The Open Group Base Specifications Issue 8
IEEE Std 1003.1-2024
Copyright © 2001-2024 The IEEE and The Open Group

NAME

atoi — convert a string to an integer

SYNOPSIS

#include <stdlib.h>

int atoi(const char *
str);

DESCRIPTION

[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard. [Option End]

The call atoi(str) shall be equivalent to:

(int) strtol(str, (char **)NULL, 10)

except that the handling of errors may differ. If the value cannot be represented, the behavior is undefined.

RETURN VALUE

The atoi() function shall return the converted value if the value can be represented.

ERRORS

No errors are defined.


The following sections are informative.

EXAMPLES

Converting an Argument

The following example checks for proper usage of the program. If there is an argument and the decimal conversion of this argument (obtained using atoi()) is greater than 0, then the program has a valid number of minutes to wait for an event.

#include <stdlib.h>
#include <stdio.h>
...
int minutes_to_event;
...
if (argc < 2 || (minutes_to_event = atoi (argv[1])) <= 0) {
   fprintf(stderr, "Usage: %s minutes\n", argv[0]); exit(1);
}
...

APPLICATION USAGE

The atoi() function is subsumed by strtol() but is retained because it is used extensively in existing code. If the number is not known to be in range, strtol() should be used because atoi() is not required to perform any error checking.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

strtol

XBD <stdlib.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 8

Austin Group Defect 1541 is applied, changing the EXAMPLES section.

End of informative text.

 

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