# atoi, atol, atoll

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< c‎ | string‎ | byte

Defined in header ``<stdlib.h>`` |  |
int atoi ( const char* str ); | (1) |
long atol ( const char* str ); | (2) |
long long atoll( const char* str ); | (3) | (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

Run this code

    #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

- C23 standard (ISO/IEC 9899:2024):

  - 7.22.1.2 The atoi, atol, and atoll functions (p: TBD)

- 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

strtolstrtoll (C99) | converts a byte string to an integer value (function)
strtoul strtoull (C99) | converts a byte string to an unsigned integer value (function)
wcstolwcstoll (C95)(C99) | converts a wide string to an integer value (function)
wcstoulwcstoull (C95)(C99) | converts a wide string to an unsigned integer value (function)
C++ documentation for atoi, atol, atoll

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