Question

I’ve looked over this about 15 times by now to no avail. I cannot understand why this is seg faulting? It doesn’t even get to the “print” statement which makes no sense. the error codes actually do work tho (when I dont have a shared memory present) I have a load.c program but it works perfectly (im 100% sure of this)

#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/types.h>
#include <sys/sem.h>
#include "header.h"

//BEGIN MAIN FUNCTION
main()
{
    int id;             //ID to data shmem
    struct StudentInfo *infoptr;    //ptr to data
    int found = 0;          //found 'boolean'
    char input[15];         //user input buffer
    struct StudentInfo *beginptr;   //ptr to beginning of data
    int rcid;           //Read count ID to shmem
    int *rcptr;         //RC ptr
    int sema_set;           //ID to shared semaphores

    //Find the shmem at our ID
        id = shmget(KEY,SEGSIZE,0);
        if(id < 0)
        {
                perror("Query: shmget failed");
                exit(1);
        }

    //set the ptr to our shared mem and attach to program
        infoptr = (struct StudentInfo *)shmat(id,0,0);
        if(infoptr <= (struct StudentInfo *)(0))
        {
                perror("Query: shmat failed");
                exit(1);
        }

    //Get our RC in shared memory 
    rcid = shmget(RCKEY,READCOUNT,0);
    if(rcid < 0)
    {
        perror("Query: shmget failed");
        exit(1);
    }
    //Set ptr to shmem and attach to process
    rcptr = (int*)shmat(rcid,0,0);
    if(rcptr <= (int*)(0))
    {
        perror("Print: Shmat failed");
        exit(1);
    }

    //Get semaphores
    sema_set = semget(SEMA_KEY,NUM_SEMAPHS,0);
    if(sema_set < 0)
    {
        perror("Query: Semget failed");
        exit(1);
    }   

    //Set program to queue up to wait
    Wait(sema_set,1);

    //Increment the read counter
    *rcptr += 1;

    //If we are the first reader, stop writers
    if(*rcptr == 1)
    Wait(sema_set,0);

    //Signal readers
    Signal(sema_set,1);

    //Set our begin ptr
    beginptr = infoptr;

    //Begin user input loop
    while(1)
    {
    //Ask user for input IT DOESN"T EVEN GET TO HERE <--
    printf("Please input a student ID :");
    scanf("%s",input);

    //While the record is not found search  
    while(strcmp(infoptr->Name,"")!=0 && found != 1)
    {
        //If record found, print the record
        if((strncmp(input,infoptr->ID,9)) == 0)
        {
            //Set found
            found = 1;

            printf("\n%s\n",infoptr->Name);
                    printf("%s\n",infoptr->telNumber);
                    printf("%s\n",infoptr->Address);
                    printf("%s\n\n",infoptr->ID);
        }
        else
            infoptr++;
    }

    //If not found, print error message
    if(found == 0)
        printf("Record not found.\n");

    //Wait on readers
    Wait(sema_set,1);
    //Decrement
    *rcptr--;
    //If no readers left
    if(*rcptr == 0)
        Signal(sema_set,0); //Signal writers
    //Signal readers
    Signal(sema_set,1);
    exit(0);        
    }
}

HEADER

#define KEY  ((key_t)(11111)) /*change it to last five digits of your SSN*/
#define SEGSIZE  sizeof(struct StudentInfo)

#define NUM_SEMAPHS 2
#define SEMA_KEY   ((key_t)(1111)) /* change this to last four digits of SSN */

#define READCOUNT sizeof(int)   //Set the size of shmem for read count
#define RCKEY ((key_t)(4003))   //Set the key of the shmem for RCount

//Struct student info
struct StudentInfo{
  char Name[20];
  char ID[15];
  char Address[50];
  char telNumber[15];
};

//Checks the semaphore whether or not to wait
void Wait(int semaph, int n);
//Signals that it's ok to run
void Signal(int semaph, int n);
//Gets the semaphore information
int GetSemaphs(key_t k, int n);

Answer 1

Your problem might come from your use of shmat. In C, never cast the return type of such a function. That you felt the need for it probably means that you had a spurious error message that came from the fact that you are missing the “sys/shm.h” header.

What happens in such cases is that gcc takes the return type for an int, usually a 32 bit quantity, and re-interprets it as a pointer. So the upper half of your address that shmat gives you is lost.

As a general rule, don’t cast away problems. Cast are rarely needed in C if all your headers are properly written. Casting the return type of a system function is almost always wrong.