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| .TH POSIX_FALLOCATE 3 2021-03-22 "GNU" "Linux Programmer's Manual" |
| .SH NAME |
| posix_fallocate \- allocate file space |
| .SH SYNOPSIS |
| .nf |
| .B #include <fcntl.h> |
| .PP |
| .BI "int posix_fallocate(int " fd ", off_t " offset ", off_t " len ); |
| .fi |
| .PP |
| .ad l |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| .BR posix_fallocate (): |
| .nf |
| _POSIX_C_SOURCE >= 200112L |
| .fi |
| .SH DESCRIPTION |
| The function |
| .BR posix_fallocate () |
| ensures that disk space is allocated for the file referred to by the |
| file descriptor |
| .I fd |
| for the bytes in the range starting at |
| .I offset |
| and continuing for |
| .I len |
| bytes. |
| After a successful call to |
| .BR posix_fallocate (), |
| subsequent writes to bytes in the specified range are |
| guaranteed not to fail because of lack of disk space. |
| .PP |
| If the size of the file is less than |
| .IR offset + len , |
| then the file is increased to this size; |
| otherwise the file size is left unchanged. |
| .SH RETURN VALUE |
| .BR posix_fallocate () |
| returns zero on success, or an error number on failure. |
| Note that |
| .I errno |
| is not set. |
| .SH ERRORS |
| .TP |
| .B EBADF |
| .I fd |
| is not a valid file descriptor, or is not opened for writing. |
| .TP |
| .B EFBIG |
| .I offset+len |
| exceeds the maximum file size. |
| .TP |
| .B EINTR |
| A signal was caught during execution. |
| .TP |
| .B EINVAL |
| .I offset |
| was less than 0, or |
| .I len |
| was less than or equal to 0, or the underlying filesystem does not |
| support the operation. |
| .TP |
| .B ENODEV |
| .I fd |
| does not refer to a regular file. |
| .TP |
| .B ENOSPC |
| There is not enough space left on the device containing the file |
| referred to by |
| .IR fd . |
| .TP |
| .B EOPNOTSUPP |
| The filesystem containing the file referred to by |
| .I fd |
| does not support this operation. |
| This error code can be returned by C libraries that don't perform the |
| emulation shown in NOTES, such as musl libc. |
| .TP |
| .B ESPIPE |
| .I fd |
| refers to a pipe. |
| .SH VERSIONS |
| .BR posix_fallocate () |
| is available since glibc 2.1.94. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .ad l |
| .nh |
| .TS |
| allbox; |
| lb lb lbx |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR posix_fallocate () |
| T} Thread safety T{ |
| MT-Safe (but see NOTES) |
| T} |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .SH CONFORMING TO |
| POSIX.1-2001. |
| .PP |
| POSIX.1-2008 says that an implementation |
| .I shall |
| give the |
| .B EINVAL |
| error if |
| .I len |
| was 0, or |
| .I offset |
| was less than 0. |
| POSIX.1-2001 says that an implementation |
| .I shall |
| give the |
| .B EINVAL |
| error if |
| .I len |
| is less than 0, or |
| .I offset |
| was less than 0, and |
| .I may |
| give the error if |
| .I len |
| equals zero. |
| .SH NOTES |
| In the glibc implementation, |
| .BR posix_fallocate () |
| is implemented using the |
| .BR fallocate (2) |
| system call, which is MT-safe. |
| If the underlying filesystem does not support |
| .BR fallocate (2), |
| then the operation is emulated with the following caveats: |
| .IP * 2 |
| The emulation is inefficient. |
| .IP * |
| There is a race condition where concurrent writes from another thread or |
| process could be overwritten with null bytes. |
| .IP * |
| There is a race condition where concurrent file size increases by |
| another thread or process could result in a file whose size is smaller |
| than expected. |
| .IP * |
| If |
| .I fd |
| has been opened with the |
| .B O_APPEND |
| or |
| .B O_WRONLY |
| flags, the function fails with the error |
| .BR EBADF . |
| .PP |
| In general, the emulation is not MT-safe. |
| On Linux, applications may use |
| .BR fallocate (2) |
| if they cannot tolerate the emulation caveats. |
| In general, this is |
| only recommended if the application plans to terminate the operation if |
| .B EOPNOTSUPP |
| is returned, otherwise the application itself will need to implement a |
| fallback with all the same problems as the emulation provided by glibc. |
| .SH SEE ALSO |
| .BR fallocate (1), |
| .BR fallocate (2), |
| .BR lseek (2), |
| .BR posix_fadvise (2) |