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.
- 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
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