The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


fputc - put a byte on a stream


#include <stdio.h>

int fputc(int
c, FILE *stream);


[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.1-2008 defers to the ISO C standard. [Option End]

The fputc() function shall write the byte specified by c (converted to an unsigned char) to the output stream pointed to by stream, at the position indicated by the associated file-position indicator for the stream (if defined), and shall advance the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the byte shall be appended to the output stream.

[CX] [Option Start] The last data modification and last file status change timestamps of the file shall be marked for update between the successful execution of fputc() and the next successful completion of a call to fflush() or fclose() on the same stream or a call to exit() or abort(). [Option End]


Upon successful completion, fputc() shall return the value it has written. Otherwise, it shall return EOF, the error indicator for the stream shall be set, [CX] [Option Start]  and errno shall be set to indicate the error. [Option End]


The fputc() function shall fail if either the stream is unbuffered or the stream's buffer needs to be flushed, and:

[CX] [Option Start] The O_NONBLOCK flag is set for the file descriptor underlying stream and the thread would be delayed in the write operation. [Option End]
[CX] [Option Start] The file descriptor underlying stream is not a valid file descriptor open for writing. [Option End]
[CX] [Option Start] An attempt was made to write to a file that exceeds the maximum file size. [Option End]
[XSI] [Option Start] An attempt was made to write to a file that exceeds the file size limit of the process. [Option End]
[CX] [Option Start] The file is a regular file and an attempt was made to write at or beyond the offset maximum. [Option End]
[CX] [Option Start] The write operation was terminated due to the receipt of a signal, and no data was transferred. [Option End]
[CX] [Option Start] A physical I/O error has occurred, or the process is a member of a background process group attempting to write to its controlling terminal, TOSTOP is set, the calling thread is not blocking SIGTTOU, the process is not ignoring SIGTTOU, and the process group of the process is orphaned. This error may also be returned under implementation-defined conditions. [Option End]
[CX] [Option Start] There was no free space remaining on the device containing the file. [Option End]
[CX] [Option Start] An attempt is made to write to a pipe or FIFO that is not open for reading by any process. A SIGPIPE signal shall also be sent to the thread. [Option End]

The fputc() function may fail if:

[CX] [Option Start] Insufficient storage space is available. [Option End]
[CX] [Option Start] A request was made of a nonexistent device, or the request was outside the capabilities of the device. [Option End]

The following sections are informative.










Standard I/O Streams, ferror, fopen, getrlimit, putc, puts, setbuf, ulimit

XBD <stdio.h>


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

Issue 5

Large File Summit extensions are added.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/37 is applied, updating the [EAGAIN] error in the ERRORS section from ``the process would be delayed'' to ``the thread would be delayed''.

Issue 7

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0168 [79] and XSH/TC1-2008/0169 [14] are applied.

End of informative text.


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