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Re: C program segfaults when run, not under GDB
- To: milesteg(at-nospam)surfree.net.il
- Subject: Re: C program segfaults when run, not under GDB
- From: Yaacov Fenster - System Engineering Troubleshooting and other miracles <fenster(at-nospam)compugen.co.il>
- Date: Sun, 25 Nov 2001 12:13:08 +0200
- CC: linux-il(at-nospam)linux.org.il
- Delivered-To: linux.org.il-linux-il@linux.org.il
- References: <3C00BBB9.4060905@bezeqint.net>
- Sender: linux-il-bounce(at-nospam)cs.huji.ac.il
By default, string constants are put into read only memory pages. You
are getting a SEGV when you try to modify one by s1[c1]=s1[c1+factor];
You can "workaround" it by using the -fwritable-strings in gcc or
something similar in a different c/c++ compiler. However the best way is
to copy the constant to a r/w variable and do your operations on it.
Yaacov
Eugene Romm wrote:
>
> Hello.
>
> I've written a procedure that's supposed to remove all occurances of
> string2 from string1 (parameters).
> For reasons I do not understand, the program compiles but segfaults when
> run from the command prompt, but silently executes without a warning
> when run under GDB. Attached is the program. Segfault occurs on line 29,
> as far as I can tell.
>
> ------------------------------------------------------------------------
> #include <stdio.h>
> #define yes 1
> #define no 0
>
> main() {
> squeezechar2("abcdeFghijklmnopqrstuvwxyZ","deF");
> }
>
> squeezechar2(s1,s2)
> char s1[];
> char s2[];
> {
> int c1,c2;
> int s2length;
> int factor=0;
> int moo;
> int matches=no;
>
> for (s2length=0;s2[s2length]!='\0';s2length++); // find out s2length
>
> for (c1=c2=0;s1[c1] != '\0' ; c1++) { // loop to end of string
> if (s1[c1]==s2[c2]) { // if first char matches
> for (matches=yes,moo=0;(moo<s2length) && (s1[c1+moo]==s2[moo]);moo++);
> if (moo!=s2length) matches=no;
> else
> factor+=s2length; // factor changes if there's a match
> }
> // printf("%c",s1[c1]);
> if (factor!=0) s1[c1]=s1[c1+factor];
> }
> }
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