ATOF(3P) POSIX Programmer's Manual ATOF(3P)

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

atof — convert a string to a double-precision number

#include <stdlib.h>
double atof(const char *str);

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.1‐2017 defers to the ISO C standard.

The call atof(str) shall be equivalent to:

strtod(str,(char **)NULL),

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

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

No errors are defined.

The following sections are informative.


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




The Base Definitions volume of POSIX.1‐2017, <stdlib.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see .

2017 IEEE/The Open Group