atoi, atol, atoll

 

C

Language

headers

Type support

Dynamic memory management

Error handling

Program utilities

Variadic function support

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

Technical Specifications

 

Strings library

Byte strings

Multibyte strings

Wide strings

 

Null-terminated byte strings

Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C99)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to numeric formats
atof atoiatolatoll strtolstrtoll	strtoulstrtoull strtofstrtodstrtold strtoimaxstrtoumax (C99)(C99)
String manipulation
strcpystrcpy_s (C11)
strncpystrncpy_s (C11)
strcatstrcat_s (C11)
strncatstrncat_s (C11)
strxfrm
String examination
strlenstrnlen_s (C11) strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtokstrtok_s (C11)
Memory manipulation
memchr
memcmp
memsetmemset_s (C11)
memcpymemcpy_s (C11)
memmovememmove_s (C11)
Miscellaneous
strerrorstrerror_sstrerrorlen_s (C11)(C11)

 

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

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 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.

Example

Run this code

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    printf("%i\n", atoi(" -123junk"));
    printf("%i\n", atoi("0"));
    printf("%i\n", atoi("junk"));         // no conversion can be performed
    printf("%i\n", atoi("2147483648"));   // UB: out of range of int
}

Output:

-123
0
0
-2147483648

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

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)
C++ documentation for atoi, atol, atoll