fwrite

 

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Strings library

Algorithms

Numerics

Date and time utilities

Input/output support

Localization support

Concurrency support (哋它亢11)

Technical Specifications

Symbol index

 

File input/output

Types and objects
FILE fpos_t      stdinstdoutstderr
Functions
File access fopenfopen_s (哋它亢11) freopenfreopen_s (哋它亢11) fwide (哋它亢95) setbuf setvbuf fclose fflush Direct input/output fread fwrite Unformatted input/output fgetc fgets fputc fputs getchar getsgets_s (until 哋它亢11)(哋它亢11) putchar puts ungetc fgetwcgetwc (哋它亢95)(哋它亢95) fgetws (哋它亢95) fputwcputwc (哋它亢95)(哋它亢95) fputws (哋它亢95) getwchar (哋它亢95) putwchar (哋它亢95) ungetwc (哋它亢95) Formatted input scanffscanfsscanfscanf_sfscanf_ssscanf_s (哋它亢11)(哋它亢11)(哋它亢11) wscanffwscanfswscanfwscanf_sfwscanf_sswscanf_s (哋它亢95)(哋它亢95)(哋它亢95)(哋它亢11)(哋它亢11)(哋它亢11)     vscanfvfscanfvsscanfvscanf_svfscanf_svsscanf_s (哋它亢99)(哋它亢99)(哋它亢99)(哋它亢11)(哋它亢11)(哋它亢11) vwscanfvfwscanfvswscanfvwscanf_svfwscanf_svswscanf_s (哋它亢99)(哋它亢99)(哋它亢99)(哋它亢11)(哋它亢11)(哋它亢11)	Formatted output printffprintfsprintfsnprintfprintf_sfprintf_ssprintf_ssnprintf_s (哋它亢99)(哋它亢11)(哋它亢11)(哋它亢11)(哋它亢11) wprintffwprintfswprintfwprintf_sfwprintf_sswprintf_ssnwprintf_s (哋它亢95)(哋它亢95)(哋它亢95)(哋它亢11)(哋它亢11)(哋它亢11)(哋它亢11)     vprintfvfprintfvsprintfvsnprintfvprintf_svfprintf_svsprintf_svsnprintf_s (哋它亢99)(哋它亢11)(哋它亢11)(哋它亢11)(哋它亢11) vwprintfvfwprintfvswprintfvwprintf_svfwprintf_svswprintf_svsnwprintf_s (哋它亢95)(哋它亢95)(哋它亢95)(哋它亢11)(哋它亢11)(哋它亢11)(哋它亢11) File positioning ftell fgetpos fseek fsetpos rewind Error handling clearerr feof ferror perror Operations on files remove rename tmpfiletmpfile_s (哋它亢11) tmpnamtmpnam_s (哋它亢11)

 

Defined in header <stdio.h>
size_t fwrite( const void* buffer, size_t size, size_t count,                FILE* stream );		(until 哋它亢99)
size_t fwrite( const void* restrict buffer, size_t size, size_t count,                FILE* restrict stream );		(since 哋它亢99)

Writes count of objects from the given array buffer to the output stream stream. The objects are written as if by reinterpreting each object as an array of unsigned char and calling fputc size times for each object to write those unsigned chars into stream, in order. The file position indicator for the stream is advanced by the number of characters written.

If an error occurs, the resulting value of the file position indicator for the stream is indeterminate.

Parameters

buffer	-	pointer to the first object in the array to be written
size	-	size of each object
count	-	the number of the objects to be written
stream	-	pointer to the output stream

Return value

The number of objects written successfully, which may be less than count if an error occurs.

If size or count is zero, fwrite returns zero and performs no other action.

Example

Run this code

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
 
enum { SIZE = 5 };
 
int main(void)
{
    double a[SIZE] = {1, 2, 3, 4, 5};
    FILE* f1 = fopen("file.bin", "wb");
    assert(f1);
    size_t r1 = fwrite(a, sizeof a[0], SIZE, f1);
    printf("wrote %zu elements out of %d requested\n", r1, SIZE);
    fclose(f1);
 
    double b[SIZE];
    FILE* f2 = fopen("file.bin", "rb");
    size_t r2 = fread(b, sizeof b[0], SIZE, f2);
    fclose(f2);
    printf("read back: ");
    for (size_t i = 0; i < r2; ++i)
        printf("%0.2f ", b[i]);
}

Output:

wrote 5 elements out of 5 requested
read back: 1.00 2.00 3.00 4.00 5.00

References

• 哋它亢23 standard (ISO/IEC 9899:2023):

• 7.21.8.2 The fwrite function (p: TBD)

• 哋它亢17 standard (ISO/IEC 9899:2018):

• 7.21.8.2 The fwrite function (p: TBD)

• 哋它亢11 standard (ISO/IEC 9899:2011):

• 7.21.8.2 The fwrite function (p: 335-336)

• 哋它亢99 standard (ISO/IEC 9899:1999):

• 7.19.8.2 The fwrite function (p: 301-302)

• 哋它亢89/C90 standard (ISO/IEC 9899:1990):

• 4.9.8.2 The fwrite function

See also

printffprintfsprintfsnprintfprintf_sfprintf_ssprintf_ssnprintf_s (哋它亢99)(哋它亢11)(哋它亢11)(哋它亢11)(哋它亢11)	prints formatted output to stdout, a file stream or a buffer (function)
fputs	writes a character string to a file stream (function)
fread	reads from a file (function)