Question

The following is a code snippet from my read from pipe function. This executes properly and verified that the data is got into buffer .

int readFrom(char *buffer)
{
int nread;

if((nread = read(readfd,buffer,100)) < 0)
{
    printf("\nerror in reading data from FIFO\n");
    return(-1);
}
buffer[nread]='\0';
return(0);
}

In the above example nread is less than 100 . I am using GCC-4.7.0.

We had an abstraction layer for the above function like below :

int pipe_input(char *parmPtr, int size)
{

char readMsg[100];


if( readFrom((char *)&readMsg) == -1)
  return ERROR;

if (strlen(readMsg) < 1)
{
    printf("Incorrect Input\n");
    return ERROR;
}
strncpy(parmPtr, readMsg, ((size < 100)?size:100));


return 0;

}

In the above function as well it was verified that read message is proper and parmptr is properly loaded with the value. But in the function Where i am trying to call pipe_input I am getting a sigsegv. This happens with GCC-4.7.0 but the same code compiled with GCC-4.2.4 executes fine. I verified the warnings, but there are no warning for the above. Any pointers would be highly helpful.

Below code snippet for calling pipe_input :

int Calling_func(void)
{
  char alpha[100] ;

  pipe_input(alpha,100);
  printf("alpha value is %s \r\n",alpha);
}

getting sigsegv at the print statement.

Answer 1

You have off-by-one errors in your code. Your array has 100 elements, but you’re not taking NULL-termination into account:

strncpy(parmPtr, readMsg, ((size < 100)?size:100));

and:

buffer[nread]='\0';

The last element is buffer[99] (since array indices start from 0, not 1,) but you can write to buffer[100]. This can result in a segfault.

You should probably declare all arrays with a size of 101 instead and see if it helps. If you’re on Linux, you should also run your program in Valgrind; it can tell you exactly how the segfault occurred.