c - 无法从被调用函数返回更新后的字符串并释放内存

Question

我有一个包含两个字符串的简单程序，我应该将它们连接到被调用函数中并返回到调用函数。

#include <stdlib.h>
#include <string.h>
#include <stdio.h>


void call_func(char *str1,char *str2)
{

  char *msg_concat;
  asprintf(&msg_concat,"%s%s", str1, str2);
  printf("%s",msg_concat);
  str2 = msg_concat;

}



int main()
{
    char *str1="new" ;

    char *full_msg ="test";
    call_func(str1,full_msg);
    printf("value of full_msg=%s",full_msg);
    return 0;

}

我已使用 asprintf() 进行连接，但主函数中未返回该值。正如我使用的指针，是不是 期望返回改变后的值？？

此外，在分配指针时，根据我的理解，复制的是引用而不是值。现在我需要释放 分配内存msg_concat 否则会出现内存泄漏。如果函数使用指针并返回 引用调用函数??

free(msg_concat) 在被调用函数的最后一行不工作 作为-

void call_func(char *str1,char *str2)
{

  char *msg_concat;
  asprintf(&msg_concat,"%s%s", str1, str2);
  printf("%s",msg_concat);
  str2 = msg_concat;
   free(msg_concat);

}

其实在我最近的项目中，我有调用函数的场景

GSList *parsed_msg = LineParser(dev_ip,encoded_msg, "\n", file_path, unique_id);

被调用的函数是

GSList *LineParser(char* dev_ip, char *msg_full, char *separator, char *file_path, int unique_id)
{
    GSList *parsed_msg = NULL;
    char connection_id[50];
    sprintf(connection_id,"%s|%d", dev_ip, unique_id);
    char *msg_concat;


    // inserting {file_path : last_not_complete_line} logic to TCP_CACHE
    //and removing the correspoing last_line
    g_mutex_lock (&mutex_hash_main);

    // char *last_line = (char *) (g_hash_table_lookup((GHashTable *) g_hash_table_lookup(TCP_CACHE, connection_id), file_path));

    GHashTable *t_filepath_msg_dict = NULL;         //for {file_path : last_not_complete_line}
    if (TCP_CACHE != NULL)
    {
        t_filepath_msg_dict = (GHashTable *)(g_hash_table_lookup(TCP_CACHE, connection_id));
        if (t_filepath_msg_dict != NULL)
        {
            char *last_line = (char *) (g_hash_table_lookup(t_filepath_msg_dict, file_path));

            if(last_line != NULL)   //if the hash has device ip, append the value to msg
            {
                zlog_debug(c,"concatenating: str1: %s and str2: %s\n", last_line, msg_full);
                asprintf(&msg_concat,"%s%s", last_line, msg_full);


                //msg_concat = concat(last_line,msg_full);
                g_hash_table_remove(t_filepath_msg_dict, file_path);

                msg_full = msg_concat;

            }   
        }

    }


    int msg_len = strlen(msg_full);
    char last_char = msg_full[msg_len - 1];
    zlog_debug(c, "len of message: %d", msg_len);
    zlog_debug(c, "last char is : %c", last_char);
    char *token=NULL;
    char *remaining_str=NULL;
    token = strtok_r(msg_full, "\n", &remaining_str);

    while(token != NULL)
    {
        if(token[0]==' ')
        {
            token = trimwhitespace_parser (token);
            if(strcmp(token,"")==0)
            {
                //insert this token to GSList
                parsed_msg = g_slist_prepend (parsed_msg, token);
                token = strtok_r(NULL, "\n", &remaining_str);
                continue;
            }
        }
        if(strcmp(remaining_str,"")==0)
        {
            if(strlen(token) > 10000)
            {
                zlog_warn(c, "Message too big(more than 10000 len). Stop looking for new line and process msg");
                g_hash_table_remove(t_filepath_msg_dict, file_path);
            }
            else
            {
                if(last_char=='\n')
                {
                    //new line is the last character. do nothing
                    zlog_debug(c, "last character is new line");
                }
                else
                {
                    zlog_debug(c, "last character is not new line");
                    //new line not received
                    if (t_filepath_msg_dict == NULL)        //insert new record
                    {
                        GHashTable *each_filepath_msg_dict = g_hash_table_new_full(g_str_hash, g_str_equal, key_str_destroy_cb_parser, value_str_destroy_cb_parser);
                        zlog_debug(c,"Inserting file_path: %s to connection_id: %s", file_path, connection_id);

                        g_hash_table_insert(each_filepath_msg_dict, strdup(file_path), strdup(token));
                        g_hash_table_insert(TCP_CACHE, strdup(connection_id), each_filepath_msg_dict);
                    }
                    else        //update existing record
                    {
                        zlog_debug(c,"Connection_id :%s is already found; appending/replacing file_path :%s", connection_id, file_path);
                        g_hash_table_insert(t_filepath_msg_dict, strdup(file_path), strdup(token));
                    }
                    g_mutex_unlock(&mutex_hash_main);
                    return parsed_msg;
                }
            }
        }
        //insert token to GSList
        parsed_msg = g_slist_prepend (parsed_msg, token);
        token = strtok_r(NULL, "\n", &remaining_str);
    }
    g_mutex_unlock(&mutex_hash_main);
    return parsed_msg;
}

我已经看到我遇到内存泄漏问题，因为我必须释放 msg_concat，因为在给定的答案中据说更改的值不会返回到调用函数。释放 asprintf msg_concat 指针的适当位置在哪里？？？

Answer 1

指针按值传递，因此最后的赋值对调用者没有影响。有两种方法可以做你想做的事:

• 将指针传递给 str2 的指针，或者
• 返回一个新的字符串给调用者。

返回结果对于函数的调用者来说稍微更明确一些，但指针到指针的方法也是有效的。

您需要解决的另一个问题是释放传递给函数的指针。如果您想释放值，则不能传递字符串文字:

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

char* call_func(char *str1,char *str2) {
    char *msg_concat = NULL;
    asprintf(&msg_concat,"%s%s", str1, str2);
    printf("%s",msg_concat);
    free(str2);
    return msg_concat;
}

int main() {
    char *str1="new" ;
    char *full_msg =strdup("test"); // Put the string in dynamic memory
    full_msg = call_func(str1, full_msg);
    printf("value of full_msg=%s", full_msg);
    free(full_msg);
    return 0;
}