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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |STANDARDS |HISTORY |NOTES |BUGS |SEE ALSO |COLOPHON | |
fopen(3) Library Functions Manualfopen(3)fopen, fdopen, freopen - stream open functions
Standard C library (libc,-lc)
#include <stdio.h>FILE *fopen(const char *restrictpath, const char *restrictmode);FILE *fdopen(intfd, const char *mode);FILE *freopen(const char *restrictpath, const char *restrictmode,FILE *restrictstream); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):fdopen(): _POSIX_C_SOURCE
Thefopen() function opens the file whose name is the string pointed to bypath and associates a stream with it. The argumentmode points to a string beginning with one of the following sequences (possibly followed by additional characters, as described below):rOpen text file for reading. The stream is positioned at the beginning of the file.r+Open for reading and writing. The stream is positioned at the beginning of the file.wTruncate file to zero length or create text file for writing. The stream is positioned at the beginning of the file.w+Open for reading and writing. The file is created if it does not exist, otherwise it is truncated. The stream is positioned at the beginning of the file.aOpen for appending (writing at end of file). The file is created if it does not exist. The stream is positioned at the end of the file.a+Open for reading and appending (writing at end of file). The file is created if it does not exist. Output is always appended to the end of the file. POSIX is silent on what the initial read position is when using this mode. For glibc, the initial file position for reading is at the beginning of the file, but for Android/BSD/MacOS, the initial file position for reading is at the end of the file. Themode string can also include the letter 'b' either as a last character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with ISO C and has no effect; the 'b' is ignored on all POSIX conforming systems, including Linux. (Other systems may treat text files and binary files differently, and adding the 'b' may be a good idea if you do I/O to a binary file and expect that your program may be ported to non-UNIX environments.) See NOTES below for details of glibc extensions formode. Any created file will have the modeS_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH(0666), as modified by the process's umask value (seeumask(2)). Reads and writes may be intermixed on read/write streams in any order. Note that ANSI C requires that a file positioning function intervene between output and input, unless an input operation encounters end-of-file. (If this condition is not met, then a read is allowed to return the result of writes other than the most recent.) Therefore it is good practice (and indeed sometimes necessary under Linux) to put anfseek(3) orfsetpos(3) operation between write and read operations on such a stream. This operation may be an apparent no-op (as infseek(..., 0L, SEEK_CUR) called for its synchronizing side effect). Opening a file in append mode (aas the first character ofmode) causes all subsequent write operations to this stream to occur at end-of-file, as if preceded by the call: fseek(stream, 0, SEEK_END); The file descriptor associated with the stream is opened as if by a call toopen(2) with the following flags: ┌──────────────┬───────────────────────────────┐ │fopen() mode│open() flags│ ├──────────────┼───────────────────────────────┤ │r │ O_RDONLY │ ├──────────────┼───────────────────────────────┤ │w │ O_WRONLY | O_CREAT | O_TRUNC │ ├──────────────┼───────────────────────────────┤ │a │ O_WRONLY | O_CREAT | O_APPEND │ ├──────────────┼───────────────────────────────┤ │r+ │ O_RDWR │ ├──────────────┼───────────────────────────────┤ │w+ │ O_RDWR | O_CREAT | O_TRUNC │ ├──────────────┼───────────────────────────────┤ │a+ │ O_RDWR | O_CREAT | O_APPEND │ └──────────────┴───────────────────────────────┘fdopen() Thefdopen() function associates a stream with the existing file descriptor,fd. Themode of the stream (one of the values "r", "r+", "w", "w+", "a", "a+") must be compatible with the mode of the file descriptor. The file position indicator of the new stream is set to that belonging tofd, and the error and end-of- file indicators are cleared. Modes "w" or "w+" do not cause truncation of the file. The file descriptor is not dup'ed, and will be closed when the stream created byfdopen() is closed. The result of applyingfdopen() to a shared memory object is undefined.freopen() Thefreopen() function opens the file whose name is the string pointed to bypath and associates the stream pointed to bystream with it. The original stream (if it exists) is closed. Themode argument is used just as in thefopen() function. Ifpath is a null pointer,freopen() changes the mode of the stream to that specified inmode; that is,freopen() reopens the pathname that is associated with the stream. The specification for this behavior was added in the C99 standard, which says: In this case, the file descriptor associated with the stream need not be closed if the call tofreopen() succeeds. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances. The primary use of thefreopen() function is to change the file associated with a standard text stream (stderr,stdin, orstdout).
Upon successful completionfopen(),fdopen(), andfreopen() return aFILE pointer. Otherwise, NULL is returned anderrno is set to indicate the error.
EINVALThemode provided tofopen(),fdopen(), orfreopen() was invalid. Thefopen(),fdopen(), andfreopen() functions may also fail and seterrno for any of the errors specified for the routinemalloc(3). Thefopen() function may also fail and seterrno for any of the errors specified for the routineopen(2). Thefdopen() function may also fail and seterrno for any of the errors specified for the routinefcntl(2). Thefreopen() function may also fail and seterrno for any of the errors specified for the routinesopen(2),fclose(3), andfflush(3).
For an explanation of the terms used in this section, seeattributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface│Attribute│Value│ ├──────────────────────────────────────┼───────────────┼─────────┤ │fopen(),fdopen(),freopen() │ Thread safety │ MT-Safe │ └──────────────────────────────────────┴───────────────┴─────────┘
fopen()freopen() C11, POSIX.1-2008.fdopen() POSIX.1-2008.
fopen()freopen() POSIX.1-2001, C89.fdopen() POSIX.1-2001.
glibc notes The GNU C library allows the following extensions for the string specified inmode:c(since glibc 2.3.3) Do not make the open operation, or subsequent read and write operations, thread cancelation points. This flag is ignored forfdopen().e(since glibc 2.7) Open the file with theO_CLOEXECflag. Seeopen(2) for more information. This flag is ignored forfdopen().m(since glibc 2.3) Attempt to access the file usingmmap(2), rather than I/O system calls (read(2),write(2)). Currently, use ofmmap(2) is attempted only for a file opened for reading.xOpen the file exclusively (like theO_EXCLflag ofopen(2)). If the file already exists,fopen() fails, and setserrno toEEXIST. This flag is ignored forfdopen(). In addition to the above characters,fopen() andfreopen() support the following syntax inmode:,ccs=string The givenstring is taken as the name of a coded character set and the stream is marked as wide-oriented. Thereafter, internal conversion functions convert I/O to and from the character setstring. If the,ccs=string syntax is not specified, then the wide-orientation of the stream is determined by the first file operation. If that operation is a wide-character operation, the stream is marked wide-oriented, and functions to convert to the coded character set are loaded.
When parsing for individual flag characters inmode (i.e., the characters preceding the "ccs" specification), the glibc implementation offopen() andfreopen() limits the number of characters examined inmode to 7 (or, before glibc 2.14, to 6, which was not enough to include possible specifications such as "rb+cmxe"). The current implementation offdopen() parses at most 5 characters inmode.
open(2),fclose(3),fileno(3),fmemopen(3),fopencookie(3),open_memstream(3)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17fopen(3)Pages that refer to this page:open(2), fclose(3), fcloseall(3), ferror(3), fflush(3), fgetc(3), fgetgrent(3), fgetpwent(3), fgetwc(3), fgetws(3), FILE(3type), fileno(3), fmemopen(3), fopencookie(3), fputwc(3), fputws(3), getline(3), getmntent(3), gets(3), libexpect(3), open_memstream(3), pmopenlog(3), popen(3), procio(3), pthread_getattr_np(3), puts(3), setbuf(3), stdin(3), stdio(3)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |