段故障？ 你能找到吗？ 因为我不能

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.

Answer 2

Valigrind and GDB are your friend

Please provide the entire code so we can compile it and help you out. Staring at source isn't a magic approach to debuggin :)

Ensure you are compiling with the debugging options (-g etc) on your compiler.

Otherwise, checkout valgrind's memcheck. When you have your compiled program, run:

valgrind ./myprogram

And you may get output similar to the following:

==584== Use of uninitialised value of size 8
==584==    at 0x400480: segfaultme (p.c:6)
==584==    by 0x40049B: main (p.c:13)
==584== 
==584== Invalid write of size 4
==584==    at 0x400480: segfaultme (p.c:6)
==584==    by 0x40049B: main (p.c:13)
==584==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==584== 
==584== 
==584== Process terminating with default action of signal 11 (SIGSEGV)
==584==  Access not within mapped region at address 0x0
==584==    at 0x400480: segfaultme (p.c:6)
==584==    by 0x40049B: main (p.c:13)
==584==  If you believe this happened as a result of a stack
==584==  overflow in your program's main thread (unlikely but
==584==  possible), you can try to increase the size of the
==584==  main thread stack using the --main-stacksize= flag.
==584==  The main thread stack size used in this run was 10485760.

Crack out GDB with:

gdb ./myprog then enter r return

And you will get some more information on exactly where the segfault occurs:

(gdb) r
Starting program: /home/aiden/tmp/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400480 in segfaultme (p=0x0) at p.c:6
6       *p = 22;
Missing separate debuginfos, use: debuginfo-install glibc-2.11.2-1.x86_64
(gdb) bt
#0  0x0000000000400480 in segfaultme (p=0x0) at p.c:6
#1  0x000000000040049c in main () at p.c:13

Entering b t return will give you a backtrace too. In the above example we can see that line 6 of pc in segfaultme() where I dereference p is the problem, and it shows segfaultme() was called by main() .

Hope this helps! Remember to get as much exposure to tools that help you!