Question

I’ve a ‘C’ program which has encountered a strange problem.. I’m getting segmentation fault in the line containing “feof(fp)”..
I am trying to run on linux..

I even used gdb command to backtrace the program..
But it was of no use..

Check my sample code..

char buf[2000],str[15],lno[5],def[15],ref[15],tmp[15],ch,ifile[20],ofile[20];

int i,j,oldi,count,c,r,d,f,t,lc=0;



FILE *fp=NULL,*fpo=NULL;

void xyzstart()
{
/*
*Some operation that is not at all concerned with the file
*
*/
}

int main()

{

printf("Enter the name of the input file\n");

gets(ifile);



fp=fopen(ifile,"r");
if(fp==NULL)

{

printf("Error");

exit(0);

}



printf("Enter the name of the output file\n");
gets(ofile);

fpo=fopen(ofile,"w");


if(fpo==NULL)

{

printf("Output file couldn't be opened\n");

exit(0);

}




while(!feof(fp))

{

fgets(buf,sizeof(buf),fp);

count++;  //Count the number of lines in a file

}



rewind(fp); //move the file pointer to the beginning of the file



while(!feof(fp)) //Error is here!! Segmentation fault (Core Dumped)!!

{

clear();  //User defined function which clears all the memory

if(count==lc)

{

nodef();  //User defined function which doesn't reads from or writes into a file

noref();  //User defined function which doesn't reads from or writes into a file

print();  //User defined function which writes the values to output file

break;

}



fgets(buf,sizeof(buf),fp); 

{

i=0;

lc++;


    while(buf[i]!=' ') //read until it encounters a space..

      {

      lno[i]=buf[i];

      i++;

      }

    lno[i]='\0';

//puts(lno);///

    }



i++;

oldi=i;

ch=buf[i];

switch(ch)

{

case 'x': xyzstart(); break;

default: printf("Nothing found");
}

}



fclose(fpo);

fclose(fp);

return 0;

}

I really don’t know what to do!! Can anyone please help me out??
Thanks in advance!

Here’s the code for both clear and xyzstart()
void clear()

{

memset(buf,'\0',sizeof(buf));

memset(lno,'\0',sizeof(lno));

memset(def,'\0',sizeof(def));

memset(ref,'\0',sizeof(ref));

i=oldi=0;

memset(str,'\0',sizeof(str));

}

void xyzstart()

{

r=d=c=0;

for(;;c++,i++)

       {

           if(buf[i]==' ')

           break;

           if(buf[i]=='(') break;

           if(buf[i]==';')break;

           if(buf[i]=='\n') break;

           if(buf[i]=='=') break;

           if(buf[i]=='+' || buf[i]=='-') break;

           str[c]=buf[i];

        }

        str[c]='\0';

if(buf[i]=='=')

           assignment();

else if(buf[i]=='+' || buf[i]=='-') //Increments or decrements

            incdec();

          else if(buf[i]=='(')

                udefined();

}

Answer 1

Some notes:

1. Do not use feof(fp) as your while loop condition; the function will only return true after you try to read past the end of the file, so you’ll wind up looping once too often. Check the result of your input operation (fgets will return NULL on failure) and then test for EOF, like so:

   
   while(fgets(buf, sizeof buf, fp) != NULL)
     count++;
   if (feof(fp))
     printf("At end of file\n");
   

2. NEVER NEVER NEVER NEVER NEVER use gets: it will introduce a point of failure in your program (and probably has in this case). It has been deprecated as of C99 and is expected to be gone from the next version of the language (yes, the mayhem caused by this one library call is scarier than the prospect of breaking over 30 years of legacy code). Use fgets or some other alternative instead.

From the code you’ve posted, I see no obvious issues; I don’t know why feof would core dump if the previous statement was a successful rewind. The only thing I can figure is that the file pointer is being overwritten somewhere (possibly by a buffer overrun in a gets call).