Read/Write structure to a file in C - GeeksforGeeks

07 Oct, 2021
Prerequisite: Structure in CFor 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
// C program for writing// struct to file#include <stdio.h>#include <stdlib.h>#include <string.h> // a struct to read and writestruct 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, "\nError opened file\n");        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 !\n");    else        printf("error writing file !\n");     // 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
// C program for reading// struct from a file#include <stdio.h>#include <stdlib.h> // struct person with 3 fieldsstruct person{    int id;    char fname[20];    char lname[20];}; // Driver programint main (){    FILE *infile;    struct person input;         // Open person.dat for reading    infile = fopen ("person.dat", "r");    if (infile == NULL)    {        fprintf(stderr, "\nError opening file\n");        exit (1);    }         // read file contents till end of file    while(fread(&input, sizeof(struct person), 1, infile))        printf ("id = %d name = %s %s\n", 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
This article is contributed by Mandeep Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
KathiNotapplicable
ruhelaa48
C-Structure & Union
C Language
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Substring in C++
Function Pointer in C
std::string class in C++
fork() in C
Enumeration (or enum) in C
Command line arguments in C/C++
Different methods to reverse a string in C/C++
TCP Server-Client implementation in C
Structures in C
Exception Handling in C++