Trying to debug a c program segmentation fault using output from valgrind - c

Question

A CLI program of mine compiles and runs fine on windows. Compiles fine on linux, but causes a segmentation fault when running.
I turned to stackoverflow for help, and found a few questions similar to what I was going to ask that suggested valgrind, which I just happen to have installed (woo!).
So I ran my program through valgrind, and got a depressingly large amount of output, but I shall start with the first error message:
==11951== Command: ./vt
==11951==
Loading...
Load default database? (y/n)y
Opened input file vtdb.~sv, reading contents...
==11951== Invalid write of size 1
==11951== at 0x400FA9: readnumberfromfile (in /home/rob/Documents/programming/c/vocabtest/vt)
==11951== by 0x400C21: getrecordsfromfile (in /home/rob/Documents/programming/c/vocabtest/vt)
==11951== by 0x401FFD: main (in /home/rob/Documents/programming/c/vocabtest/vt)
==11951== Address 0x53b05bb is 0 bytes after a block of size 11 alloc'd
==11951== at 0x4C28FAC: malloc (vg_replace_malloc.c:236)
==11951== by 0x400EAC: readnumberfromfile (in /home/rob/Documents/programming/c/vocabtest/vt)
==11951== by 0x400C21: getrecordsfromfile (in /home/rob/Documents/programming/c/vocabtest/vt)
==11951== by 0x401FFD: main (in /home/rob/Documents/programming/c/vocabtest/vt)
==11951==
...finished.
1180 entries read from vtdb.~sv.
The problem seems to be in readnumberfromfile, and I've looked through it, and I can't seem to find what's wrong with it!
Can anyone shed some light?
int readnumberfromfile (int maxvalue,char separator)
{
int number, i=0;
char ch;
char * buff = (char *)malloc(11);//allocate enough space for an 10-digit number and a terminating null
if (!buff) {printf("Memory allocation failed!\n");return 0;}//return 0 and print error if alloc failed
if (!maxvalue) maxvalue=MAXINTVALUE;
ch=getc(inputfile);
while (!isdigit(ch))
{
if (ch == separator||ch=='\n'||ch==EOF) {fprintf(stderr,"Format error in file\n");return 0;}//if no number found(reached separator before digit), print error and return 0
ch = getc(inputfile);//cycle forward until you reach a digit
}
while (i<11 && ch!=separator && ch!='\n')//stop when you reach '~', end of line, or when number too long
{
buff[i++]=ch;
ch = getc(inputfile); //copy number from file to buff, one char at a time
}
buff[i] = '\0';//terminate string
number = atoi(buff)<=maxvalue ? atoi(buff) : maxvalue;//convert string to number and make sure it's in range
free(buff);
return number;
}
This is called from getrecordsfromfile if that's of any use:
void getrecordsfromfile(char * inputfilename,char separator)
{
int counter = 0;
struct vocab * newvocab;
struct listinfo * newvocablist;
if (!(inputfile = fopen(inputfilename, "r")))
{
printf("Unable to read input file. File does not exist or is in use.\n");
}
else
{
printf("Opened input file %s, reading contents...\n",inputfilename);
while (!feof(inputfile))
{
newvocab = (struct vocab *)malloc(sizeof(struct vocab));
if (!newvocab)
{
printf("Memory allocation failed!\n");
return;
}
else
{
newvocab->question=readtextfromfile(MAXTEXTLENGTH,separator);
newvocab->answer=readtextfromfile(MAXTEXTLENGTH,separator);
newvocab->info=readtextfromfile(MAXTEXTLENGTH,separator);
newvocab->hint=readtextfromfile(MAXTEXTLENGTH,separator);
newvocab->right=readnumberfromfile(1,separator);
newvocab->counter=readnumberfromfile(0,separator);
newvocab->known=readnumberfromfile(3,separator);
switch (newvocab->known)
{
case 0: newvocablist = &n2l;break;
case 1: newvocablist = &norm;break;
case 2: newvocablist = &known;break;
case 3: newvocablist = &old;break;
}
addtolist(newvocab,newvocablist);
if (newvocab->question==NULL||newvocab->answer==NULL)
{
printf("Removing empty vocab record created from faulty input file...\n");
removefromlist(newvocab,newvocablist,1);
}
else counter++;
}
}
fclose(inputfile);
printf("...finished.\n%i entries read from %s.\n\n",counter,inputfilename);
}
return;
}
Full source can be gitted from https://github.com/megamasha/Vocab-Tester
A couple of notes: I am trying to help myself, I have done my research, looked at similar questions and found out about valgrind myself.
I am still a relative beginner though, and while I appreciate solutions (WHAT to do to fix it), yet more useful is knowledge (HOW to fix or avoid it myself next time). I am here (and very keen) to learn.

Answer 1

buff[i] = '\0';//terminate string
in here i == 11, since you allocated only 11 chars, and while condition ends when i=11.
so, you access a memory you did not allocate.
the behavior for this situation is not defined.
you can solve this by allocating one extra character on your malloc.

Answer 2

int number, i=0;
...
while (i<11 ...
You are reading up to eleven digits for i = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. And then trying to stick the \0 in twelfth slot buff[11].
It's called an "off by one error".
So the fix depends on what you want to change. If you want to accept 11 characters, change the malloc of buff. If you want to only accept 10, then change the while condition.

Answer 3

Invalid write of size 1
You're probably writing a char
Address 0x53b05bb is 0 bytes after a block of size 11 alloc'd
You've only just overflowed something of size 11
Both in readnumberfromfile
This is suspiciously related (by the sizes):
char * buff = (char *)malloc(11);
This will be done with i = 11 after the loop, which is past the end of the allocation:
buff[i] = '\0'
As wormsparty says you can get valgrind to be more helpful, by getting debug symbols in your binary.

Answer 4

For later, if you compile with -g, valgrind will show you exactly at which line the segfault happened.