n"); 

int n_bytes = write(file_descriptor, msg, strlen(msg) + 1); 

if (n_bytes <= 0) 
{ 
    perror("write"); 
    exit(EXIT_FAILURE); 
} 

if (DEBUG_MODE) 
    printf("\nsuccessfully written %d bytes to server:: %s", n_bytes, msg); 

// char* message_back = calloc(BUFFER_SIZE + 1, sizeof(char)); 
if (*p_server_message == NULL) 
{ 
    *p_server_message = calloc(BUFFER_SIZE + 1, sizeof(char)); 
} 

if (*p_server_message == NULL) 
{ 
    perror("Could not allocate memory for p_server_message"); 
    exit(EXIT_FAILURE); 
} 

char* tmp_message_back = calloc(BUFFER_SIZE + 1, sizeof(char)); 

if (tmp_message_back == NULL) 
{ 
    if (DEBUG_MODE) 
     perror("Could not allocate memory for tmp_message_back"); 

    exit(EXIT_FAILURE); 
} 

int n_read = 0; 
int n_total_read = 0; 

while ((n_read = read(file_descriptor, tmp_message_back, BUFFER_SIZE)) > 0) 
{ 
    n_total_read += n_read; 
    tmp_message_back[n_read] = '\0'; 

    /* 
    char* strcat(char* destination, const char* source); 

    Appends a copy of the source string to the destination string. 
    The terminating null character in destination is overwritten by the first character of source, 
    and a null-character is included at the end of the new string formed by the concatenation of both in destination. 
    */ 
    *p_server_message = strcat(*p_server_message, tmp_message_back); 

    // void* realloc(void* ptr, size_t size); 
    /* 
    char* new_message_back = realloc(*p_server_message, n_total_read + (BUFFER_SIZE * sizeof(char))); 

    if (new_message_back == NULL) 
    { 
     perror("Could not allocate memory for p_server_message while receiving bytes from server"); 

     free(tmp_message_back); 

     return; 
    } 

    *p_server_message = new_message_back; 
    */ 
    *p_server_message = realloc(*p_server_message, n_total_read + (BUFFER_SIZE * sizeof(char))); 
    (*p_server_message)[n_total_read] = '\0'; 

    if (DEBUG_MODE) 
     printf("\nread %d (%d in total), size:: %zu ===> | %s |\t| %s |\n", n_read, n_total_read, strlen(*p_server_message), *p_server_message, tmp_message_back); 
} 

*p_server_message = realloc(*p_server_message, n_total_read); 

if (DEBUG_MODE) 
{ 
    printf("\n\n\n\n-- read %d in total, size:: %zu\n", n_total_read, strlen(*p_server_message)); 
    printf("\n\np_server_message:: %s\n\n", *p_server_message); 
} 

free(tmp_message_back); 
// free(message_back); 
} 

那麼爲什麼它仍然會產生未初始化緩衝區的分段錯誤？

我可以解決這個問題，如果我分配和初始化內存（使用calloc）在函數內部，無論緩衝區是否已經被初始化。

通過取消註釋行::

// if (*p_server_message == NULL) 
{ 
    *p_server_message = calloc(BUFFER_SIZE + 1, sizeof(char)); 
} 

但我無法理解這背後的原因。