Segmentation fault problem

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October 8th, 2012, 11:27 AM

elenizi

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Segmentation fault problem

hi i want to make a parser that reads from a file and saves thetokens into a struct i am using this code but i keep getting segmentation fault

Code:

#include <stdio.h>
#include <string.h>
#include <strings.h>

typedef struct{
  int label;
  int id;
  int r1;
  int r2;
  int r3;
}instruction;

int main(int argc,char *argv[])
{
    FILE *input;
    char str[1000];   
    char delim[] = " ";
    char *result;

 input = fopen(argv[1], "r"); // error check this!
 int i=0;
    instruction *instr = (instruction*)(malloc(100*sizeof(instruction)));
    while  (fgets(str,1000,input)!=NULL){
  
        result = strtok(str, delim);
        instr[i].label = atoi(result);

        result = strtok(NULL, delim);
        instr[i].id = atoi(result);
 
	result = strtok(str, delim);
        instr[i].r1 = atoi(result);  
         
	result = strtok(NULL, delim);        
        instr[i].r2 = atoi(result); 

	result = strtok(str,delim);
        instr[i].r3 = atoi(result); 
 
       i++;
    }
    fclose(input);
    return 0;
}

October 8th, 2012, 12:20 PM

bdb

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All your strtok() calls except the first, should have NULL for their first argument.

October 8th, 2012, 12:49 PM

salem

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Also, at each step, you should have
if ( result != NULL )
before passing result onto atoi()

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October 8th, 2012, 01:08 PM

elenizi

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tried your advice but the same error appears =/

October 8th, 2012, 01:21 PM

b49P23TIvg

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By chance when you invoke your program do you pass it a file name argument? If you don't your argv[1] is NULL.

Code:

$ supreme_parser filename.goes.here.my_funky_language

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October 8th, 2012, 02:00 PM

salem

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Don't just post "I tried and it didn't work". You need to post actual code changes and actual test commands and results.

> input = fopen(argv[1], "r"); // error check this!
So DO IT!
- check argv[1] actually exists as a command line argument.
- check that the file was opened successfully.

> instruction *instr = (instruction*)(malloc(100*sizeof(instruction)));
CHECK IT!
While you're at it
- remove the cast, so it's instruction *instr = malloc(100*sizeof(instruction));
- add #include <stdlib.h>

If you get some error message about "cannot convert void*", then change your filename / build process so it compiles C code and not C++ code.

Casting malloc in C is as best a waste of time, and at worst a dangerous bug.
At it's most dangerous, the cast masks your failure to include stdlib.h (as you have done), at which point the compiler assumes extern int malloc();
But this is a lie (it returns a pointer). On machines where pointers are larger than ints (which is getting fairly common at the moment with 64-bit machines becoming widely available), the implicit int and cast will lose information, giving you a garbage pointer for your trouble (and handing you a segfault for your reward).

Segmentation fault problem

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