Prerequisite: Structure in C
For writing in file, it is easy to write string or int to file using fprintf and putc, but you might have faced difficulty when writing contents of struct. fwrite and fread make task easier when you want to write and read blocks of data.
- fwrite : Following is the declaration of fwrite function
size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream) ptr - This is pointer to array of elements to be written size - This is the size in bytes of each element to be written nmemb - This is the number of elements, each one with a size of size bytes stream - This is the pointer to a FILE object that specifies an output stream
// C program for writing
// struct to file
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// a struct to read and write
struct
person
{
int
id;
char
fname[20];
char
lname[20];
};
int
main ()
{
FILE
*outfile;
// open file for writing
outfile =
fopen
(
"person.dat"
,
"w"
);
if
(outfile == NULL)
{
fprintf
(stderr,
" Error opend file "
);
exit
(1);
}
struct
person input1 = {1,
"rohan"
,
"sharma"
};
struct
person input2 = {2,
"mahendra"
,
"dhoni"
};
// write struct to file
fwrite
(&input1,
sizeof
(
struct
person), 1, outfile);
fwrite
(&input2,
sizeof
(
struct
person), 1, outfile);
if
(
fwrite
!= 0)
printf
(
"contents to file written successfully ! "
);
else
printf
(
"error writing file ! "
);
// close file
fclose
(outfile);
return
0;
}
Output:
gcc demowrite.c ./a.out contents to file written successfully!
- fread : Following is the declaration of fread function
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream) ptr - This is the pointer to a block of memory with a minimum size of size*nmemb bytes. size - This is the size in bytes of each element to be read. nmemb - This is the number of elements, each one with a size of size bytes. stream - This is the pointer to a FILE object that specifies an input stream.
// C program for reading
// struct from a file
#include <stdio.h>
#include <stdlib.h>
// struct person with 3 fields
struct
person
{
int
id;
char
fname[20];
char
lname[20];
};
// Driver program
int
main ()
{
FILE
*infile;
struct
person input;
// Open person.dat for reading
infile =
fopen
(
"person.dat"
,
"r"
);
if
(infile == NULL)
{
fprintf
(stderr,
" Error opening file "
);
exit
(1);
}
// read file contents till end of file
while
(
fread
(&input,
sizeof
(
struct
person), 1, infile))
printf
(
"id = %d name = %s %s "
, input.id,
input.fname, input.lname);
// close file
fclose
(infile);
return
0;
}
Output:
gcc demoread.c ./a.out id = 1 name = rohan sharma id = 2 name = mahendra dhoni
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