I don't know the cause of this segmentation fault

This is a discussion on I don't know the cause of this segmentation fault within the C Programming forums, part of the General Programming Boards category; Hello. For one of my classes, I have to write a program that reads in a file that contains a ...

  1. #1
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    I don't know the cause of this segmentation fault

    Hello. For one of my classes, I have to write a program that reads in a file that contains a string and looks for occurrences of substrings asked by the user. I am running this on Ubuntu by the way.

    Anyway, the terminal commands are

    echo "abcdefgh" > bar
    ./file bar bc

    where file is the name of my program, bar is the file that "abcdefgh" is saved to, and bc is the string the user wants to find. The code for my program is below

    Code:
    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    #include <errno.h>
    
    
    int main(int argc, char *argv[])
    {
        FILE *pFile;
        char *textFile=NULL;
        textFile=(char *)malloc(sizeof(char)*10);
        pFile=fopen(argv[1],"r");
        
        int count;
        
        while(fgets(textFile,sizeof(textFile)-1,pFile)!=NULL)
        {
    
    
        if(argc>1)
        {
        for(count=1;((textFile=strstr(textFile,argv[count]))!=NULL);count++)
        {
        textFile++;
        }//end for
        }//end if
        }//end while
                
    printf("The string appeared: %d times\n",count);
    
    
    fclose(pFile);
    
    
    free(textFile);
    
    
    return 0;
    
    
    }//end main
    I am receiving the Segmentation Fault error but have no idea where the problem is or how to fix it. If someone could tell me, that would be great.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Well there are so many places it could go wrong.

    > textFile=(char *)malloc(sizeof(char)*10);
    You don't check for success here.

    > pFile=fopen(argv[1],"r");
    You don't check for success here.

    > while(fgets(textFile,sizeof(textFile)-1,pFile)!=NULL)
    You're lying to fgets about the true size of the buffer. sizeof() in a pointer does NOT tell you how much data it is pointing to.

    > for(count=1;((textFile=strstr(textFile,argv[count]))!=NULL);count++)
    Your loop pays no attention to argc, so you could be accessing argv out of bounds

    > textFile++;
    Modifying a pointer to something malloc'ed

    > free(textFile);
    Freeing a modified pointer.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    Algorithm Dissector iMalc's Avatar
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    One more Salem missed, the string that is being read in does not have a null-terminator appended.

    So yeah, there's 7 reasons why it would segfault all up so far. That's pretty impressive for a mere 41 lines of code.

    This is also a very very bad practice:
    Code:
        }//end for
        }//end if
        }//end while
    If you have to write what the curly bracket is for then you're doing it wrong.
    Learn about indentation and how each open curly bracket means the following code is tabbed out further and each closing curly bracket means that code is following tabbed out less. This is the first thing you need to get right when you start programming. It is not something to just get right later.
    Last edited by iMalc; 02-05-2012 at 12:44 AM.
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  4. #4
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    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

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