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Difference between revisions of "cpp/io/c/fseek"

From cppreference.com
< cpp‎ | io‎ | c
m (s/rw/rb in the example fopen since it's talking about bytes)
m (→‎Notes: added POSIX requirements on error.)
 
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{{cpp/title | fseek }}
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{{cpp/title|fseek}}
 
{{cpp/io/c/navbar}}
 
{{cpp/io/c/navbar}}
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{{ddcl | header=cstdio |
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{{ddcl|header=cstdio|
 
int fseek( std::FILE* stream, long offset, int origin );
 
int fseek( std::FILE* stream, long offset, int origin );
 
}}
 
}}
  
−
Sets the file position indicator for the file stream {{tt|stream}}.
+
Sets the file position indicator for the file stream {{c|stream}}.
  
−
If the {{tt|stream}} is open in binary mode, the new position is exactly {{tt|offset}} bytes measured from the beginning of the file if {{tt|origin}} is {{lc|SEEK_SET}}, from the current file position if {{tt|origin}} is {{lc|SEEK_CUR}}, and from the end of the file if {{tt|origin}} is {{lc|SEEK_END}}. Binary streams are not required to support {{lc|SEEK_END}}, in particular if additional null bytes are output.
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If the {{c|stream}} is open in binary mode, the new position is exactly {{c|offset}} bytes measured from the beginning of the file if {{c|origin}} is {{lc|SEEK_SET}}, from the current file position if {{c|origin}} is {{lc|SEEK_CUR}}, and from the end of the file if {{c|origin}} is {{lc|SEEK_END}}. Binary streams are not required to support {{lc|SEEK_END}}, in particular if additional null bytes are output.
  
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If the {{tt|stream}} is open in text mode, the only supported values for {{tt|offset}} are zero (which works with any {{tt|origin}}) and a value returned by an earlier call to {{lc|std::ftell}} on a stream associated with the same file (which only works with {{tt|origin}} of {{lc|SEEK_SET}}).
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If the {{c|stream}} is open in text mode, the only supported values for {{c|offset}} are zero (which works with any {{c|origin}}) and a value returned by an earlier call to {{lc|std::ftell}} on a stream associated with the same file (which only works with {{c|origin}} of {{lc|SEEK_SET}}).
  
−
If the {{tt|stream}} is wide-oriented, the restrictions of both text and binary streams apply (result of {{lc|std::ftell}} is allowed with SEEK_SET and zero offset is allowed from SEEK_SET and SEEK_CUR, but not SEEK_END)
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If the {{c|stream}} is wide-oriented, the restrictions of both text and binary streams apply (result of {{lc|std::ftell}} is allowed with {{lc|SEEK_SET}} and zero offset is allowed from {{lc|SEEK_SET}} and {{lc|SEEK_CUR}}, but not {{lc|SEEK_END}}).
  
 
In addition to changing the file position indicator, {{tt|fseek}} undoes the effects of {{lc|std::ungetc}} and clears the end-of-file status, if applicable.
 
In addition to changing the file position indicator, {{tt|fseek}} undoes the effects of {{lc|std::ungetc}} and clears the end-of-file status, if applicable.
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===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | stream | file stream to modify}}
+
{{par|stream|file stream to modify}}
−
{{par | offset | number of characters to shift the position relative to origin}}  
+
{{par|offset|number of characters to shift the position relative to origin}}
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{{par | origin | position to which {{tt|offset}} is added. It can have one of the following values: {{lc|SEEK_SET}}, {{lc|SEEK_CUR}}, {{lc|SEEK_END}}}}  
+
{{par|origin|position to which {{c|offset}} is added. It can have one of the following values: {{lc|SEEK_SET}}, {{lc|SEEK_CUR}}, {{lc|SEEK_END}}}}
 
{{par end}}
 
{{par end}}
  
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POSIX allows seeking beyond the existing end of file. If an output is performed after this seek, any read from the gap will return zero bytes. Where supported by the filesystem, this creates a ''sparse file''.
 
POSIX allows seeking beyond the existing end of file. If an output is performed after this seek, any read from the gap will return zero bytes. Where supported by the filesystem, this creates a ''sparse file''.
  
−
POSIX also requires that fseek first performs {{lc|fflush}} if there are any unwritten data (but whether the shift state is restored is implementation-defined). The standard C++ file streams guarantee both flushing and unshifting: {{lc|std::basic_filebuf::seekoff}}
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POSIX also requires that {{tt|fseek}} first performs {{lc|std::fflush|fflush}} if there are any unwritten data (but whether the shift state is restored is implementation-defined). The standard C++ file streams guarantee both flushing and unshifting: {{lc|std::basic_filebuf::seekoff}}.
 +
 
 +
POSIX specifies, that [https://pubs.opengroup.org/onlinepubs/9699919799/functions/fseek.html {{tt|fseek}}] should return {{c|-1}} on error, and set {{lc|errno}} to indicate the error.
  
 
===Example===
 
===Example===
 
{{example
 
{{example
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|
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|code=
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| code=
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#include <cassert>
 
#include <cstdio>
 
#include <cstdio>
 
#include <cstdint>
 
#include <cstdint>
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#include <vector>
 
 
#include <fstream>
 
#include <fstream>
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#include <cassert>
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#include <vector>
  
 
int main()
 
int main()
 
{
 
{
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     std::ofstream("dummy.nfo") << "sample data\n";
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     std::ofstream("dummy.nfo") << "8 bytes\n"; // create the file
−
 
+
  
 
     std::FILE* fp = std::fopen("dummy.nfo", "rb");
 
     std::FILE* fp = std::fopen("dummy.nfo", "rb");
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     std::fseek(fp, 0, SEEK_END); // seek to end
 
     std::fseek(fp, 0, SEEK_END); // seek to end
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     std::size_t filesize = std::ftell(fp);
+
     const std::size_t filesize = std::ftell(fp);
 +
    std::vector<std::uint8_t> buffer(filesize);
  
 
     std::fseek(fp, 0, SEEK_SET); // seek to start
 
     std::fseek(fp, 0, SEEK_SET); // seek to start
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    std::vector<uint8_t> buffer(filesize);
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     std::fread(buffer.data(), sizeof(std::uint8_t), buffer.size(), fp);
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     std::fread(buffer.data(), sizeof(uint8_t), buffer.size(), fp);
+
  
 
     std::fclose(fp);
 
     std::fclose(fp);
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     std::printf("i've read %zi bytes\n", filesize);
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     std::printf("I've read %zi bytes\n", filesize);
 
}
 
}
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| output=
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|p=true
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i've read 12 bytes
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|output=
 +
I've read 8 bytes
 
}}
 
}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/io/c/dsc fsetpos}}
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{{dsc inc|cpp/io/c/dsc fsetpos}}
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{{dsc inc | cpp/io/c/dsc fgetpos}}
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{{dsc inc|cpp/io/c/dsc fgetpos}}
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{{dsc inc | cpp/io/c/dsc ftell}}
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{{dsc inc|cpp/io/c/dsc ftell}}
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{{dsc inc | cpp/io/c/dsc rewind}}
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{{dsc inc|cpp/io/c/dsc rewind}}
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{{dsc see c | c/io/fseek}}
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{{dsc see c|c/io/fseek}}
 
{{dsc end}}
 
{{dsc end}}
  
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[[de:cpp/io/c/fseek]]
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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[[es:cpp/io/c/fseek]]
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[[fr:cpp/io/c/fseek]]
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[[it:cpp/io/c/fseek]]
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[[ja:cpp/io/c/fseek]]
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[[pt:cpp/io/c/fseek]]
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[[ru:cpp/io/c/fseek]]
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[[zh:cpp/io/c/fseek]]
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Latest revision as of 00:02, 14 May 2024

 
C++
 
Input/output library
 
C-style I/O
Types and objects
Functions
File access
Direct input/output
Unformatted input/output
Formatted input
(C++11)(C++11)(C++11)    
(C++11)(C++11)(C++11)    
 
Defined in header <cstdio>
int fseek( std::FILE* stream, long offset, int origin );

Sets the file position indicator for the file stream stream.

If the stream is open in binary mode, the new position is exactly offset bytes measured from the beginning of the file if origin is SEEK_SET, from the current file position if origin is SEEK_CUR, and from the end of the file if origin is SEEK_END. Binary streams are not required to support SEEK_END, in particular if additional null bytes are output.

If the stream is open in text mode, the only supported values for offset are zero (which works with any origin) and a value returned by an earlier call to std::ftell on a stream associated with the same file (which only works with origin of SEEK_SET).

If the stream is wide-oriented, the restrictions of both text and binary streams apply (result of std::ftell is allowed with SEEK_SET and zero offset is allowed from SEEK_SET and SEEK_CUR, but not SEEK_END).

In addition to changing the file position indicator, fseek undoes the effects of std::ungetc and clears the end-of-file status, if applicable.

If a read or write error occurs, the error indicator for the stream (std::ferror) is set and the file position is unaffected.

Contents

[edit] Parameters

stream - file stream to modify
offset - number of characters to shift the position relative to origin
origin - position to which offset is added. It can have one of the following values: SEEK_SET, SEEK_CUR, SEEK_END

[edit] Return value

​0​ upon success, nonzero value otherwise.

[edit] Notes

After seeking to a non-end position in a wide stream, the next call to any output function may render the remainder of the file undefined, e.g. by outputting a multibyte sequence of a different length.

POSIX allows seeking beyond the existing end of file. If an output is performed after this seek, any read from the gap will return zero bytes. Where supported by the filesystem, this creates a sparse file.

POSIX also requires that fseek first performs fflush if there are any unwritten data (but whether the shift state is restored is implementation-defined). The standard C++ file streams guarantee both flushing and unshifting: std::basic_filebuf::seekoff.

POSIX specifies, that fseek should return -1 on error, and set errno to indicate the error.

[edit] Example

Run this code
#include <cassert>
#include <cstdio>
#include <cstdint>
#include <fstream>
#include <vector>
 
int main()
{
    std::ofstream("dummy.nfo") << "8 bytes\n"; // create the file
 
    std::FILE* fp = std::fopen("dummy.nfo", "rb");
    assert(fp);
 
    std::fseek(fp, 0, SEEK_END); // seek to end
    const std::size_t filesize = std::ftell(fp);
    std::vector<std::uint8_t> buffer(filesize);
 
    std::fseek(fp, 0, SEEK_SET); // seek to start
    std::fread(buffer.data(), sizeof(std::uint8_t), buffer.size(), fp);
 
    std::fclose(fp);
    std::printf("I've read %zi bytes\n", filesize);
}

Possible output: 

I've read 8 bytes

[edit] See also

 moves the file position indicator to a specific location in a file
(function) [edit]
gets the file position indicator
(function) [edit]
returns the current file position indicator
(function) [edit]
moves the file position indicator to the beginning in a file
(function) [edit]
C documentation for fseek
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