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segmentation fault

This is a discussion on segmentation fault within the C Programming forums, part of the General Programming Boards category; I am trying to read the contents of a file into a char array and then print that array and ...

  1. #1
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    segmentation fault

    I am trying to read the contents of a file into a char array and then print that array and so far it actually works but it not only prints the array but also "Segmentation fault. (core dumped)". Here is the code I am using.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    int main(int argc, char *argv[]){
        if (argc != 3){
            printf("Usage: %s <input filename> <file to write to>\n", argv[0]);
        }
        else {
            FILE *ifile = fopen(argv[1], "r");
            FILE *ofile = fopen(argv[2], "w");
            if (!ifile){
                printf("Error: Could not open input file \n");
            }
            else {
                if(!ofile){
                    printf("Error: Could not write to file %s", argv[2]);
                }
                else {
                    int length = 0;
                    int i = 0;
                    while(fgetc    (ifile) != EOF){
                        length++;
                    }
                    char input[] = "";
                    rewind(ifile);
                    for(i = 0; i < length; i++){
                        input[i] = fgetc(ifile);
                    }
                    printf("%s",input);
                }
            }
        }
        return 0;
    }
    I am guessing it does not work because I didn't initialize the length of the input[] array correctly, but how would I go about doing that?

  2. #2
    SAMARAS std10093's Avatar
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    You have have
    Code:
    char input[] = "";
    so automatically, the length is computed by the compiler to preserve enough space to fit what is assigned to it, thus ""
    What you need is
    Code:
    char input[length] = "";
    If would suggest you to make sure that length is a positive number. You could do that after line 22.
    Code - functions and small libraries I use


    It’s 2014 and I still use printf() for debugging.


    "Programs must be written for people to read, and only incidentally for machines to execute. " —Harold Abelson

  3. #3
    and the hat of wrongness Salem's Avatar
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    You're right, your input array lacks a length, and lacks the code to place a \0 at the end of it.

    If your compiler supports variable length arrays, you can do this
    char input[length+1];
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  4. #4
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    ok so i tried
    Code:
    char input[length] = "";
    but i am getting a compiler error:
    Code:
    :23:26: error: variable-sized object ‘input’ may not be initialized

  5. #5
    SAMARAS std10093's Avatar
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    Salem explained the problem, but you posted the same time so I guess you didn't notice it

    You have to dynamically allocate your memory, or set from start a constant number that is logical for your operation ( not too small, not too big )
    Code - functions and small libraries I use


    It’s 2014 and I still use printf() for debugging.


    "Programs must be written for people to read, and only incidentally for machines to execute. " —Harold Abelson

  6. #6
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    i also tried length+1 but that doesnt work either
    so do i have to allocate memory? or is there a way to enable variable lengths (i am using gcc)

  7. #7
    SAMARAS std10093's Avatar
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    length + 1 was meant for the terminating null. You have to remember this for your dynamic allocation too!

    I provided you two options... Not one

    If you interesting read here for the gcc
    Code - functions and small libraries I use


    It’s 2014 and I still use printf() for debugging.


    "Programs must be written for people to read, and only incidentally for machines to execute. " —Harold Abelson

  8. #8
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    what option would i have to use for gcc though? i read it but it says nothing about variable size

  9. #9
    SAMARAS std10093's Avatar
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    Maybe this is better.
    Code - functions and small libraries I use


    It’s 2014 and I still use printf() for debugging.


    "Programs must be written for people to read, and only incidentally for machines to execute. " —Harold Abelson

  10. #10
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    nope, still doesn't work. i now tried every version of c (c99/c90/c89/etc) and it doesnt work with any of them it is still giving me the error
    Code:
    main.c: In function ‘int main(int, char**)’:main.c:23:30: error: variable-sized object ‘input’ may not be initialized
    when using
    Code:
    char input[length + 1] = "";
    and when i am using
    Code:
    char input[length + 1];
    it gives me this:
    Code:
    /tmp/ccEoODxU.o:(.eh_frame+0x13): undefined reference to `__gxx_personality_v0'collect2: ld returned 1 exit status
    i also tried using the
    Code:
    -pedantic
    flag but that changed nothing
    what am i doing wrong

  11. #11
    SAMARAS std10093's Avatar
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    Go to post number 5.

    (If i were you I would do that. It is not such a big deal )
    Code - functions and small libraries I use


    It’s 2014 and I still use printf() for debugging.


    "Programs must be written for people to read, and only incidentally for machines to execute. " —Harold Abelson

  12. #12
    and the hat of wrongness Salem's Avatar
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    > __gxx_personality_v0
    What is your command line for compiling?
    Because this suggests you're using g++ to compile your C program.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  13. #13
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    but how would i dynamically allocate memory? sorry, i am kind of new to all this

  14. #14
    a_capitalist_story
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    Code:
    /tmp/ccEoODxU.o:(.eh_frame+0x13): undefined reference to `__gxx_personality_v0'collect2: ld returned 1 exit status
    Are you compiling this using g++ instead of gcc? I think that's the cause of this error.

  15. #15
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    no, i am using gcc with
    Code:
    gcc -Wall main.c

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