to a File in C (original) (raw)

Last Updated : 9 Apr, 2026

For writing in the file, it is easy to write a string or an int to a file using **fprintf and **putc, but you might have faced difficulty when writing the contents of the struct. **fwrite and **fread make tasks easier when you want to write and read blocks of data.

In C language, structures are written/read in **binary format, which ensures exact byte-by-byte storage and avoids issues like text formatting.

For writing/reading structures, the file should be opened in the binary mode ("rb", "wb" or "ab" ).

rb: for binary reading
wb: for binary writing
ab: for appending binary

Writing Structures to a File using fwrite

The first step for writing structures in a file is to open the file in "wb" or "ab" mode. Then, we can use the fwrite() function to easily write a structure to a file. The fwrite() function writes to the file stream in the form of a binary data block.

Syntax of fwrite()

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)

**Parameters:

**ptr: pointer to the block of memory to be written.
**size: the size of each element to be written (in bytes).
**nmemb: number of elements.
**stream: FILE pointer to the output file stream.

**Return Value: Number of objects written.

**Example

C `

#include <stdio.h> #include <stdlib.h>

// a struct to be read and written struct person { int id; char fname[20]; char lname[20]; };

int main() { FILE* outfile;

// open file for writing
outfile = fopen("person.bin", "wb");
if (outfile == NULL) {
    fprintf(stderr, "\nError opened file\n");
    exit(1);
}

struct person input1 = { 1, "rohan", "sharma" };

// write struct to file
int flag = 0;
flag = fwrite(&input1, sizeof(struct person), 1,
              outfile);
if (flag) {
    printf("Contents of the structure written "
           "successfully");
}
else
    printf("Error Writing to File!");

// close file
fclose(outfile);

return 0;

}

Output

Contents of the structure written successfully

Reading Structure from a File using fread

To read structure from a file we need to open the file in "rb" binary read mode and then we can simply read structures using fread() function. This function reads a block of memory from the given stream.

Syntax of fread()

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)

**Parameters:

**ptr: pointer to the block of memory to read.
**size: the size of each element to read (in bytes).
**nmemb: number of elements.
**stream: FILE pointer to the input file stream.

**Return Value: Number of objects read.

**Example

C `

#include <stdio.h> #include <stdlib.h>

// struct person with 3 fields struct person { int id; char fname[20]; char lname[20]; };

int main() { FILE* infile;

// Open file for read + write
infile = fopen("person1.dat", "wb+");
if (infile == NULL) {
    fprintf(stderr, "\nError opening file\n");
    exit(1);
}

struct person people[] = {
    {1, "Rohan", "Sharma"},
    {2, "Amit", "Verma"},
    {3, "Neha", "Singh"}
};

int count = sizeof(people) / sizeof(people[0]);

for (int i = 0; i < count; i++) {
    fwrite(&people[i], sizeof(struct person), 1, infile);
}

struct person read_struct;

// Move file pointer to start
rewind(infile);

printf("Reading records:\n");

while (fread(&read_struct, sizeof(struct person), 1, infile) == 1) {
    printf("ID: %d, Name: %s %s\n",
           read_struct.id,
           read_struct.fname,
           read_struct.lname);
}

fclose(infile);
return 0;

}

Output

Reading records: ID: 1, Name: Rohan Sharma ID: 2, Name: Amit Verma ID: 3, Name: Neha Singh