atoi, atol, atoll

From cppreference.com
< c‎ | string‎ | byte
Defined in header <stdlib.h>
int       atoi( const char *str );
long      atol( const char *str );
long long atoll( const char *str );
(since C99)

Interprets an integer value in a byte string pointed to by str. The implied radix is always 10.

Discards any whitespace characters until the first non-whitespace character is found, then takes as many characters as possible to form a valid integer number representation and converts them to an integer value. The valid integer value consists of the following parts:

  • (optional) plus or minus sign
  • numeric digits

If the value of the result cannot be represented, i.e. the converted value falls out of range of the corresponding return type, the behavior is undefined.

Parameters

str - pointer to the null-terminated byte string to be interpreted

Return value

Integer value corresponding to the contents of str on success.

If no conversion can be performed, 0 is returned.

Notes

The name stands for "ASCII to integer".

Example

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    printf("%i\n", atoi(" -123junk"));
    printf("%i\n", atoi(" +321dust"));
    printf("%i\n", atoi("0"));
    printf("%i\n", atoi("0042")); // treated as a decimal number with leading zeros
    printf("%i\n", atoi("0x2A")); // only leading zero is converted discarding "x2A"
    printf("%i\n", atoi("junk")); // no conversion can be performed
    printf("%i\n", atoi("2147483648")); // UB: out of range of int
}

Possible output:

-123
321
0
42
0
0
-2147483648

References

  • C17 standard (ISO/IEC 9899:2018):
  • 7.22.1.2 The atoi, atol, and atoll functions (p: 249)
  • C11 standard (ISO/IEC 9899:2011):
  • 7.22.1.2 The atoi, atol, and atoll functions (p: 341)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.20.1.2 The atoi, atol, and atoll functions (p: 307)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.10.1.2 The atoi function
  • 4.10.1.3 The atol function

See also

converts a byte string to an integer value
(function)
converts a byte string to an unsigned integer value
(function)
(C95)(C99)
converts a wide string to an integer value
(function)
converts a wide string to an unsigned integer value
(function)