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About c : strcpy-to-mmap-address-retuns-bus-error

Question Detail

I created a process which calls mmap with MAP_SHARED flag set,when i attempt to copy a string to that address i receive Bus error core dumped,could some one please explain the reason behind it and how to fix it. Following is my code

int main()
{
int fd=0;
char* ret = NULL;
void *map_addr = NULL;

fd = open(“./shared_file.txt”, O_RDWR, S_IRUSR | S_IWUSR);

if(fd == -1) {
printf(“errno = %d\n”,errno);
printf(“Aborting process1###########\n”);
abort();
}

map_addr = mmap(NULL, 5*sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

if(map_addr == MAP_FAILED) {
printf(“mmap failed error no =%d\n”,errno);
close(fd);
return -1;
}

printf(“map_addr = %p#################\n”,(int*)map_addr);
printf(“processid = %d#################\n”,(int)getpid());

ret = strcpy((char*)map_addr,”Stack Overflow”);

if(ret == (char*)map_addr)
printf(“strcpy success\n”);

/*if(msync(map_addr, sizeof(int), MS_SYNC))
printf(“msync failed errno = %d\n”,errno);*/

close(fd);

sleep(120);

return (0);
}

Question Answer

The cause of a bus error is usually an attempt to dereference a pointer that has not been initialized properly and contains junk data that are not accessible in a multiple of 4 or 1 or as related to datatype sizes.

First you should check if the shared_file.txt file size is >= 20 bytes(assuming sizeof int is 4 bytes) as specified in the mmap() length argument(where you put 5*(sizeof(int))) in the line below:

map_addr = mmap(NULL, 5*sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

If file size is less than 20 bytes, you could use fallocate call to pre allocate the memory.

If shared_file.txt file size is <= 20 bytes and then you ask mmap to map 20 bytes, it could cause a bus error when you write beyond the actual no. of bytes available in file, because that would be access to an undefined portion of a memory. MMAP_FAILED will not be returned in this case, during memory initialization. A quick check is to see if you can write a single character in the mmap char* pointer. If you can't( you will get a SIGBUS), that means file size is zero.

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