stdio.h(0P)

✨ PROLOG ✨

okay bestie, before anything else — this page is part of the POSIX Programmer's Manual, and here's the lil disclaimer: the Linux implementation of this interface may differ (so like, go check the corresponding Linux manual page for the deets on Linux behavior), or the interface might not be implemented on Linux at all. 💀 not every platform is gonna have your back on this one.

---

🎀 NAME 🎀

stdio.h — standard buffered input/output

that's it, that's the vibe. it's the header that runs your whole file-reading, file-writing, printing-to-terminal lifestyle. main character energy fr fr.

---

🌸 SYNOPSIS 🌸

#include <stdio.h>

just that one line. iconic. 💅

---

🦋 DESCRIPTION 🦋

so here's the tea: some of the functionality described on this reference page actually extends the ISO C standard. if you want those extra symbols to be visible in this header, your application has to define the appropriate feature test macro — go read the System Interfaces volume of POSIX.1‐2017, Section 2.2, The Compilation Environment for the whole story. 🔥

🧚 the data types (defined through typedef)

• FILE — a structure containing information about a file. it's literally the whole personality of your stream.
• fpos_t — a non-array type containing all information needed to specify uniquely every position within a file. yes, every position, uniquely. she's thorough.
• off_t — as described in <sys/types.h>.
• size_t — as described in <stddef.h>.
• ssize_t — as described in <sys/types.h>.
• va_list — as described in <stdarg.h>.

🍓 the macros that expand to integer constant expressions

• BUFSIZ — size of <stdio.h> buffers. this shall expand to a positive value. (positive! she's not gonna be weird about it.)
• L_ctermid — maximum size of character array to hold ctermid() output.
• L_tmpnam — maximum size of character array to hold tmpnam() output.

💜 the macros that expand to integer constant expressions with distinct values

• _IOFBF — input/output fully buffered.
• _IOLBF — input/output line buffered.
• _IONBF — input/output unbuffered.

⭐ and ALSO these macros that 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.

(yes bestie, there are two separate groups of distinct-valued constant-expression macros, and they are NOT the same group. keep up 💅)

🌈 the macros that 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} — minimum number of unique filenames generated by tmpnam(). also: maximum number of times an application can call tmpnam() reliably. the value of {TMP_MAX} is at least 25.

on XSI-conformant systems, the value of {TMP_MAX} is at least 10000. 😭 the XSI girlies are eating good.

🙈 the negative-value macro

• EOF — end-of-file return value. this one shall expand to an integer constant expression with type int and a negative value. so when you see a negative number, it's not a bug, it's just EOF saying hi.

🫶 and also

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

🧵 the string-constant macro

• P_tmpdir — default directory prefix for tempnam().

👑 the standard stream macros

these 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 trio. the holy trinity. the ones you've been typing since day one. 💖

---

💅 the functions

the following shall be declared as functions and may also be defined as macros. function prototypes shall be provided.

void     clearerr(FILE *);
char    *ctermid(char *);
int      dprintf(int, const char *restrict, ...)
int      fclose(FILE *);
FILE    *fdopen(int, const char *);
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);
int      fileno(FILE *);
void     flockfile(FILE *);
FILE    *fmemopen(void *restrict, size_t, const char *restrict);
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);
int      fseeko(FILE *, off_t, int);
int      fsetpos(FILE *, const fpos_t *);
long     ftell(FILE *);
off_t    ftello(FILE *);
int      ftrylockfile(FILE *);
void     funlockfile(FILE *);
size_t   fwrite(const void *restrict, size_t, size_t, FILE *restrict);
int      getc(FILE *);
int      getchar(void);
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);
char    *gets(char *);
FILE    *open_memstream(char **, size_t *);
int      pclose(FILE *);
void     perror(const char *);
FILE    *popen(const char *, const char *);
int      printf(const char *restrict, ...);
int      putc(int, FILE *);
int      putchar(int);
int      putc_unlocked(int, FILE *);
int      putchar_unlocked(int);
int      puts(const char *);
int      remove(const char *);
int      rename(const char *, const char *);
int      renameat(int, const char *, int, const char *);
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, ...);
char    *tempnam(const char *, const char *);
FILE    *tmpfile(void);
char    *tmpnam(char *);
int      ungetc(int, FILE *);
int      vdprintf(int, const char *restrict, va_list);
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);

also? inclusion of the <stdio.h> header may also make visible all symbols from <stddef.h>. surprise bonus content. 🎁

the following sections are informative.

---

💜 APPLICATION USAGE 💜

real talk, this one's important: 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. so like… the guarantee has an asterisk, bestie. plan accordingly. 🫶

---

🔥 RATIONALE 🔥

there's a whole conflict between the ISO C standard and the POSIX definition of the {TMP_MAX} macro, and it's 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. so yeah — drama in the standards committee. periodt. 💅

---

🌈 FUTURE DIRECTIONS 🌈

None. she said what she said, and what she said was nothing. 💀

---

👑 SEE ALSO 👑

• <stdarg.h>
• <stddef.h>
• <sys_types.h>

and also, from the System Interfaces volume of POSIX.1‐2017, Section 2.2, The Compilation Environment: clearerr(), ctermid(), fclose(), fdopen(), feof(), ferror(), fflush(), fgetc(), fgetpos(), fgets(), fileno(), flockfile(), fmemopen(), fopen(), fprintf(), fputc(), fputs(), fread(), freopen(), fscanf(), fseek(), fsetpos(), ftell(), fwrite(), getc(), getchar(), getc_unlocked(), getdelim(), getopt(), gets(), open_memstream(), pclose(), perror(), popen(), putc(), putchar(), puts(), remove(), rename(), rewind(), setbuf(), setvbuf(), stdin, system(), tempnam(), tmpfile(), tmpnam(), ungetc(), vfprintf(), vfscanf()

go read all of them. or like, at least the ones you actually need. i'm not your mom. 🧚

---

🎀 COPYRIGHT 🎀

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 http://www.opengroup.org/unix/online.html .

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 https://www.kernel.org/doc/man-pages/reporting_bugs.html .

---

and that's the whole page, bestie — brought to you by IEEE/The Open Group, 2017, stdio.h(0P) ✨ go forth and fopen() responsibly 🫶💖