Strange Segmentation Fault

**gw247** · 11-03-2011

I'm working on a program that takes, as input, a file written in C and converts it to IA-32 assembly. If the inp pointer is NULL, it does what it needs to do. If it isn't, it displays Segmentation Fault and exits. However, it still creates the other files that are needed but does not write text to them.

Segmentation Faults are difficult to trace. I'd be extremely grateful for any help rendered.

Thank you

Code:

int main(int argc,char *argv[]) 
{ 
int valid=0,k=0,l=0,i,x,y,z; 
FILE *inp, *outp, *tpin, *tpout; 
char ch, temp[SIZE], decl[SIZE]; 
nodetype *list,*newnode,*prev; 
prev=NULL; 
//fflush(outp); 
 
intro("This program takes a program written in ‘C’ and converts it into a program written in assembly language."); 
 
//valid=readC(inp,argv[1]); 
//if(valid==1) 
//DEBUG 
 
inp=fopen(argv[1],"r"); 
 
if(inp!=NULL) 
    { 
    
    outp=fopen("asmcode.txt","w");

    fprintf(outp,".file\t\"main.c\"\n\t.section\t.rodata\n");

    tpin=fopen("declarein.txt","w");            //Stores input placeholder information

    tpout=fopen("declareout.txt","w");            //Stores output placeholder information

    while(!feof(inp))                    //grabs a line from the input file 
        {

        //do{ 
        fgets(temp,SIZE,inp); 
        //temp[i]=ch; 
        //i++; 
        //}while(ch!='\n'); 
        //temp[i]='\0';

        printf("jjj\n");
 
        if((temp[0]=='s')&&(temp[1]=='c')&&(temp[2]=='a')&&(temp[3]=='n')&&(temp[4]=='f')&&(temp[5]=='(')) 
            {

 
            for(x=6,y=0;x<10,y<4;x++,y++) 
                decl[y]=temp[x]; 
            fprintf(outp,".LC%d:\n\t.string\t",l);    //Stores input assembly declarations in main output file 
            fprintf(tpin,"%s",decl);        //Keeps a copy of the placeholder in declarein.txt 
            fputs("\n",tpin); 
            l++; 
            for(z=0;z<4;z++) 
                fprintf(outp,"%s\n\t.text\n",decl[z]); 
            } 
        else if((temp[0]=='p')&&(temp[1]=='r')&&(temp[2]=='i')&&(temp[3]=='n')&&(temp[4]=='t')&&(temp[5]=='f')&&(temp[6]=='(')) 
            {

            for(x=7,y=0;x!='\0';x++,y++) 
                decl[y]=temp[x]; 
            decl[y]='\0'; 
            fprintf(outp,".LC%d:\n\t.string\t",k);    //Stores output assembly declarations in main output file 
            fprintf(tpout,"%s",decl);        //Keeps a copy of the placeholder in declarein.txt 
            fputs("\n",tpout); 
            k++; 
            for(z=0;z<4;z++) 
                fprintf(outp,"%s\n\t.text\n",decl[z]); 
            } 
        else for(x=0;x!='\n';x++) 
                    if(temp[x]=='=') 
                        {     
                            newnode=(nodetype*)malloc(sizeof(nodetype)); 
                            newnode->next=list; 
                            list=newnode; 
                            for(y=0,z=0;y<x;y++,z++) 
                            newnode->nm[z]=temp[y]; 
                            newnode->val=temp[x++]; 
                        } 
    } 
    
    fclose(tpin); 
    fclose(tpout); 
    fprintf(outp,".globl\tmain\n\t.type\tmain@function\nmain:\n\tpushl\t%cebp\n\tmov\t%cesp,%cebp\n",'%','%','%'); 
    fseek(inp,0L,0); 
    while(!feof(inp)) 
        { 
        fgets(temp,SIZE,inp); 
        if((temp=="int main()")||(temp[0]=='#')||(temp[0]==' ')||(temp[0]=='{')||(temp[0]=='}')) 
            fseek(inp,1L,1); 
        //convert(inp,outp,list); 
        fseek(inp,1L,1); 
        } 
    fprintf(outp,"\n\tmov\t$0,%ceax\n\tleave\n\tret\n\t.size\tmain,.-main\n\t.ident\t\"GCC:(Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2\"\n\t.section\t.note.GNU-stack,\"\",@progbits",'%'); 
    } 
else { 
    printf("\n\nInvalid input file"); 
    exit(1); 
    } 
fclose(outp); 
fclose(inp); 
terminate(); 
return 0; 
}

**anduril462** · 11-03-2011

Seg faults are not terribly difficult to trace if you know how to use a debugger. Learn to use one, it will save you hours of headache.

Your code is rife with potential seg faults and other errors and bad practices. Not to mention, we would need the full, compilable code to iron out all your issues. But what you've provided has enough problems to keep you busy for a while:

You don't check if argv[1] is valid (or if argc > 1) before using it
You open 4 files, but only check if one file pointer is null before use
You malloc memory, but don't check if malloc succeeded (or failed, returning null) before using that memory
You didn't provide us with the definition of nodetype, so for all we know, your loops that mess with newnode->nm[z] have overflows.
You control loops with feof. Read this: Cprogramming.com FAQ > Why it's bad to use feof() to control a loop
You use horrible variable names like k, l, i, x, y, z, and don't comment your code well, so we have no idea what you're trying to do and can't check for logic errors.
You compare strings with == (line 86) instead of strcmp. You compare sequences of characters with a giant list of char by char ==. Try using strncmp or memcmp for this.
You never initialize list to null, so the list you build is never properly terminated.
Your loop on line 51 probably doesn't do what you think it does. It prints decl, then decl starting at the 2nd char, then again starting at the 3rd, and finally starting at the 4th.
Your loop on line 57 does not check if you overrun the bounds of decl or temp

That's what I found in my 5 minute overview. You have some serious work to do improving your C skills. Perhaps you should put this project aside for the time being and go back to the basics.

Read some tutorials (like ours here: Cprogramming.com - Programming Tutorials: C++ Made Easy and C Made Easy) and Google for more. Crack open those textbooks and class notes. Work the practice problems. Then, get paper and pencil, and plan your solution to this problem thoroughly before touching the keyboard. Once it's planned, code in very small sections, around 5-10 lines, and test as you go, making sure everything works perfectly before moving on to the next part.

**gw247** · 11-04-2011

Originally Posted by anduril462

Seg faults are not terribly difficult to trace if you know how to use a debugger. Learn to use one, it will save you hours of headache.

I'm gonna go through my basics first

Originally Posted by anduril462

You don't check if argv[1] is valid (or if argc > 1) before using it
You open 4 files, but only check if one file pointer is null before use
You malloc memory, but don't check if malloc succeeded (or failed, returning null) before using that memory
You control loops with feof. Read this: Cprogramming.com FAQ > Why it's bad to use feof() to control a loop
You use horrible variable names like k, l, i, x, y, z, and don't comment your code well, so we have no idea what you're trying to do and can't check for logic errors.
You compare strings with == (line 86) instead of strcmp. You compare sequences of characters with a giant list of char by char ==. Try using strncmp or memcmp for this.
You never initialize list to null, so the list you build is never properly terminated.
Your loop on line 51 probably doesn't do what you think it does. It prints decl, then decl starting at the 2nd char, then again starting at the 3rd, and finally starting at the 4th.
Your loop on line 57 does not check if you overrun the bounds of decl or temp

Checked and solved. Thank you.

I appreciate you taking the time to go through my code. This was a real eye-opener. I'm gonna start solidifying my basics. Thank you again.

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