c: segfault while trying to assign a character to an element of a string.

schneidz · 07-14-2024, 09:02 AM

i am essentially trying to remove \n from a string (if it exists).

Code:

1:#include <stdio.h>
2:#include <string.h>
3:#include <stdlib.h>
4:
5:int main(int argc, char *argv[])
6:{
7:  char *ex1 = "123 hello world\n";
8:  char *pp;
9:  int i1;
10:
11:  i1= strtol(ex1, &pp, 10);
12:  printf("ex1 = %s: i1 = %d -- pp = \"%s\" -- strcspn-pp = %d\n",ex1,i1,pp,strcspn(pp,"\n"));
13://  pp[strcspn(pp, "\n")] = '\0';
14:  pp[6] = 'a';
15:  printf("\n\n-----apres call: pp = \"%s\"\n\n\n", pp);
16:}

Code:

~ $ gcc storage/shared/stuff/schneidz-test.c && ./a.out
ex1 = 123 hello world
: i1 = 123 -- pp = " hello world
" -- strcspn-pp = 12
Segmentation fault

if i comment out line 14 it will print the same pp as in line 12.

pan64 · 07-14-2024, 09:19 AM

your variable pp points to nowhere.

schneidz · 07-14-2024, 10:52 AM

Quote:

Originally Posted by pan64

your variable pp points to nowhere.

?
the 2nd parameter of strtol is supposed to store the start of the next chunk after the return.

Code:

pp = " hello world
"

intestinal fortitude · 07-14-2024, 10:59 AM

Your problem is that even though ex1 is of type 'char *', the data is it is pointing to is 'const char *' (i.e. it is read-only). Since 'pp' is populated from a pointer into ex1, 'pp' is also pointing to a read-only string.

When you try to assign a value to the character pointed to by pp, you get undefined behavior. In this case, a segfault.

To fix your problem, change the definition of ex1:
char ex1[] = "123 hello world\n";

Quote:

your variable pp points to nowhere.

The call to strtol() puts a valid address into 'pp'.

intestinal fortitude · 07-14-2024, 11:07 AM

Quote:

Originally Posted by schneidz

?
the 2nd parameter of strtol is supposed to store the start of the next chunk after the return.

Code:

pp = " hello world
"

It will have the position where strotol() stopped converting data. In your example, that is the space character after "123".

When I have a string and I want to remove a newline from the end of the string (and only at the end of the string):

Code:

char my_string[] = "This is a test\n";
size_t l;

l = strlen(my_string);
if (l > 0 && my_string[l-1]=='\n')
    my_string[--l] = 0;

NevemTeve · 07-14-2024, 12:47 PM

At compilation use options `-Wall -W -Werror`

teckk · 07-14-2024, 02:00 PM

Quote:

i am essentially trying to remove \n from a string (if it exists).

Code:

#include <stdio.h>
#include <string.h>

char* deleteCharC(char* s, char ch) {
    int i, j;
    int len = strlen(s);
    for (i = j = 0; i < len; i++) {
        if (s[i] != ch) {
            s[j++] = s[i];
        }
    }
    s[j] = '\0';
    return s;
}

int main() {
    char ch = '\n';
    
    char a[] = "123 hello world\n";
    printf("%s\n", deleteCharC(a, ch));
    
    char b[] = "456 hello world\n\n\n";
    printf("%s\n", deleteCharC(b, ch));

    char c[] = "789 hello world";
    printf("%s\n", deleteCharC(c, ch));
    
    return 0;
}

//gcc rstring.c -o rstring

Code:

gcc rstring.c -o rstring
./rstring
123 hello world
456 hello world
789 hello world

schneidz · 07-14-2024, 02:38 PM

Quote:

Originally Posted by NevemTeve

At compilation use options `-Wall -W -Werror`

f.y.i.:

Code:

~ $ gcc -Wall -W -Werror storage/shared/stuff/schneidz-test.c
storage/shared/stuff/schneidz-test.c:12:76: error: format specifies type 'int' but the argument has type 'unsigned long' [-Werror,-Wformat]
   12 |   printf("ex1 = %s: i1 = %d -- pp = \"%s\" -- strcspn-pp = %d\n",ex1,i1,pp,strcspn(pp,"\n"));
      |                                                            ~~              ^~~~~~~~~~~~~~~~
      |                                                            %lu
storage/shared/stuff/schneidz-test.c:5:14: error: unused parameter 'argc' [-Werror,-Wunused-parameter]
    5 | int main(int argc, char *argv[])
      |              ^
storage/shared/stuff/schneidz-test.c:5:26: error: unused parameter 'argv' [-Werror,-Wunused-parameter]
    5 | int main(int argc, char *argv[])
      |                          ^
3 errors generated.

NevemTeve · 07-14-2024, 11:46 PM

That's it. Now you only have to fix every errors.
E.g. for `argc` unused put this line into main:

Code:

(void)argc;

Also the first version of the program had this line:

Code:

char *ex1 = "123 hello world\n";

With the said options the compiler explains the problem clearly: assigning a const pointer to a nonconst pointer-variable.

intestinal fortitude · 07-15-2024, 06:05 AM

GCC with '-Wall' and '-Wall -Wextra' did not give a warning about the constant string. To get a warning about the string, they have to use '-Wwrite-strings'

ETA: I am using Slackware with GCC 14.1.0

NevemTeve · 07-15-2024, 06:29 AM

Oh, you are right, the compiler ignores this problem, unless you use yet another option:

Code:

$ gcc schneidz.c -W -Wall -Wwrite-strings -Werror -o schneidz && ./schneidz
schneidz.c: In function ‘main’:
schneidz.c:7:15: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
    7 |   char *ex1 = "123 hello world\n";
      |               ^~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Edit: Compiling as C++ also works:

Code:

$ g++ schneidz.c
schneidz.c: In function ‘int main(int, char**)’:
schneidz.c:7:15: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
    7 |   char *ex1 = "123 hello world\n";
      |               ^~~~~~~~~~~~~~~~~~~