In GCC C is their any way to catch a runtime system error in C code?

schmitta · 01-16-2014, 02:29 PM

I am getting a core dump when running my C program. Is there any C catch mechanism such as signal to catch that error so I can print out the error that the operating system found to a file as well as close the output file? How does that work? Thank you. Alvin...

dwhitney67 · 01-16-2014, 03:15 PM

Just use the debugger.

Code:

$ cc -g -O0 YourProgram.c

$ gdb ./a.out

(gdb) run

[crash occurs]

(gdb) bt

(gdb) quit

[fix your code]

schmitta · 01-16-2014, 03:35 PM

Thank you dwhitney67 That helped a lot.

schmitta · 01-16-2014, 04:03 PM

A problem:
I did

Code:

$ c99 -g -Oapsbasic apsbasic.c

$ gdb ./apsbasic

(gdb) run

but it said:

Code:

Starting program: /home/schmitta/APS_PRODUCTS/USBTOEMBED/software/apsbasic/apsbasic 
usage: run <filename>
[Inferior 1 (process 2844) exited with code 01]
(gdb)

so I typed:

Code:

run ./apsbasic ./pt01.bas

and it said:

Code:

Starting program: /home/schmitta/APS_PRODUCTS/USBTOEMBED/software/apsbasic/apsbasic ./apsbasic ./pt01.bas
usage: run <filename>
[Inferior 1 (process 2849) exited with code 01]
(gdb)

So it would not let me enter the file name ./pt01.bas

So I typed and it answered:

Code:

(gdb) run ./apsbasic
Starting program: /home/schmitta/APS_PRODUCTS/USBTOEMBED/software/apsbasic/apsbasic ./apsbasic

  APSBASIC -- PC to USB to Embedded PIC32 Control Program
     Copyright (c) 2013 APSTRON LLC all rights reserved
                  www.apsproducts.net
             type help for more information


Program received signal SIGSEGV, Segmentation fault.
get_token () at apsbasic.c:2912
2912		*temp2='\0';
(gdb)

So it does not look like it will allow me to enter command line fields after the ./apsbasic module name.

piyush.sharma · 01-17-2014, 02:33 AM

try

Code:

gdb --args apsbasic arg1

schmitta · 01-17-2014, 09:44 AM

Thank you piyush.sharma I will try that.

schmitta · 01-17-2014, 10:26 AM

I have a symbol table structure called "struct symbol" defined globally in a header file. In one of my functions I try to print out the sizeof the structure in "sizeof(symbol)". I get the following error code:

Code:

apsbasic.c:2299:64: error: ‘symbol’ undeclared (first use in this function)

Why does the above not work? Thank you. Alvin...

dwhitney67 · 01-17-2014, 12:25 PM

It would be helpful if you posted ALL of your code (or at least the relevant parts), rather than just the error.

However, if I had to guess... and that's all I can do right now (crystal ball is not functioning at the moment), I would say that you are using a statement like "sizeof(symbol)" versus using "sizeof(struct symbol)".

schmitta · 01-19-2014, 02:37 PM

I was using sizeof(struct symbol); but someone told me that was wrong. Thanks. I eventually instantiated (in C not c++) struct symbol a; and then did sizeof(a); multiplied it by the offset index of the array of structures -- struct symbol{ ... }symbtab[1000]; and then found the address of of the first structure in the array (&symbtab[0]) - added it all together as pointers and came up with a pointer (struct symbol *x

and assigned the added up mess to the pointer (x=&symbtab[0]+index*sizeof(a)

. Then i used the pointer formed to address the the item in the structure I wanted (char *nameptr=(*x).name; where name is a pointer to a string that is the symbol in the symbol table I was looking for). If you have any suggestions it would be greatly appreciated.

dwhitney67 · 01-19-2014, 05:41 PM

First, pick a programming language: C and C++ are not the same, although they share, for the most part, the same syntax.

In C you need to specify the keyword 'struct' when referring to a structure, unless you type-cast it away using a 'typedef'. Using the latter can sometimes lead to confusion when code is analyzed/maintained, thus avoid it if you can.

Code:

// My preferred style; the structure below must be referred to as "struct Symbol".
//
struct Symbol
{
   char* name;
};


// Alternate style, which I personally do not like; here, the structure can be referred to as "Symbol".
//
typedef struct
{
    char* name;
} Symbol;

As for what you are attempting to accomplish, I could not discern such from what you posted. It would seem that your attempts to access specific locations within your structure array may be incorrect. When performing pointer arithmetic on structures, the offset used is assumed to a multiple of the size of the structure.

Thus when computing the offset within your structure, perhaps you meant to do something like:

Code:

struct Symbol* x = &symtab[0] + index;

rtmistler · 01-20-2014, 10:27 AM

To add to the original recommendation for using gdb. What you could also do is enable core file creation.

In the shell:

Code:

ulimit -c unlimited

Also can be done in your program:

Code:

#include <sys/time.h>
#include <sys/resource.h>

struct rlimit rlim = { RLIM_INFINITY, RLIM_INFINITY };

setrlimit(RLIMIT_CORE, &rlim);

Check the result of the setrlimit() call to ensure that it did happen.

When your program fails, it will generate a core file. You then can enter into gdb like the following:

Code:

gdb <binary-name> core

Same result is you can perform backtrace on that. As said before, make sure you use -ggdb in your compilation statement and consider disabling optimization -O0.

As far as the question about the structs, if you have an array of structures and you know the index, then index into the array first and get your structure member before you start performing pointer operations. A possible issue is longword alignment, for instance if you have a structure which is something like:

Code:

struct Eg {
    char array[5];
    char *pArray;
    short smallVar;
    double longVar;
};

If the machine natural word is 32 bits, the pointer will be a 32-bit value; however the short may not be that, but instead by 16-bits, and another problem there is the array of 5 characters may be aligned at a 16-bit word boundary versus a 32-bit word boundary. One thing a person can do there is to use directives for packing; however the behaviors I don't trust and if I know it's important, I then force alignment by declaring that array to be 32 characters and putting in say a filler "short unused" variable between smallVar and longVar. In the end, never rely on normal pointer progression through a struct so that you can declare like a char * and then step through that entire structure; you didn't say you were doing that either, just noting this potential problem. Instead it sounds as if you have an array of structures, say 1000 of the example shown above and say you wish to look at each character in the sub-element named "array". Then you do something like:

Code:

struct Eg myList[1000];
char *p_temp;
int index;

for(index = 0; index < sizeof(myList); index++) {
    p_temp = Eg.array;
    // Then use p_temp to look into the array, but remember that the size of that array is "5" or sizeof(Eg.array)
}

dwhitney67 · 01-20-2014, 11:50 AM

Quote:

Originally Posted by rtmistler

Code:

struct Eg myList[1000];
char *p_temp;
int index;

for(index = 0; index < sizeof(myList); index++) {
    p_temp = Eg.array;
    // Then use p_temp to look into the array, but remember that the size of that array is "5" or sizeof(Eg.array)
}

The code above looks wrong. If you want to index into the array of structures, to get the number of elements (which we know to be 1000), you would use sizeof(myList) / sizeof(struct Eg). Also, you want to access the array, not the structure definition.

Code:

struct Eg myList[1000];

for (size_t index = 0; index < sizeof(myList) / sizeof(struct Eg); index++) {
    char* p_temp = myList[index].array;
    // ...
}

P.S. If you are going to use std-99 style comments with C code, then you might as well extend the std-99 practice to declaring variables closest to where they are used.

rtmistler · 01-20-2014, 12:03 PM

Thanks for the clarifications. I'm a hybrid of K&R, C99, and just adapt as time, styles, and examples grow. I probably also wouldn't use the sizeof a struct anyways and would've just used 1000 directly in my code. Also a big proponent of testing it out if I'm not sure. Code standards can be whatever they want, but what it really does for your case is the most important thing.

schmitta · 01-20-2014, 07:01 PM

Thank you both for your extended comments and information. I will study it all.

I use to program mini computers where we had a linkage editor with names of variables and programs and their addresses in storage. We also had debuggers to manipulate items in memory. When I get a storage dump or just when I want to see what happened I would like to have the address of where in storage the program was loaded as well as a storage viewing routine or something to dump out the hex of storage as well as the ascii translation of the code. I am using Ubuntu. Is there a way to get the function and variable names and their offsets in storage as well as a program to dump out the storage in a formatted way? Thank you. Alvin....

dwhitney67 · 01-20-2014, 07:15 PM

Just use 'gdb' (or the GUI front-end called 'ddd') to debug and analyze your program.