Segmentation Fault (works on Windows but not Unix)

Question

The following is a simple program that reads a .dat file that i included below. when i gcc compile from a windows command line everything works fine when i run the exe but when i run from a unix cmd line..... ".out" i get a segmentation fault. Why is this the case

#include<stdio.h>

int main(){

    FILE *fp;
    char ch;
    char c;
    char word[15];
    int count = 0;
    int a;
    int boolean_comma = 0;

    fp = fopen("lab1.dat","r");

    if( fp == NULL) {
        printf("ERROR");
    }else
        while(!feof(fp)){

            ch = fgetc(fp);
            word[count] = ch;

            /*printf("%c",word[count]);*/

            if(ch == ','){

                count -= 1;
                boolean_comma = 1;
            }/*END IF*/

            if(ch == ' '){
                if(count == 0){
                    count-=1;
                }
                if(count == 4 && boolean_comma == 1){
                    printf("****, ");
                }

                if(count == 4 && boolean_comma == 0){

                    printf("**** ");
                }/*END IF*/

                else{

                    if(boolean_comma != 1){
                        for(a = 0; a < count; a++){

                            c = word[a];            
                            printf("%c",c);

                        }/*END FOR*/
                    }
                    boolean_comma = 0;
                    printf(" "); 
                    count = 0;
                }/*END ELSE*/

                count = 0;
                /*END IF*/}
            else{
                count++;
            }/*END ELSE*/

            printf("%i",count);
        }/*END WHILE*/

    fclose(fp);

    return 0;

}//end main

FILE

p1data.dat
Mary had a little lamb, its fleas
as white as snow, And everywhere that Mary went, the
fleas were sure to go!

Which line is it getting the segmentation fault on? The debugger should tell you this. — Barmar, Sep 09 '15 at 20:50
Segmentation faults generally occur when you use an uninitialized pointer or write outside the bounds of an array. Since you have no pointers (other than `fp`, which you use correctly), it must be the `word[]` array. Make sure you never access too far. — Barmar, Sep 09 '15 at 20:51

score 4 · Accepted Answer · edited May 23 '17 at 11:58

As well as the obvious upper limit, it seems that when count gets to 0, it can go negative.

if(count == 0){
    count-=1;
}

This will cause a seg fault with this

word[count] = ch;

Additionally you are using the dreaded feof

while(!feof(fp))

instead of

int ch;
while((ch = fgetc(fp)) != EOF)

Note the int type. Don't use char (or float) types unless you have a specific reason to. Library functions use int for character input and output, not char.

score 0 · Answer 2 · answered Sep 09 '15 at 21:01

0

It seems that when count gets to 15,then word[count]= ch; writes out of the bound of word

answered Sep 09 '15 at 21:01

blakelead

1,780
2
17
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1

In the given example `count` will not be negative but you are right for the general case. – blakelead Sep 09 '15 at 21:08

score 0 · Answer 3 · answered Sep 09 '15 at 21:11

I've just compile your file on my ubuntu 15.04, create lab1.dat file with your content and run it

vim a.c
vim lab1.dat
gcc a.c
./a.out
123456789101112131415p1data.dat
Mary 0123had 01a 0123456little 012344****,  0123its 012345678fleas
as 012345white 012as 012344****,  0123And 012345678910everywhere 01234****    01234**** 012344****,  0123456789the

My gcc -V output is

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.9.2-10ubuntu13' --with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.9 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.9.2 (Ubuntu 4.9.2-10ubuntu13)

So, program successful fun without any problem. Could you please specify linux distribution, gcc -V output and recheck if you have problem with your code.

Segmentation Fault (works on Windows but not Unix)

3 Answers3