atoi, atol, atoll

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

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

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

Compiler support
Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Strings library
Algorithms
Numerics
Date and time utilities
Input/output support
Localization support
Concurrency support (C11)
Technical Specifications
Symbol index

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