man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

putc_unlocked(3C)

Name

fputc, putc, putc_unlocked, putchar, putchar_unlocked, putw - put a byte on a stream

Synopsis

#include <stdio.h>

int fputc(int c, FILE *stream);
int putc(int c, FILE *stream);
int putc_unlocked(int c, FILE *stream);
int putchar(int c);
int putchar_unlocked(int c);
int putw(int w, FILE *stream);

Description

The fputc() function writes 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 advances the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the byte is appended to the output stream.

The st_ctime and st_mtime fields of the file will be marked for update between the successful execution of fputc() and the next successful completion of a call to fflush(3C) or fclose(3C) on the same stream or a call to exit(3C) or abort(3C).

The putc() routine behaves like fputc(), except that it is implemented as a macro. It runs faster than fputc(), but it takes up more space per invocation and its name cannot be passed as an argument to a function call.

The call putchar(c) is equivalent to putc(c, stdout). The putchar() routine is implemented as a macro.

The putc_unlocked() and putchar_unlocked() routines are variants of putc() and putchar(), respectively, that do not lock the stream. It is the caller's responsibility to acquire the stream lock before calling these routines and releasing the lock afterwards; see flockfile(3C) and stdio(3C). These routines are implemented as macros.

The putw() function writes the word (that is, type int) w to the output stream (at the position at which the file offset, if defined, is pointing). The size of a word is the size of a type int and varies from machine to machine. The putw() function neither assumes nor causes special alignment in the file.

The st_ctime and st_mtime fields of the file will be marked for update between the successful execution of putw() and the next successful completion of a call to fflush(3C) or fclose(3C) on the same stream or a call to exit(3C) or abort(3C).

Return Values

Upon successful completion, fputc(), putc(), putc_unlocked(), putchar(), and putchar_unlocked() return the value that was written. Otherwise, these functions return EOF, the error indicator for the stream is set, and errno is set to indicate the error.

Upon successful completion, putw() returns 0. Otherwise, it returns a non-zero value, sets the error indicator for the associated stream, and sets errno to indicate the error.

An unsuccessful completion will occur, for example, if the file associated with stream is not open for writing or if the output file cannot grow.

Errors

The fputc(), putc(), putc_unlocked(), putchar(), putchar_unlocked(), and putw() functions will fail if either the stream is unbuffered or the stream 's buffer needs to be flushed, and:

EAGAIN

The O_NONBLOCK flag is set for the file descriptor underlying stream and the process would be delayed in the write operation.

EBADF

The file descriptor underlying stream is not a valid file descriptor open for writing.

EFBIG

An attempt was made to write to a file that exceeds the maximum file size or the process' file size limit.

EFBIG

The file is a regular file and an attempt was made to write at or beyond the offset maximum.

EINTR

The write operation was terminated due to the receipt of a signal, and no data was transferred.

EIO

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 process is neither ignoring nor blocking SIGTTOU and the process group of the process is orphaned. This error may also be returned under implementation-dependent conditions.

ENOSPC

There was no free space remaining on the device containing the file.

EPIPE

An attempt is made to write to a pipe or FIFO that is not open for reading by any process. A SIGPIPE signal will also be sent to the calling thread.

The fputc(), putc(), putc_unlocked(), putchar(), putchar_unlocked(), and putw() functions may fail if:

ENOMEM

Insufficient storage space is available.

ENXIO

A request was made of a non-existent device, or the request was outside the capabilities of the device.

Usage

Functions exist for the putc(), putc_unlocked(), putchar(), and putchar_unlocked() macros. To get the function form, the macro name must be undefined (for example, #undef putc).

When the macro forms are used, putc() and putc_unlocked() evaluate the stream argument more than once. In particular, putc(c, *f++); does not work sensibly. The fputc() function should be used instead when evaluating the stream argument has side effects.

Because of possible differences in word length and byte ordering, files written using putw() are implementation-dependent, and possibly cannot be read using getw(3C) by a different application or by the same application running in a different environment.

The putw() function is inherently byte stream oriented and is not tenable in the context of either multibyte character streams or wide-character streams. Application programmers are encouraged to use one of the character-based output functions instead.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
fputc(), putc(), putc_unlocked(), putchar(), and putchar_unlocked() are Standard.
MT-Level
See NOTES below.

See Also

getrlimit(2), ulimit(2) write(2), Intro(3), abort(3C), exit(3C), fclose(3C), ferror(3C), fflush(3C), flockfile(3C), printf(3C), putc(3C), puts(3C), setbuf(3C), stdio(3C), attributes(5), standards(5)

Notes

The fputc(), putc(), putchar(), and putw() routines are MT-Safe in multithreaded applications. The putc_unlocked() and putchar_unlocked() routines are unsafe in multithreaded applications.