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Showing content from https://cplusplus.com/reference/cstdlib/atol/ below:

function

atol

long int atol ( const char * str );

Convert string to long integer

Parses the C-string str interpreting its content as an integral number, which is returned as a value of type long int.

The function first discards as many whitespace characters (as in isspace) as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many base-10 digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed and zero is returned.

Parameters

str: C-string containing the representation of an integral number.

Return Value On success, the function returns the converted integral number as a long int value.
If no valid conversion could be performed, a zero value is returned.
If the converted value would be out of the range of representable values by a long int, it causes undefined behavior. See strtol for a more robust cross-platform alternative when this is a possibility.

Example

/* atol example */
#include <stdio.h>      /* printf, fgets */
#include <stdlib.h>     /* atol */

int main ()
{
  long int li;
  char buffer[256];
  printf ("Enter a long number: ");
  fgets (buffer, 256, stdin);
  li = atol(buffer);
  printf ("The value entered is %ld. Its double is %ld.\n",li,li*2);
  return 0;
}

Output:

Enter a number: 567283
The value entered is 567283. Its double is 1134566.

Data races The array pointed by str is accessed.

Exceptions (C++)No-throw guarantee: this function never throws exceptions.

If str does not point to a valid C-string, or if the converted value would be out of the range of values representable by an long int, it causes undefined behavior.