fwrite

 

C

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headers

Type support

Dynamic memory management

Error handling

Program utilities

Variadic function support

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

Technical Specifications

 

File input/output

Functions
File access
fopenfopen_s (C11) freopenfreopen_s (C11) fwide (C95)	setbuf setvbuf fclose fflush
Direct input/output
fread
fwrite
Unformatted input/output
fgetc fgets fputc fputs getchar getsgets_s (until C11)(since C11) putchar puts ungetc	fgetwcgetwc (C95)(C95) fgetws (C95) fputwcputwc (C95)(C95) fputws (C95) getwchar (C95) putwchar (C95) ungetwc (C95)
Formatted input
scanffscanfsscanfscanf_sfscanf_ssscanf_s (C11)(C11)(C11) wscanffwscanfswscanfwscanf_sfwscanf_sswscanf_s (C95)(C95)(C95)(C11)(C11)(C11)	vscanfvfscanfvsscanfvscanf_svfscanf_svsscanf_s (C99)(C99)(C99)(C11)(C11)(C11) vwscanfvfwscanfvswscanfvwscanf_svfwscanf_svswscanf_s (C99)(C99)(C99)(C11)(C11)(C11)
Formatted output
printffprintfsprintfsnprintfprintf_sfprintf_ssprintf_ssnprintf_s (C99)(C11)(C11)(C11)(C11) wprintffwprintfswprintfwprintf_sfwprintf_sswprintf_ssnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11)	vprintfvfprintfvsprintfvsnprintfvprintf_svfprintf_svsprintf_svsnprintf_s (C99)(C11)(C11)(C11)(C11) vwprintfvfwprintfvswprintfvwprintf_svfwprintf_svswprintf_svsnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11)
File positioning
ftell
fgetpos
fseek
fsetpos
rewind
Error handling
clearerr
feof
ferror
perror
Operations on files
remove
rename
tmpfiletmpfile_s (C11)
tmpnamtmpnam_s (C11)

 

Defined in header <stdio.h>
size_t fwrite( const void *buffer, size_t size, size_t count,                FILE *stream );		(until C99)
size_t fwrite( const void *restrict buffer, size_t size, size_t count,                FILE *restrict stream );		(since C99)

Writes count of objects in the given array buffer to the output stream stream. Objects are not interpreted in any way.

Parameters

buffer	-	pointer to the first object 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.

Example

fwrite with error checking

Run this code

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    // write buffer to file
    FILE *f1;
    char buffer[] = { 'x' , 'y' , 'z' };
    f1 = fopen("file.bin", "wb");
    int ret_code = fwrite(buffer, sizeof(char), sizeof(buffer), f1);
    if (ret_code < (int)sizeof(buffer))
       if (ferror(f1))
       {
          perror("fwrite()");
          fprintf(stderr,"fwrite() failed in file %s at line # %d\n", __FILE__,__LINE__-5);
          exit(EXIT_FAILURE);
       }
    fclose(f1);
 
    // read the same data and print it to the standard output
    FILE *f2;
    char rbuf[10];
    f2 = fopen("file.bin", "rb");
    const char* res = fgets(rbuf, sizeof(rbuf), f2);
    fclose(f2);
 
    if (res) { // points to rbuf on read success, NULL on failure
        puts(res);
    }
 
    return EXIT_SUCCESS;
}

Output:

xyz

References

• C11 standard (ISO/IEC 9899:2011):

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

• C99 standard (ISO/IEC 9899:1999):

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

• C89/C90 standard (ISO/IEC 9899:1990):

• 4.9.8.2 The fwrite function

See also

printffprintfsprintfsnprintfprintf_sfprintf_ssprintf_ssnprintf_s (C99)(C11)(C11)(C11)(C11)	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)
C++ documentation for fwrite