Mindbender
asked on
free() behaviour questions
OK, I have a unix shell. I am using gcc compiler for cygwin and on a unix box @ uni.
I am wondering why, if I have the shell's control loop only exiting when 'exit' is typed in to the command line, a bunch of free()s that shouldn't have even been reached cause segmentation faults before the shell script has exited. The free()s relate to the input string and this is the problem, I beleive. The free()s are before a return of the regular int on exiting the main function.
Whatchya reckon?? Guidance appreciated.
I am wondering why, if I have the shell's control loop only exiting when 'exit' is typed in to the command line, a bunch of free()s that shouldn't have even been reached cause segmentation faults before the shell script has exited. The free()s relate to the input string and this is the problem, I beleive. The free()s are before a return of the regular int on exiting the main function.
Whatchya reckon?? Guidance appreciated.
SOLUTION
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ASKER
Ahh, thanks! Why isn't that referenced by the getdomainname man page I wonder?
Hi Mindbender,
> get the login name into a C program
You could also use the environment for that:
getenv("USER")
>Why isn't that referenced by the getdomainname man page I wonder?
Because everything's related ... somehow.
Cheers!
Stefan
> get the login name into a C program
You could also use the environment for that:
getenv("USER")
>Why isn't that referenced by the getdomainname man page I wonder?
Because everything's related ... somehow.
Cheers!
Stefan
ASKER
Why does the free() cause segmentation faults before they're processed??
Code example
char* function() {
char* stringA = (char*)malloc(MAX_STRING);
char* stringB = (char*)malloc(MAX_STRING);
char* stringC = (char*)malloc(2*MAX_STRING
stringA = "something";
stringB = "somethingelse";
stringC = strcat(stringA,stringB);
free(stringA);
free(stringB);
return(stringC);
}
Why doesn't that work?
Code example
char* function() {
char* stringA = (char*)malloc(MAX_STRING);
char* stringB = (char*)malloc(MAX_STRING);
char* stringC = (char*)malloc(2*MAX_STRING
stringA = "something";
stringB = "somethingelse";
stringC = strcat(stringA,stringB);
free(stringA);
free(stringB);
return(stringC);
}
Why doesn't that work?
one problem with this ( which may or may not be related to yours segfault)
strcat doesnt concatenate two strings and return the result ,
its appends the second argument to the first ....
man strcat NAME
strcat, strncat - concatenate two strings
SYNOPSIS
#include <string.h>
char *strcat(char *dest, const char *src);
char *strncat(char *dest, const char *src, size_t n);
DESCRIPTION
The strcat() function appends the src string to the dest string overwriting the `\0' character at the end of dest, and
then adds a terminating `\0' character. The strings may not overlap, and the dest string must have enough space for the
result.
The strncat() function is similar, except that only the first n characters of src are appended to dest.
RETURN VALUE
The strcat() and strncat() functions return a pointer to the resulting string dest.
strcat doesnt concatenate two strings and return the result ,
its appends the second argument to the first ....
man strcat NAME
strcat, strncat - concatenate two strings
SYNOPSIS
#include <string.h>
char *strcat(char *dest, const char *src);
char *strncat(char *dest, const char *src, size_t n);
DESCRIPTION
The strcat() function appends the src string to the dest string overwriting the `\0' character at the end of dest, and
then adds a terminating `\0' character. The strings may not overlap, and the dest string must have enough space for the
result.
The strncat() function is similar, except that only the first n characters of src are appended to dest.
RETURN VALUE
The strcat() and strncat() functions return a pointer to the resulting string dest.
ASKER
OK, that's true, but that is just because I was too lazy to properly write it out ;) Sorry. I was intending to purely demonstrate the way I had used free() calls. If the free calls are put after the return, it all works fine.
The issue here is that I am inexperienced with C....can I put return() anywhere in a function and have it still work? That would explain it well enough for me. I am used to putting return as the last statement in a function. There is no set text for this unit, so I went without.
The issue here is that I am inexperienced with C....can I put return() anywhere in a function and have it still work? That would explain it well enough for me. I am used to putting return as the last statement in a function. There is no set text for this unit, so I went without.
ASKER CERTIFIED SOLUTION
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anything after return in a fucntion is not executed ..
return , just returns the control to the function caller
return , just returns the control to the function caller
>....can I put return() anywhere in a function and have it still work?
When the execution encounters "return" statement, it will return from the function at that point.
The subsequent statements will not executed!
That's the reason your code worked when you moved "return" to before "free"!
Actually the "free" statements nver got executed and hence it didn't crash!!
-ssnkumar
When the execution encounters "return" statement, it will return from the function at that point.
The subsequent statements will not executed!
That's the reason your code worked when you moved "return" to before "free"!
Actually the "free" statements nver got executed and hence it didn't crash!!
-ssnkumar
ASKER
I guess I should try it, eh? The reason I am not is because I am in a Winblows lab right now using Java (no unix access unless I feel like SSH'ing nutmeg), but I will. If I answer my own question, what do I do? I want to give points to avizit, but it isn't the correct answer for the main question. Been a LONG time since I have been here (about year in a serious sense).
ASKER
OK then, but why did the free() statements cause segmentation faults?
free() can only free what has been allocated by malloc etc ,
free cannot free just any pointers ..
so when you have some other value in stringA which has not be allocated by malloc/calloc etc you get segfault
free cannot free just any pointers ..
so when you have some other value in stringA which has not be allocated by malloc/calloc etc you get segfault
ASKER
Wow, I can't keep up with you guys. OK, thankyou, that explains my problems. Let me try it out and I will award points.
Look at my first comment:
>This address is not assigned by you using malloc. So, you cannot also free this address.
>That is the reason:
> free(stringA);
>gives a segmentation fault!
Hope this helps you.....
-ssnkumar
>This address is not assigned by you using malloc. So, you cannot also free this address.
>That is the reason:
> free(stringA);
>gives a segmentation fault!
Hope this helps you.....
-ssnkumar
Hi Mindbender,
If you had read my first comment, it would have become clear for you.
I have clearly explained what is happening with your code. Why it segfaults and also how to overcome it.
I don't know if you checked it.
-ssnkumar
If you had read my first comment, it would have become clear for you.
I have clearly explained what is happening with your code. Why it segfaults and also how to overcome it.
I don't know if you checked it.
-ssnkumar
ASKER
Christ, what a mess. To clarify my responses:
I will probably accept a split answer, 125 to ssnkumar, 50 to avizit.
I have been flicking between apps, and not paying a large amount of attention to this page. I completely missed ssnkumar's explanation, which I will probably accept as an answer.
I will probably accept a split answer, 125 to ssnkumar, 50 to avizit.
I have been flicking between apps, and not paying a large amount of attention to this page. I completely missed ssnkumar's explanation, which I will probably accept as an answer.
Thanks for reading my comments also.
I didn't bargain for the points!
I felt bad that, you completely missed out my comments.
Happy to know that it helped you:-)
-ssnkumar
I didn't bargain for the points!
I felt bad that, you completely missed out my comments.
Happy to know that it helped you:-)
-ssnkumar
ASKER
Thanks guys, points awarded!
ASKER