Surprising Segfault

[sangamesh]

Hi,

I've the following code in main() function.. The scanf is written to read multiple strings in a single array of char:
-----
--

else
            {
                printf("\n\n MODULAR VIEW \n");
                printf("cmd> Type help to get help \n");
                printf("cmd>");                
                scanf("%[^\n]",usr_in);          
                help_module(usr_in);
            }

The prototype of the called function is:

void help_module(char *);

The variable declaration is:

char user_in[80]

This segfaults during execution.:

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MODULAR VIEW
cmd> Type help to get help
Segmentation fault

The surprising point here is that - its getting segfault before scanf. Means its just printing the printf statements and then segfault. Why so? I've checked by putting some more printf's before the function call and scanf. But anytime its not reading any value through scanf. Simply segfaults. What may be the reason here?

[Salem]

When is a segfault not a surprise

Anyway, instead of posting random lines of code expecting us to guess what you did wrong, run the code in the debugger and it will stop you at the precise point of the segfault.

Then you can start looking around the current code state to see which pointer is NULL or garbage.

[sangamesh]

Compiled with "-g" option:

 icc -g lsub.c -o lsubg

This is the output of gdb:

(gdb) file lsubg
Reading symbols from /windows/D/san/work/work/lsubC/lsubg...(no debugging symbols found)...done.
(gdb) run
Starting program: /windows/D/san/work/work/lsubC/lsubg
Detaching after fork from child process 17905.
Detaching after fork from child process 17906.


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 MODULAR VIEW 
cmd> Type help to get help 
cmd>
cmd again 

Program received signal SIGSEGV, Segmentation fault.
0x000000396e67efa2 in strcmp () from /lib64/libc.so.6
Missing separate debuginfos, use: debuginfo-install glibc-2.10.1-5.x86_64 libgcc-4.4.1-2.fc11.x86_64
(gdb)

But if I change the scanf statement from

scanf("%[^\n]",usr_in);

to

scanf("%s", usr_in);

It runs well. But it stores only the first word. Any hints why this scanf giving segfault

[Salem]

The faulting function call is strcmp, not scanf

The first thing you do is 'bt' to get the stack trace showing where you called strcmp from.
Examine in detail the parameters you're passing - one of them isn't a valid address.

[sangamesh]

The faulting function call is strcmp, not scanf

The first thing you do is 'bt' to get the stack trace showing where you called strcmp from.
Examine in detail the parameters you're passing - one of them isn't a valid address.

The strcmp function is used in the following way:

The char array contains set of strings.

char*  help_str[] = {"help","help one","help one two", "help one two three", "help one two three four"};

And there is a string which is inputted from user: user_val (Declared as char user_val[50]). This string is passed to the user-defined function, in which the strcmp operation is perfomed against the items present in the array help_str. In this step, its giving seg-fault.

strcmp(usr_val,help_str[j]);

[Salem]

And the loop that is counting j is what?

If you step off the end of that array, it will fault.

You should post more code, rather than just single lines.

[patrink]

But if I change the scanf statement from

scanf("%[^\n]",usr_in);

to

scanf("%s", usr_in);

It runs well. But it stores only the first word. Any hints why this scanf giving segfault

Just a tip, scanf separates strings when it encounters a space or newline (and maybe something else, correct me if I'm wrong)

typing in:

char Variable1[80], Variable2[80], Variable3[80];

scanf("%s", Variable1);
scanf("%s%s", Variable2, Variable3);

printf("%s\n%s%s", Variable1, Variable2, Variable3);

(if we type in "Hello World" into the console) it would return

Hello
Hello World!

Scanf sees a space and ends off the variable there, thats why it would require 2 variables to actually take in "Hello World!"

To get around this (and a safe way to do so) is to use fgets. An example of this would be something such as:

char Variable[80];
fgets(Variable, sizeof Variable, stdin);

Its not only faster than scanf (atleast thats what I heard because scanf has to check if its taking in an int,double,float,string, and formatting) its also safer because you're limiting how much data can be taken in (that "sizeof Variable") which will make sure the buffer doesn't overflow. Correct me if I'm wrong with any of this, its all of the top of my head.