Seg fault with struct behavior

Question

I currently have a struct for storing port and destination address info which I want to populate with data read from a simple text file.

typedef struct {
    int listen_port;
    int forward_port;
    char *forward_addr;
} Route;

If I just assign 30 it works fine.

E.G.

while((read = getline(&line, &length, f)) != -1) {
    Route *srv = (Route*) malloc(sizeof(Route));
    srv->listen_port = 30;  //works fine
    /* rest of code */
}

But if I try to use strtok to tokenize a line read from file 80,127.0.0.1,8080 and then assigned that to the srv->listen_port, I get a seg fault.

E.G.

while((read = getline(&line, &length, f)) != -1) {
    int port = atoi(strtok(line, ","));
    Route *srv = (Route*) malloc(sizeof(Route));
    srv->listen_port = port;  //seg fault
    /* unreachable code */
}

Can someone explain why this is happening? I'm stumped as to why it isn't working.

Thanks in advance.

Do I need to? I can print the contents of line without malloc'ing — user1231232141214124, Mar 20 '14 at 21:40
My Crystal ball tells me the problem is in the `getline` function. Show the code. — Jabberwocky, Mar 20 '14 at 21:41
I recommend you disregard any hints anybody has given you. Instead, load up your debugger, let your program crash, and then get a back trace. There is no more valuable programming skill you should learn sooner than debugging. — San Jacinto, Mar 20 '14 at 21:46
It was `getline` with `char*` causing the problems. When I switched to `fgets` and `char line[BUFFER]`, everything worked fine. — user1231232141214124, Mar 20 '14 at 22:00
@San Jacinto - Can you recommend any good resources for learning the C debugging tools? My C experience is lacking. — user1231232141214124, Mar 20 '14 at 22:02
@redFIVE Learning C is much more difficult than learning whatever debugger you want to use. Pretty much any C book will give you the basics. K&R taught 2 generations or more of C programmers. Regarding learning gdb, google is actually very helpful. Once you learn how to load a program, run it, pause it, set breakpoints, step into/over, and view the values of variables, you've accomplished 95% of what you need to debug any userspace program. Again, highly recommend you forget printf exists and learn to use the debugger. Save yourself time by investing an hour and learning the basics of gdb. — San Jacinto, Mar 21 '14 at 11:16
BTW, if you try gdb, use the -tui option. It will make your experience much more enjoyable. — San Jacinto, Mar 21 '14 at 11:18

score 3 · Accepted Answer · edited May 23 '17 at 11:44

3

See

atoi — how to identify the difference between zero and error?

Also, strtok can just return NULL if line is a zero length string, so there is potentially atoi(NULL)

This code's behavior can be so undefined...

1) intialize line to NULL, getline will allocate space for you ( if it is not NULL, it will assume you already allocated it )

2) free line after use

3) use strtol or scanf to get the integer, since you and the compiler can't be sure the input line will contain a number in ascii digits

4) check the return value of malloc , it returns NULL in case of failure

As a general rule, treat every possible error case, it is a necessary best practice in C

edited May 23 '17 at 11:44

Community

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answered Mar 20 '14 at 21:46

Gábor Buella

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Yeah my C is extremely rusty. `atoi` wasn't liking the `strtok` from the `getline`. But once I changed it from `char* line` to `char line[BUFFER]` in combination with `fgets`, everything worked like a charm. I appreciate the help. – user1231232141214124 Mar 20 '14 at 21:57

Seg fault with struct behavior

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