atoi, atol, atoll - cppreference.com

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# 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`.

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

### [edit] Parameters

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

### [edit] Return value

Integer value corresponding to the contents of `str` on success. If the converted value falls out of range of corresponding return type, the return value is undefined. If no conversion can be performed, ​0​ is returned.

### [edit] Example

Run this code

    #include <stdio.h>
    #include <stdlib.h>

    int main(void)
    {
        printf("%i\n", atoi(" -123junk"));
    }

Output:

    -123

### [edit] References

- 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

### [edit] See also

strtolstrtoll (C99) | converts a byte string to an integer value (function) [edit]
strtoul strtoull (C99) | converts a byte string to an unsigned integer value (function) [edit]
C++ documentation for atoi, atol, atoll

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