Question

looking for a way to read arguments from a configuration file I found a nice way to do it but the thing is that something really weird happens when I try to count the lines.

This is the code:

FILE *file = fopen("config", "r");

char line[100];
int linenum = 0;
//int foo;  Uncomment and it starts to working, doesn't matter if you rename it.

while(fgets(line, sizeof(line), file) != NULL) {
    char option[4];
    char arg[100];

    if (line[linenum] == '#') 
        continue;
    linenum++;

    if (sscanf(line, "%s %s", option, arg) != 2)
        fprintf(stderr, "Syntax error, line %i\n", linenum);

The config file looks like this:

#config file
option1
option2
option3

So the result is:

Syntax error, line 1
Syntax error, line 0
Syntax error, line 0

But if I declare an int variable with any name before while loop it starts to working!

The result is:

Syntax error, line 1
Syntax error, line 2
Syntax error, line 3

What in the world is happening here? my mind is gonna blow, maybe it’s something dumb but I don’t see any reason for this.

Answer 1

Any time you see strange behavior like this, you should start looking for memory overrun errors. Typical symptoms include:

1. Code that works or stops working if you add or remove unused variables.
2. Variables that change for no apparent reason.
3. Code that sometimes runs normally and crashes other times, using the same input.
4. Code that produces different results between invocations, using the same input.

In this case, as the other answers point out, you have two potential memory-related errors:

The line if (line[linenum] == '#') continue;

should be if (line[0] == '#') continue;

Otherwise the index will be invalid when linenum > 99.

And the line if (sscanf(line, "%s %s", option, arg) != 2){

will overrun the option and arg variables if the input strings are too long. (The error you’re seeing occurs because the text (e.g. option1) is longer than 4 characters and overflows option. You can fix this in several ways:

• Increase the size of option. You’ll need to know the maximum length of the input strings in order to choose an appropriate size.
• Limit the number of characters that are copied into option:
  if (sscanf(line, "%3s %99s", option, arg) != 2){
  This may cause unexpected output for long input strings, because arg will contain the remainder of the string in option if it’s truncated.
• According to http://linux.die.net/man/3/sscanf you can use the a modifier to tell sscanf to allocate memory for the strings as it reads them:
  char **str1, **str2;
if (sscanf(line, "%as %as", &str1, &str2) != 2){