C working with strings and SEGFAULT - c

Question

Hello and sorry for my bad english.
I am starting with C language, but I didnt get pointers well...
I searched for similar topics, but I didnt get it from them, so I created own topic.
I have got main function, where I call function newSpeak.
There is my code of newSpeak, but there isnt everything...
char * newSpeak ( const char * text, const char * (*replace)[2] )
{
int i;
char * alpha;
for(i=0;i<4;i++)
{
alpha=strstr(text, replace[0][4]);
if(alpha[0])
strncpy (alpha,replace[1][0],10);
}
return 0;
}
Thanks for answering
EDIT: I have found source of the problem.
It works, when I dont use for cycle and run it once. But it doesnt work even when the condition in for cycle is i<1, which should make it run only once...This is strange for me...

Answer 1

alpha=strstr(text, replace[0][4]);
if(alpha[0])
// looks crashy
man strstr:
These functions return a pointer to the beginning of the substring,
or NULL if the substring is not found.
EDIT:
It is difficult to tell what you are trying to do, but below find an arbitrary adaptation of your code. If it were my program, I would write it very differently. I mention that because I do not want someone to read this and think it is the way it should be done.
#include <stdio.h>
#include <string.h>
void newSpeak (char *text, const char *replace[4][2])
{
int i, j;
char *alpha;
for (i = 0; i < 4; i++) {
if (alpha = strstr(text, replace[i][0])) {
for (j = 0; alpha[j] && replace[i][1][j]; j++)
alpha[j] = replace[i][1][j];
}
}
}
int main ()
{
char buf[100] = "abc";
const char *replace[4][2] = {
{ "a", "e" },
{ "b", "f" },
{ "c", "g" },
{ "d", "h" },
};
newSpeak(buf, replace);
puts(buf);
}

Answer 2

The line
strncpy (alpha,replace[1][0],10);
should have generated a compiler warning (and NEVER ignore compiler warnings). The function prototype is
char *strncpy( char *dest, char *source, int n);
But you are passing it replace[1][0] which is a character. It might work if you passed
strncpy( alpha, &replace[1][0], 10);
Even then I still worry. It could be that since alpha is pointing to a block of memory in the block pointed to by text which is a const char*, that you are not allowed to modify that memory.
EDIT I think my first point is wrong - I misread your prototype. But I'm pretty sure the second point is valid (and probably the reason for the segfault).
second edit
It is possible that text does not have sufficient memory allocated to have 10 characters copied into it from replace. Realize that the thing you are matching against (replace[0][4]) and the thing you are copying (replace[1][0]]) are not the same thing; also, you are looping over i but not using that value ... makes me wonder if there is a typo (I am not clairvoyant and cannot figure out what you wanted to change from loop to loop).
You need to check the size of the thing you are copying into:
strncpy(alpha, replace[1][0], (strlen(alpha)<10)?strlen(alpha):10);
would ensure you are copying no more than 10 characters, and no more than there's space in alpha.
This is "on top of" everything else already pointed out (of which using if (alpha!=NULL) instead of if(alpha[0]) is a big one.)
EDIT 3 - I think I figured out the majority of the problems with your code now... see http://codepad.org/YK5VyGAn for a small "working" sample.
Issues with your code included:
You declare text as const char*, then proceed to modify it
You declare replace as const char* (*replace)[2], then address element replace[0][4] (4 > 2...)
You assign the return value of strstr to alpha; this could be NULL (no match), yet you test for alpha[0] (which will fail if alpha == NULL).
When you copied the replacement string, you copied "up to 10 characters" - regardless of whether (a) the target string could accommodate this, and (b) the source string had this many characters. The result might be that you copy the full source string (including the terminating '\0') so that you will not find another match afterwards (you have "deleted" the rest of the string). And then you will run into the "strstr returns NULL" error...
Not sure (without seeing your input string or "replace" strings) which of these actually caused your code to fail - I have written a small program that addresses all of these mistakes. You can find it at http://codepad.org/4jSOnmPy - reproduced here:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MIN(a,b) (a>b)?(b):(a)
char * newSpeak (const char *text, const char *(*replace)[5] ){
int ii=0, n;
char *alpha, *beta;
printf("length of input string is %d\n", strlen(text));
beta = malloc(strlen(text)+1);
printf("allocated %d bytes\n", strlen(text)+1);
fflush(stdout);
strcpy(beta, text);
printf("copy OK: beta now %s\n", beta);
fflush(stdout);
for(ii = 0; ii < 4; ii++) {
// alpha=strstr(beta, replace[0][0]);
alpha=strstr(beta, "a");
printf("alpha is '%s'\n", alpha);
fflush(stdout);
if(alpha!=NULL) {
char *rs;
rs = replace[1][ii];
printf("ii = %d; alpha now: '%s'\n", ii, alpha);
fflush(stdout);
n = MIN(strlen(alpha), strlen(rs));
printf("n is now %d\n", n);
fflush(stdout);
printf("going to copy at most %d characters from '%s' into '%s'\n", n, rs, alpha);
fflush(stdout);
strncpy (alpha,rs,n);
printf("beta is now '%s'\n", beta);
fflush(stdin);
}
else printf("no match found\n");
}
return beta;
}
int main(void) {
char* r[2][5]={{"a","b","c","d", "e"}, {"o","e","i","u","s"}};
char* myText = "this is a vary sally strang";
printf("NewSpeak: %s\n", "hello world");
printf("converted: %s\n", newSpeak(myText, r));
return 0;
}
Output:
NewSpeak: hello world
length of input string is 27
allocated 28 bytes
copy OK: beta now this is a vary sally strang
alpha is 'a vary sally strang'
ii = 0; alpha now: 'a vary sally strang'
n is now 1
going to copy at most 1 characters from 'o' into 'a vary sally strang'
beta is now 'this is o vary sally strang'
alpha is 'ary sally strang'
ii = 1; alpha now: 'ary sally strang'
n is now 1
going to copy at most 1 characters from 'e' into 'ary sally strang'
beta is now 'this is o very sally strang'
alpha is 'ally strang'
ii = 2; alpha now: 'ally strang'
n is now 1
going to copy at most 1 characters from 'i' into 'ally strang'
beta is now 'this is o very silly strang'
alpha is 'ang'
ii = 3; alpha now: 'ang'
n is now 1
going to copy at most 1 characters from 'u' into 'ang'
beta is now 'this is o very silly strung'
converted: this is o very silly strung
Note - I added lots of "useless" output, including fflush(stdout); statements. This is a good way to ensure that debug printout shows you exactly how far into a program you got, and what was going on before it crashed - without the fflush it's possible you are missing many lines of output (because they never "made it to the screen").
It's obvious from the above that if your replacement strings are a different length than the string they replace, you will get some strange overwriting (I left both search and replace string length at 1 but there is no reason why that should be so).
I hope this helps!