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.

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


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.


#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




  • C11 standard (ISO/IEC 9899:2011):
    • The atoi, atol, and atoll functions (p: 341)
  • C99 standard (ISO/IEC 9899:1999):
    • The atoi, atol, and atoll functions (p: 307)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The atoi function
    • The atol function

See also

converts a byte string to an integer value
converts a byte string to an unsigned integer value
C++ documentation for atoi, atol, atoll

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