posix_fallocate(3)

1posix_fallocate(3)         Library Functions Manual         posix_fallocate(3)
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NAME

6       posix_fallocate - allocate file space
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <fcntl.h>
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14       int posix_fallocate(int fd, off_t offset, off_t len);
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16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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18       posix_fallocate():
19           _POSIX_C_SOURCE >= 200112L
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DESCRIPTION

22       The function posix_fallocate() ensures that disk space is allocated for
23       the file referred to by the file descriptor fd for the bytes in the
24       range starting at offset and continuing for len bytes.  After a suc‐
25       cessful call to posix_fallocate(), subsequent writes to bytes in the
26       specified range are guaranteed not to fail because of lack of disk
27       space.
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29       If the size of the file is less than offset+len, then the file is in‐
30       creased to this size; otherwise the file size is left unchanged.
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RETURN VALUE

33       posix_fallocate() returns zero on success, or an error number on fail‐
34       ure.  Note that errno is not set.
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ERRORS

37       EBADF  fd is not a valid file descriptor, or is not opened for writing.
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39       EFBIG  offset+len exceeds the maximum file size.
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41       EINTR  A signal was caught during execution.
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43       EINVAL offset was less than 0, or len was less than or equal to 0, or
44              the underlying filesystem does not support the operation.
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46       ENODEV fd does not refer to a regular file.
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48       ENOSPC There is not enough space left on the device containing the file
49              referred to by fd.
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51       EOPNOTSUPP
52              The filesystem containing the file referred to by fd does not
53              support this operation.  This error code can be returned by C
54              libraries that don't perform the emulation shown in NOTES, such
55              as musl libc.
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57       ESPIPE fd refers to a pipe.
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ATTRIBUTES

60       For an explanation of the terms used in this section, see at‐
61       tributes(7).
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63       ┌──────────────────┬───────────────┬───────────────────────────────────┐
64       │Interface         │ Attribute     │ Value                             │
65       ├──────────────────┼───────────────┼───────────────────────────────────┤
66       │posix_fallocate() │ Thread safety │ MT-Safe (but see NOTES)           │
67       └──────────────────┴───────────────┴───────────────────────────────────┘
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STANDARDS

70       POSIX.1-2008.
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HISTORY

73       glibc 2.1.94.  POSIX.1-2001
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75       POSIX.1-2008 says that an implementation shall give the EINVAL error if
76       len was 0, or offset was less than 0.  POSIX.1-2001 says that an
77       implementation shall give the EINVAL error if len is less than 0, or
78       offset was less than 0, and may give the error if len equals zero.
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CAVEATS

81       In the glibc implementation, posix_fallocate() is implemented using the
82       fallocate(2) system call, which is MT-safe.  If the underlying
83       filesystem does not support fallocate(2), then the operation is
84       emulated with the following caveats:
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86       •  The emulation is inefficient.
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88       •  There is a race condition where concurrent writes from another
89          thread or process could be overwritten with null bytes.
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91       •  There is a race condition where concurrent file size increases by
92          another thread or process could result in a file whose size is
93          smaller than expected.
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95       •  If fd has been opened with the O_APPEND or O_WRONLY flags, the
96          function fails with the error EBADF.
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98       In general, the emulation is not MT-safe.  On Linux, applications may
99       use fallocate(2) if they cannot tolerate the emulation caveats.  In
100       general, this is only recommended if the application plans to terminate
101       the operation if EOPNOTSUPP is returned, otherwise the application
102       itself will need to implement a fallback with all the same problems as
103       the emulation provided by glibc.
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