Read/Write Structure From/to a File in C (original) (raw)
Last Updated : 10 Jan, 2025
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
- **ptr: pointer to the block of memory to be written.
- **size: the size of each element to be written (in bytes).
- **nmemb: umber of elements.
- **stream: FILE pointer to the output file stream.
Return Value
- Number of objects written.
Example
C `
// C program for writing // struct to file #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
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
- **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 written.
Example
C `
// 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;
// Open person.dat for reading
infile = fopen("person1.dat", "wb+");
if (infile == NULL) {
fprintf(stderr, "\nError opening file\n");
exit(1);
}
struct person write_struct = { 1, "Rohan", "Sharma" };
// writing to file
fwrite(&write_struct, sizeof(write_struct), 1, infile);
struct person read_struct;
// setting pointer to start of the file
rewind(infile);
// reading to read_struct
fread(&read_struct, sizeof(read_struct), 1, infile);
printf("Name: %s %s \nID: %d", read_struct.fname,
read_struct.lname, read_struct.id);
// close file
fclose(infile);
return 0;}
`
Output
Name: Rohan Sharma ID: 1