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PROLOG |NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |EXAMPLES |APPLICATION USAGE |RATIONALE |FUTURE DIRECTIONS |SEE ALSO |COPYRIGHT | |
FOPEN(3P) POSIX Programmer's ManualFOPEN(3P)This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
fopen — open a stream
#include <stdio.h> FILE *fopen(const char *restrictpathname, const char *restrictmode);
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.1‐2017 defers to the ISO C standard. Thefopen() function shall open the file whose pathname is the string pointed to bypathname, and associates a stream with it. Themode argument points to a string. If the string is one of the following, the file shall be opened in the indicated mode. Otherwise, the behavior is undefined.r orrb Open file for reading.w orwb Truncate to zero length or create file for writing.a orab Append; open or create file for writing at end-of- file.r+ orrb+ orr+b Open file for update (reading and writing).w+ orwb+ orw+b Truncate to zero length or create file for update.a+ orab+ ora+b Append; open or create file for update, writing at end-of-file. The character'b'shall have no effect, but is allowed for ISO C standard conformance. Opening a file with read mode (r as the first character in themode argument) shall fail if the file does not exist or cannot be read. Opening a file with append mode (a as the first character in themode argument) shall cause all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls tofseek(). When a file is opened with update mode ('+'as the second or third character in themode argument), both input and output may be performed on the associated stream. However, the application shall ensure that output is not directly followed by input without an intervening call tofflush() or to a file positioning function (fseek(),fsetpos(), orrewind()), and input is not directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream shall be cleared. Ifmode isw,wb,a,ab,w+,wb+,w+b,a+,ab+, ora+b, and the file did not previously exist, upon successful completion,fopen() shall mark for update the last data access, last data modification, and last file status change timestamps of the file and the last file status change and last data modification timestamps of the parent directory. Ifmode isw,wb,a,ab,w+,wb+,w+b,a+,ab+, ora+b, and the file did not previously exist, thefopen() function shall create a file as if it called thecreat() function with a value appropriate for thepath argument interpreted frompathname and a value of S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH for themode argument. Ifmode isw,wb,w+,wb+, orw+b, and the file did previously exist, upon successful completion,fopen() shall mark for update the last data modification and last file status change timestamps of the file. After a successful call to thefopen() function, the orientation of the stream shall be cleared, the encoding rule shall be cleared, and the associatedmbstate_tobject shall be set to describe an initial conversion state. The file descriptor associated with the opened stream shall be allocated and opened as if by a call toopen() with the following flags: ┌──────────────────┬───────────────────────────┐ │fopen() Mode│open() Flags│ ├──────────────────┼───────────────────────────┤ │r orrb │ O_RDONLY │ │w orwb │ O_WRONLY|O_CREAT|O_TRUNC │ │a orab │ O_WRONLY|O_CREAT|O_APPEND │ │r+ orrb+ orr+b │ O_RDWR │ │w+ orwb+ orw+b │ O_RDWR|O_CREAT|O_TRUNC │ │a+ orab+ ora+b │ O_RDWR|O_CREAT|O_APPEND │ └──────────────────┴───────────────────────────┘Upon successful completion,fopen() shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, anderrno shall be set to indicate the error.
Thefopen() function shall fail if:EACCESSearch permission is denied on a component of the path prefix, or the file exists and the permissions specified bymode are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created.EINTRA signal was caught duringfopen().EISDIRThe named file is a directory andmode requires write access.ELOOPA loop exists in symbolic links encountered during resolution of thepath argument.EMFILEAll file descriptors available to the process are currently open.EMFILE{STREAM_MAX} streams are currently open in the calling process.ENAMETOOLONG The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.ENFILEThe maximum allowable number of files is currently open in the system.ENOENTThemode string begins with'r'and a component ofpathname does not name an existing file, ormode begins with'w'or'a'and a component of the path prefix ofpathname does not name an existing file, orpathname is an empty string.ENOENTorENOTDIR Thepathname argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters. Ifpathname without the trailing <slash> characters would name an existing file, an[ENOENT]error shall not occur.ENOSPCThe directory or file system that would contain the new file cannot be expanded, the file does not exist, and the file was to be created.ENOTDIR A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or thepathname argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.ENXIOThe named file is a character special or block special file, and the device associated with this special file does not exist.EOVERFLOW The named file is a regular file and the size of the file cannot be represented correctly in an object of typeoff_t.EROFSThe named file resides on a read-only file system andmode requires write access. Thefopen() function may fail if:EINVALThe value of themode argument is not valid.ELOOPMore than {SYMLOOP_MAX} symbolic links were encountered during resolution of thepath argument.EMFILE{FOPEN_MAX} streams are currently open in the calling process.ENAMETOOLONG The length of a component of a pathname is longer than {NAME_MAX}.ENOMEMInsufficient storage space is available.ETXTBSY The file is a pure procedure (shared text) file that is being executed andmode requires write access.The following sections are informative.Opening a File The following example tries to open the file namedfilefor reading. Thefopen() function returns a file pointer that is used in subsequentfgets() andfclose() calls. If the program cannot open the file, it just ignores it. #include <stdio.h> ... FILE *fp; ... void rgrep(const char *file) { ... if ((fp = fopen(file, "r")) == NULL) return; ... }None.
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Section 2.5,Standard I/O Streams,creat(3p),fclose(3p),fdopen(3p),fmemopen(3p),freopen(3p),open_memstream(3p) The Base Definitions volume of POSIX.1‐2017,stdio.h(0p)
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 athttp://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, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017FOPEN(3P)Pages that refer to this page:stdio.h(0p), close(3p), fclose(3p), fdopen(3p), feof(3p), ferror(3p), fgetpos(3p), fgets(3p), fgetwc(3p), fgetws(3p), fileno(3p), fmemopen(3p), fputc(3p), fputs(3p), fputwc(3p), fputws(3p), fread(3p), freopen(3p), fseek(3p), fsetpos(3p), ftell(3p), fwrite(3p), lockf(3p), open_memstream(3p), puts(3p), setbuf(3p), setvbuf(3p), stdin(3p), tempnam(3p), tmpfile(3p), tmpnam(3p)
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