<stdio.h>

The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group

A newer edition of this document exists here

NAME

stdio.h - standard buffered input/output

SYNOPSIS

#include <stdio.h>

DESCRIPTION

^[] [Option Start] Some of the functionality described on this reference page extends the ISO C standard. Applications shall define the appropriate feature test macro (see XSH The Compilation Environment ) to enable the visibility of these symbols in this header. [Option End]

The <stdio.h> header shall define the following data types through typedef:

FILE

A structure containing information about a file.

fpos_t

A non-array type containing all information needed to specify uniquely every position within a file.

off_t

As described in <sys/types.h>.

size_t

As described in <stddef.h>.

ssize_t

^[] [Option Start] As described in <sys/types.h>.

va_list

^[] [Option Start] As described in <stdarg.h>. [Option End]

The <stdio.h> header shall define the following macros which shall expand to integer constant expressions:

BUFSIZ

Size of <stdio.h> buffers. ^[] [Option Start] This shall expand to a positive value. [Option End]

L_ctermid

^[] [Option Start] Maximum size of character array to hold ctermid() output. [Option End]

L_tmpnam

^[] [Option Start] Maximum size of character array to hold tmpnam() output. [Option End]

The <stdio.h> header shall define the following macros which shall expand to integer constant expressions with distinct values:

_IOFBF

Input/output fully buffered.

_IOLBF

Input/output line buffered.

_IONBF

Input/output unbuffered.

The <stdio.h> header shall define the following macros which shall expand to integer constant expressions with distinct values:

SEEK_CUR

Seek relative to current position.

SEEK_END

Seek relative to end-of-file.

SEEK_SET

Seek relative to start-of-file.

The <stdio.h> header shall define the following macros which shall expand to integer constant expressions denoting implementation limits:

{FILENAME_MAX}

Maximum size in bytes of the longest pathname that the implementation guarantees can be opened.

{FOPEN_MAX}

Number of streams which the implementation guarantees can be open simultaneously. The value is at least eight.

{TMP_MAX}

^[] [Option Start] Minimum number of unique filenames generated by tmpnam(). Maximum number of times an application can call tmpnam() reliably. The value of {TMP_MAX} is at least 25. [Option End]
^[] [Option Start] On XSI-conformant systems, the value of {TMP_MAX} is at least 10000. [Option End]

The <stdio.h> header shall define the following macro which shall expand to an integer constant expression with type int and a negative value:

EOF

End-of-file return value.

The <stdio.h> header shall define NULL as described in <stddef.h>.

The <stdio.h> header shall define the following macro which shall expand to a string constant:

P_tmpdir

^[] [Option Start] Default directory prefix for tempnam(). [Option End]

The <stdio.h> header shall define the following macros which shall expand to expressions of type "pointer to FILE" that point to the FILE objects associated, respectively, with the standard error, input, and output streams:

stderr

Standard error output stream.

stdin

Standard input stream.

stdout

Standard output stream.

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.
void clearerr(FILE *);
^[][Option Start]
char *ctermid(char *);
int dprintf(int, const char *restrict, ...)
[Option End]
int fclose(FILE *);
^[][Option Start]
FILE *fdopen(int, const char *);
[Option End]
int feof(FILE *);
int ferror(FILE *);
int fflush(FILE *);
int fgetc(FILE *);
int fgetpos(FILE *restrict, fpos_t *restrict);
char *fgets(char *restrict, int, FILE *restrict);
^[][Option Start]
int fileno(FILE *);
void flockfile(FILE *);
FILE *fmemopen(void *restrict, size_t, const char *restrict);
[Option End]
FILE *fopen(const char *restrict, const char *restrict);
int fprintf(FILE *restrict, const char *restrict, ...);
int fputc(int, FILE *);
int fputs(const char *restrict, FILE *restrict);
size_t fread(void *restrict, size_t, size_t, FILE *restrict);
FILE *freopen(const char *restrict, const char *restrict,
 FILE *restrict);
int fscanf(FILE *restrict, const char *restrict, ...);
int fseek(FILE *, long, int);
^[][Option Start]
int fseeko(FILE *, off_t, int);
[Option End]
int fsetpos(FILE *, const fpos_t *);
long ftell(FILE *);
^[][Option Start]
off_t ftello(FILE *);
int ftrylockfile(FILE *);
void funlockfile(FILE *);
[Option End]
size_t fwrite(const void *restrict, size_t, size_t, FILE *restrict);
int getc(FILE *);
int getchar(void);
^[][Option Start]
int getc_unlocked(FILE *);
int getchar_unlocked(void);
ssize_t getdelim(char **restrict, size_t *restrict, int,
 FILE *restrict);
ssize_t getline(char **restrict, size_t *restrict, FILE *restrict);
[Option End]
^[][Option Start]
char *gets(char *);
[Option End]
^[][Option Start]
FILE *open_memstream(char **, size_t *);
int pclose(FILE *);
[Option End]
void perror(const char *);
^[][Option Start]
FILE *popen(const char *, const char *);
[Option End]
int printf(const char *restrict, ...);
int putc(int, FILE *);
int putchar(int);
^[][Option Start]
int putc_unlocked(int, FILE *);
int putchar_unlocked(int);
[Option End]
int puts(const char *);
int remove(const char *);
int rename(const char *, const char *);
^[][Option Start]
int renameat(int, const char *, int, const char *);
[Option End]
void rewind(FILE *);
int scanf(const char *restrict, ...);
void setbuf(FILE *restrict, char *restrict);
int setvbuf(FILE *restrict, char *restrict, int, size_t);
int snprintf(char *restrict, size_t, const char *restrict, ...);
int sprintf(char *restrict, const char *restrict, ...);
int sscanf(const char *restrict, const char *restrict, ...);
^[][Option Start]
char *tempnam(const char *, const char *);
[Option End]
FILE *tmpfile(void);
^[][Option Start]
char *tmpnam(char *);
[Option End]
int ungetc(int, FILE *);
^[][Option Start]
int vdprintf(int, const char *restrict, va_list);
[Option End]
int vfprintf(FILE *restrict, const char *restrict, va_list);
int vfscanf(FILE *restrict, const char *restrict, va_list);
int vprintf(const char *restrict, va_list);
int vscanf(const char *restrict, va_list);
int vsnprintf(char *restrict, size_t, const char *restrict,
 va_list);
int vsprintf(char *restrict, const char *restrict, va_list);
int vsscanf(const char *restrict, const char *restrict, va_list);
^[] [Option Start] Inclusion of the <stdio.h> header may also make visible all symbols from <stddef.h>. [Option End]

The following sections are informative.

APPLICATION USAGE

Since standard I/O streams may use an underlying file descriptor to access the file associated with a stream, application developers need to be aware that {FOPEN_MAX} streams may not be available if file descriptors are being used to access files that are not associated with streams.

RATIONALE

There is a conflict between the ISO C standard and the POSIX definition of the {TMP_MAX} macro that is addressed by ISO/IEC 9899:1999 standard, Defect Report 336. The POSIX standard is in alignment with the public record of the response to the Defect Report. This change has not yet been published as part of the ISO C standard.

FUTURE DIRECTIONS

None.

CHANGE HISTORY

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

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Threads Extension.

Large File System extensions are added.

The constant L_cuserid and the external variables optarg, opterr, optind, and optopt are marked as extensions and LEGACY.

The cuserid() and getopt() functions are marked LEGACY.

Issue 6

The constant L_cuserid and the external variables optarg, opterr, optind, and optopt are removed as they were previously marked LEGACY.

The cuserid(), getopt(), and getw() functions are removed as they were previously marked LEGACY.

Several functions are marked as part of the Thread-Safe Functions option.

This reference page is updated to align with the ISO/IEC 9899:1999 standard. Note that the description of the fpos_t type is now explicitly updated to exclude array types.

Extensions beyond the ISO C standard are marked.

Issue 7

Austin Group Interpretation 1003.1-2001 #172 is applied, adding rationale about a conflict for the definition of {TMP_MAX} with the ISO C standard.

SD5-XBD-ERN-99 is applied, adding APPLICATION USAGE.

The dprintf(), fmemopen(), getdelim(), getline(), open_memstream(), and vdprintf() functions are added from The Open Group Technical Standard, 2006, Extended API Set Part 1.

The renameat() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 2.

The gets(), tmpnam(), and tempnam() functions and the L_tmpnam macro are marked obsolescent.

This reference page is clarified with respect to macros and symbolic constants, and a declaration for the off_t type is added.

POSIX.1-2008, Technical Corrigendum 1, XBD/TC1-2008/0065 [291,427] is applied.

End of informative text.

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