substr() function, not present in standard C library.
Syntax: char *substr(const char *str, long start_index, long end_index);
The description of function substr() is in the header file "substr.h".
The code was compiled using the following gcc flags:
-Wall -Werror -Wextra -Wundef -Wunreachable-code -Winit-self -Wparentheses -Wconversion -Wsign-conversion -Wsign-compare -Werror-implicit-function-declaration -Wmissing-prototypes -Wmissing-declarations -Wformat-security
The code is below:
substr.c
#include "substr.h"
#include <stdlib.h>
#include <string.h>
/* The description of function substr() is in the header file "substr.h". */
char *substr(const char *str, long start_index, long end_index)
{
char *substring = NULL;
long len = 0;
long substr_len = 0;
if ((!str) || (!*str))
return NULL;
if ((start_index < 0) || (end_index < 0) || (end_index < start_index))
return NULL;
len = (long)(strlen(str));
if ((start_index > (len - 1)) || (end_index > (len - 1)))
return NULL;
substr_len = end_index - start_index + 1;
substring = malloc((size_t)(substr_len + 1)); // extra 1 byte for null byte
if (!substring)
return NULL;
memmove(substring, str + start_index, (size_t)(substr_len));
substring[substr_len] = 0;
return substring;
} // end of substr
substr.h
#ifndef SUBSTR_H
#define SUBSTR_H
/*
* char *substr(const char *str, long start_index, long end_index):
*
* Function substr() allocates memory and returns a pointer to a string / character
* array which is a substring of 'str' starting from index 'start_index' till
* 'end_index' (inclusive). This substring is terminated by null byte at the end.
* If 'str' is NULL or 'str' is empty or 'start_index' is less than 0 or 'end_index'
* is less than 0 or 'end_index' is less than 'start_index' or 'start_index' is
* greater than length of 'str' - 1 or 'end_index' is greater than length of
* 'str' - 1 then NULL is returned.
*
* The returned pointer points to a memory region containing the substring and this
* memory region was allocated using malloc. So, it is the user's responsibility to
* free the allocated memory.
*
*/
char *substr(const char *str, long start_index, long end_index);
#endif
test_substr.c
#include "substr.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *get_input_from_stdin_and_discard_extra_characters(char *str, long size);
#define ARRAY_SIZE 256
int main(void)
{
char str[ARRAY_SIZE] = {0};
long start_index = -1;
long end_index = -1;
char *arg_str = NULL;
char *sub_str = NULL;
while (1) {
arg_str = str;
system("clear");
printf("\nPlease input a string to split (max 255 characters) (To enter NULL"
" string, type NULL and press ENTER): ");
get_input_from_stdin_and_discard_extra_characters(str, ARRAY_SIZE);
if (strcmp(str, "NULL") == 0) {
arg_str = NULL;
} else {
if (!*str)
printf("(Length of string entered = %zu.)\n", strlen(arg_str));
else
printf("(Length of string entered = %zu. Index 0 to %zu.)\n", strlen(arg_str), strlen(arg_str) - 1);
}
printf("\nPlease input the start index from where to start copying the"
" substring: ");
scanf("%ld", &start_index);
// now clear the stdin input buffer
get_input_from_stdin_and_discard_extra_characters(NULL, 0);
printf("\nPlease input the end index at which to stop copying the substring: ");
scanf("%ld", &end_index);
// now clear the stdin input buffer
get_input_from_stdin_and_discard_extra_characters(NULL, 0);
printf("\n");
printf("\n-----------------");
printf("\nInput parameters:");
printf("\n-----------------\n");
printf("str = \"%s\"\n", arg_str?arg_str:"(null string)");
printf("start index = %ld\n", start_index);
printf("end index = %ld\n", end_index);
printf("\n");
printf("\n-------");
printf("\nResult:");
printf("\n-------\n");
sub_str = substr(arg_str, start_index, end_index);
if (sub_str) {
printf("substr = \"%s\"\n\n", sub_str);
free(sub_str);
} else {
printf("substr() returned NULL.\n\n");
}
printf("\n\nPlease press ENTER to continue..");
// now clear the stdin input buffer
get_input_from_stdin_and_discard_extra_characters(NULL, 0);
} // end of while(1) loop
} // end of main
/*
* get_input_from_stdin_and_discard_extra_characters(char *str, long size):
*
* Function get_input_from_stdin_and_discard_extra_characters() reads at most
* 'size - 1' characters into 'str' from stdin and then appends the null
* character ('\0'). If 'size' is 0 then this function will discard all input
* and return NULL. So, to discard all input, this function can be called with
* 'str' having value NULL and 'size' having value 0.
* In all cases, reading input stops after encountering a newline ('\n') or EOF
* even if 'size - 1' characters have not been read. If a newline ('\n') or EOF
* is read then it is replaced by null character ('\0'). If there are extra
* characters in input, they are read and discarded.
* In all cases, 'str' or NULL is returned.
*/
char *get_input_from_stdin_and_discard_extra_characters(char *str, long size)
{
int c = 0;
long i = 0;
// If size is 0 then this function will discard all input and return NULL.
// No need to check str if size is 0.
if (size == 0) {
// discard all input
while ((c = getchar()) && (c != '\n') && (c != EOF));
return NULL;
}
if (!str)
return str;
if (size < 0)
return NULL;
for (i = 0; i < (size - 1); i = i + 1) {
c = getchar();
if ((c == '\n') || (c == EOF)) {
str[i] = 0;
return str;
}
str[i] = (char)(c);
} // end of for loop
str[i] = 0;
// discard rest of input
while ((c = getchar()) && (c != '\n') && (c != EOF));
return str;
} // end of get_input_from_stdin_and_discard_extra_characters
strndup()is available on almost all platforms for a long time already, and will be in the C23 standard. \$\endgroup\$