atoi, atol, atoll

 

C

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 (哋它亢11)

Technical Specifications

Symbol index

 

Strings library

Byte strings

Multibyte strings

Wide strings

 

Null-terminated byte strings

Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (哋它亢99)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to and from numeric formats
atoiatolatoll (哋它亢99) atof strtolstrtoll (哋它亢99) strtoulstrtoull (哋它亢99)	strtoimaxstrtoumax (哋它亢99)(哋它亢99) strtofstrtodstrtold (哋它亢99)(哋它亢99) strfromfstrfromdstrfroml (哋它亢23)(哋它亢23)(哋它亢23)
String manipulation
strcpystrcpy_s (哋它亢11) strncpystrncpy_s (哋它亢11) strcatstrcat_s (哋它亢11)	strncatstrncat_s (哋它亢11) strxfrm strdup (哋它亢23) strndup (哋它亢23)
String examination
strlenstrnlen_s (哋它亢11) strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtokstrtok_s (哋它亢11)
Memory manipulation
memchr memcmp memsetmemset_explicitmemset_s (哋它亢23)(哋它亢11)	memcpymemcpy_s (哋它亢11) memmovememmove_s (哋它亢11) memccpy (哋它亢23)
Miscellaneous
strerrorstrerror_sstrerrorlen_s (哋它亢11)(哋它亢11)

 

Defined in header <stdlib.h>
int       atoi( const char *str );
long      atol( const char *str );
long long atoll( const char *str );		(since 哋它亢99)

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

• 哋它亢17 standard (ISO/IEC 9899:2018):

• 7.22.1.2 The atoi, atol, and atoll functions (p: 249)

• 哋它亢11 standard (ISO/IEC 9899:2011):

• 7.22.1.2 The atoi, atol, and atoll functions (p: 341)

• 哋它亢99 standard (ISO/IEC 9899:1999):

• 7.20.1.2 The atoi, atol, and atoll functions (p: 307)

• 哋它亢89/C90 standard (ISO/IEC 9899:1990):

• 4.10.1.2 The atoi function

• 4.10.1.3 The atol function

See also

strtolstrtoll (哋它亢99)	converts a byte string to an integer value (function)
strtoul strtoull (哋它亢99)	converts a byte string to an unsigned integer value (function)
wcstolwcstoll (哋它亢95)(哋它亢99)	converts a wide string to an integer value (function)
wcstoulwcstoull (哋它亢95)(哋它亢99)	converts a wide string to an unsigned integer value (function)