Delete space from string in ansi C

Question

I'm trying to write simple function trim space in the string in ansi C.

My str_utis.h:

#include <string.h>

const char* trim_str(char *input_str); 

My str_utils.c:

const char* trim_str(char* input_str){
    char* str = NULL;
    int len = strlen(input_str);
    int i = 0;
    for (i = 0; i < len - 1; i++){
        if (input_str[i] == ' ')
            ;
        else
            str += input_str[i];
    }
    return str;
}

When i try to execute it i got segfault:

int main(int argc, char** argv) {
    const char* a = trim_str("Hey this is string");
    printf("%s", a);
    return 0;
}

why is it wrong? how can i write it correctly?

Thank you.

Answer 1

You cannot modify a string literal. It's UB. Copy the string out and modify its contents. Change

char* str = NULL;
int len = strlen(input_str);

to

size_t len = strlen( input_str );
char *str = malloc( len + 1 );

and then proceed to copy out the non-whitespace contents.

Answer 2

str isn't allocated, and you can't use += to append to a string anyway. Read up on strncat.

Answer 3

Go through this. this will remove blank spaces and tabs from both ends of the input string.

const char* trim_str(char* input_str){
    char* str = NULL;
    int len = strlen(input_str);
    str = (char *)malloc(len+1);
    int i = 0;
    while(i < len && (input_str[i]==' ' || input_str[i]=='\t')){
        ++i;
    }
    int j = 0;
    while(i< len && input_str[i]!=' ' && input_str[i]!='\t'){
      str[j]= input_str[i];
      ++j;
      ++i;
    }
   str[j] = '\0';

   return str;
  }

Answer 4

first of all you need to get the new string size without spaces: You don't want to allocate big strings if you don't have to.

   const char* trim_str(char* input_str){
        char* str = NULL;
        int len = strlen(input_str);
        int i = 0;
        int newSize = 0; 
        for (i = 0; i < len - 1; i++){
            if (input_str[i] == ' ')
                ;
            else
                newSize++;
        }
        str = malloc( newSize+ 1 );
        str[newSize] = '\0'

        // put the code of the copy bytes here...

        return str;
    }

Answer 5

char *trimdupstr(char *str)
{
size_t len, src,dst;
char *new;

if (!str) return NULL;
len = strlen (str);
new = malloc(len+1);
if (!new) return NULL;

for(src=dst=0; new[dst] = str[src++];   ) {
        if (new[dst] != ' ') dst++;
        }
return new;
}

And this one also removes tabs and CR/LF's:

char *trimdupstr(char *str)
{
size_t len, src,dst;
char *new;

if (!str) return NULL;
len = strlen (str);
new = malloc(len+1);

for (src=dst=0; str[src]; dst += len) {
        len = strspn(str+src, "\t \r\n" );
        src += len;
        len = strcspn(str+src, "\t \r\n" );
        memcpy (new+dst, str+src, len);
        src += len;
        }
new[dst] = 0;
return new;
}

Answer 6

This is a pretty safe method.

void rtrim(char instrng[]) {
assert (instrng != NULL);
if (strlen(instrng) == 0) {
    return;
}
while (instrng[strlen(instrng)-1] == '\n' || instrng[strlen(instrng)-1] == ' ' ) {
    instrng[strlen(instrng)-1] = '\0';
}
return; 
}