UNIX消息队列分段故障

Question

i've got a problem with the exchange of messages between two different process. I must use only dynamic arrays. I also tried using shared memory but i haven't solved my problem.

this is the code of the sender:

  #include <stdio.h>
  #include <string.h>      
  #include <unistd.h>
  #include <stdlib.h> 
  #include <sys/ipc.h>    
  #include <sys/msg.h>    
  #include <errno.h>    
  #define KEYMSG 12345678

typedef struct msg
{
    long int type;
    char* str1;
    char* str2;
}msg;

int main()
{
    msg mymsg;
    mymsg.str1=(char*)malloc(30*sizeof(char));
    mymsg.str2=(char*)malloc(30*sizeof(char));
    strcpy(mymsg.str1,"this is a test ");
    strcpy(mymsg.str2,"test ok ");
    mymsg.type=1;


    if((msgget(KEYMSG,IPC_CREAT|0666))==-1)    
    {
        fprintf(stderr,"ERROR CREATION\n");
        fflush(stderr);
    }
    else
    {
        fprintf(stdout,"CREATION OK\n-----------\n");
        fflush(stdout);
    }

    if((msgsnd(msgget(KEYMSG,0666),&mymsg,sizeof(msg),IPC_NOWAIT))<0)
    {
        fprintf(stderr,"msg not sent\n");
        fflush(stderr);
    }
    else
    {
        fprintf(stdout, "\nmsg sent:\n");
        fprintf(stdout, "%s\t%s\n\n",mymsg.str1,mymsg.str2);
        fflush(stdout);
    }   

and this is the code of the reader:

#include <stdio.h>
#include <string.h>      
#include <unistd.h>
#include <stdlib.h> 
#include <sys/ipc.h>    
#include <sys/msg.h>
#include <errno.h>  
#include <sys/shm.h>
#define KEYMSG 12345678

typedef struct msg
{
    long int type;
    char* str1;
    char* str2;
}msg;

int main()
{
    msg mymsgrcv;
    mymsgrcv.str1=(char*)malloc(30*sizeof(char));
    mymsgrcv.str2=(char*)malloc(30*sizeof(char));

    if((msgrcv(msgget(KEYMSG,0666),&mymsgrcv,sizeof(msg),1,IPC_NOWAIT))==-1)
    {
        fprintf(stderr,"READ ERROR\n");
        fflush(stderr);
    }
    else
    {
        fprintf(stdout, "\nmsg read:\n");
        fprintf(stdout, "%s\t%s\n\n",mymsgrcv.str1,mymsgrcv.str2);
        fflush(stdout);
    }

    return 0;   
}

Answer 1

1.) In your message you have pointers to strings but their addresses are just local to one process. The addresses have no meaning to the other process. You have to pass the string and not just a pointer to it.

Define the message eg like this:

   typedef struct msg
   {
     long int type;
     char str1[32];
     char str2[32];
   }msg;

2.) Always do error checking on pointers returned by malloc(3) !

Answer 2

So you wrote a piece of software under Linux which fails with a segfault. How to find it?

First step is to enable debug info in your compiler ( -g in gcc).

Then install valgrind.

Now let's say your program is called homework (so you run it as ./homework ), so run:

valgrind homework

and valgrind will tell you where the problem is.

Answer 3

In the reader part you may get a heap overflow since mymsgrcv.str1 and mymsgrcv.str2 may be empty strings (they do not carry an end char \\0 ).

You may either use valgrind or gcc address sanitizer (with this compile line: gcc -g -Wall -fsanitize=address -fno-omit-frame-pointer reader.c -o reader ).

Here is what gcc address sanitizer reports:

==22502==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60300000effe at pc 0x7fca63d99111 bp 0x7fff4ff40520 sp 0x7fff4ff3fcd0
READ of size 31 at 0x60300000effe thread T0
    #0 0x7fca63d99110  (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x5e110)
    #1 0x7fca63d99a64 in __interceptor_vfprintf (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x5ea64)
    #2 0x7fca63d99b69 in __interceptor_fprintf (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x5eb69)
    #3 0x400c15 in main /home/jvet/TMP/reader.c:32
    #4 0x7fca639b3b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
    #5 0x400988  (/home/jvet/TMP/reader+0x400988)

0x60300000effe is located 0 bytes to the right of 30-byte region [0x60300000efe0,0x60300000effe)
allocated by thread T0 here:
    #0 0x7fca63dcf37a in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x9437a)
    #1 0x400ad7 in main /home/jvet/TMP/p2.c:21
    #2 0x7fca639b3b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)

Answer 4

Whenever you want to send string over IPC, prefer to use array fixed size this will make IPC message of bigger size even though you have very less data to send

To remove such issues you can make tailing of data using variable size array and size of data that is followed by message header.