segmentation fault in file operations in c

Question

#include<stdio.h>

/* this is a lexer which recognizes constants , variables ,symbols, identifiers , functions , comments and also header files . It stores the lexemes in 3 different files . One file contains all the headers and the comments . Another file will contain all the variables , another will contain all the symbols. */

int main()
{
    int i;
    char a,b[20],c;
    FILE *fp1;

    fp1=fopen("source.txt","r"); //the source file is opened in read only mode which will passed through the lexer

    //now lets remove all the white spaces and store the rest of the words in a file 

    if(fp1==NULL)
    {
        perror("failed to open source.txt");
        //return EXIT_FAILURE;
    }
    i=0;
    while(1)
    {


        a=fgetc(fp1);

        if(a !="")
        {
            b[i]=a;
        }
        else
        {


            fprintf(fp1, "%.20s\n", b);
            i=0;
            continue;
        }
        i=i+1;                  

        /*Switch(a)
        {
            case EOF :return eof;
            case '+':sym=sym+1;

            case '-':sym=sym+1;

            case '*':sym=sym+1;

            case '/':sym=sym+1;

            case '%':sym=sym+1;

            case '
        */
    }
return 0;
}

how does this code end up in segmentation fault?

Answer 1

There is no check to test the End of File.

Change

while(1) 

to

while (!feof(fp1))

EDIT:

But the reason for seg-fault is the missing NULL char at the end of b. You can add the NULL char at the end of b just before writing it to the file:

b[i] = 0; // i must <=19
fprintf(fp1, "%.20s\n", b);

This way you also need to ensure that you don't stuff more than 19 char into b that way you always have space to write the NULL char.

Answer 2

And also, the string b is probably not null terminated. At some point in your code you need:

 b[i] = '\0';

Answer 3

Use your while like this:

...
 while((a=fgetc(fp1))!=EOF)
    {
        if(a !="")
        {
            b[i]=a;
        }
...

You will check every-time if a is different from EOF. If a equals EOF, the processing will stop.

I would consider reading a bigger buffer with fread() / fgets(), and parse the resulting string, instead of reading character by character. I think this may improve the overall performance of your parser.

Answer 4

SegFault?? usually INFINITE LOOP...

---

Use the following...

#include<stdio.h>

/* this is a lexer which recognizes constants , variables ,symbols, identifiers , functions , comments and also header files . It stores the lexemes in 3 different files . One file contains all the headers and the comments . Another file will contain all the variables , another will contain all the symbols. */

int main()
{
    int i;
    char a,b[20],c;
    FILE *fp1;

    fp1=fopen("source.txt","r"); //the source file is opened in read only mode which will passed through the lexer

    //now lets remove all the white spaces and store the rest of the words in a file 

    if(fp1==NULL)
    {
        perror("failed to open source.txt");
        //return EXIT_FAILURE;
    }
    i=0;
    while(!feof(fp1))
    {


        a=fgetc(fp1);

        if(a !="")
        {
            b[i]=a;
        }
        else
        {

            b[i]='\0';
            fprintf(fp1, "%.20s\n", b);
            i=0;
            continue;
        }
        i=i+1;                  

        /*Switch(a)
        {
            case EOF :return eof;
            case '+':sym=sym+1;

            case '-':sym=sym+1;

            case '*':sym=sym+1;

            case '/':sym=sym+1;

            case '%':sym=sym+1;

            case '
        */
    }
return 0;
}

GoodLUCK!!

CVS @ 2600Hertz

Answer 5

It looks like it needs a check to be sure it is not writing beyond the end of array b. If it reads over 20 characters, it will write past it and corrupt the stack.