Read/Write structure to a file in C

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.

1. 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
   

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   // 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, "\nError opend 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; }

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   Output:

   gcc demowrite.c
   ./a.out
   contents to file written successfully!
   

2. 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.

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   // 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, "\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; }

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   Output:

   gcc demoread.c
   ./a.out
   id = 1   name = rohan sharma
   id = 2   name = mahendra dhoni
   

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