除内存分配外的分段错误

Question

I can't seem to figure out why my assignment to an char * [] is giving me a segmentation fault when I try and assign a value to [0].

So what I mean is, I created a

char * temp[255]

which should allocated space in the heap for the temp. Thus when I do

while(buffer)
{
    for(num = 0; num < i; num++)
    {
        if(strcmp(temp[i], buffer) == 0)
        {
            break;
        }
    }

    /* send menu choice to server indicating client wants list of all files on server */
    if (sendto(sock, menuOption, sizeof(menuOption), 0, (struct sockaddr *) &serverAddr, sizeof(serverAddr)) < 0)
    {
        printf("sendto() sent a different number of bytes than expected");
    }

    /* receive a response from the server */
    fromSize = sizeof(fromAddr);
    if ((recvDataLen = recvfrom(sock, buffer, sizeof(buffer), 0, (struct sockaddr *) &fromAddr, &fromSize)) < 0)
    {   
        printf("recvfrom() failed\n");
    }

    if(serverAddr.sin_addr.s_addr != fromAddr.sin_addr.s_addr)
    {
        fprintf(stderr,"Error: received a packet from unknown source.\n");
        exit(1);
    }

    /* null terminate the received data */
    buffer[recvDataLen] = '\0';
    strcpy(temp[i], buffer);
    i++;
    printf("%s\n", buffer);    /* Print the echoed arg */
}

I should not be getting a segmentation fault, because i is initialized to 0 (I have not shown the initialization here because i is set at the top of my file along with socket attributes). So in the first iteration of the while loop, the for loop does nothing because num and i are both zero, thus it proceeds to send data to the server and retrieve data from the server. That data is put into buffer and should be then copied into temp. However it just gives me a segmentation fault and crashes. Any ideas why?

Answer 1

Your char* temp[255]; is not what you think. It is an array of 255 pointers . Not strings. Your pointers are uninitialized so they point to completly random places, which causes your segmentation fault. To Avoid this you need to allocate memory for every pointer.

char* temp[255] = {0}; // set all pointers to 0
//...
for(num = 0; num < i; num++)
{
   if(temp[i] != 0 && strcmp(temp[i], buffer) == 0) // if a pointer is 0 (uninitialized), do not access it
   {
      break;
   }
}
//...
temp[i] = malloc(size); // probably (strlen(buffer) + 1) (except you use something else for buffer)
if(temp[i] == NULL)
{
// something bad happend.
}
strcpy(temp[i], buffer);

Answer 2

You have declared an array of 255 pointers:

char * temp[255];

So you can do the following with these:

char actual_data[5] = {0};
strcpy (actual_data, "Heya");
temp[0] = actual_data;     //temp[0] now points to "heya"

But you cannot do:

strcpy (temp[0], actual_data);

This is because temp[0] is a pointer and does not have any memory of its own other than 4/8 bytes (equal to an integer). If you want to strcpy then you need to allocate memory. You can do something like:

char actual_data[5] = {0};
strcpy (actual_data, "Heya");
char *temp[255];
for (i=0; i<255; i++) 
{
     temp[i] = (char *)malloc (sizeof(actual_data) * sizeof(char));
     strcpy (temp[i], actual_data);
} 

This is just an example where all the 255 pointers will point to actual_data (hence "heya").