Read/Write Structure From/to a File in C

For writing in the 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 the struct. fwrite and fread make tasks easier when you want to write and read blocks of data.

Writing Structure to a File using fwrite

We can use fwrite() function to easily write a structure in a file. fwrite() function writes the to the file stream in the form of binary data block.

Syntax of fwrite()

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

Parameters

Return Value

Example

C

// C program for writing // struct to file // a struct to be read and written struct person < char fname[20]; char lname[20]; FILE * outfile; // open file for writing outfile = fopen ( "person.bin" , "wb" ); if (outfile == NULL) < fprintf (stderr, "\nError opened file\n" ); struct person input1 = < 1, "rohan" , "sharma" >; // write struct to file int flag = 0; flag = fwrite (&input1, sizeof ( struct person), 1, printf ( "Contents of the structure written " "successfully" ); printf ( "Error Writing to File!" ); // close file fclose (outfile); Output 
Contents of the structure written successfully

Reading Structure from a File using fread

We can easily read structure from a file 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

Return Value

Example

C

// C program for reading // struct from a file // struct person with 3 fields struct person < char fname[20]; char lname[20]; // Driver program FILE * infile; // Open person.dat for reading infile = fopen ( "person1.dat" , "wb+" ); if (infile == NULL) < fprintf (stderr, "\nError opening file\n" ); struct person write_struct = < 1, "Rohan" , "Sharma" >; // writing to file struct person read_struct; // setting pointer to start of the file rewind (infile); // reading to read_struct printf ( "Name: %s %s \nID: %d" , read_struct.fname, read_struct.lname, read_struct.id); // close file fclose (infile); Output 
Name: Rohan Sharma ID: 1

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