atoi, atol, atoll

int       atoi( const char *str );

long      atol( const char *str );

long long atoll( const char *str );

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

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
}

-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