Read the contents of a file into character buffer using C program

Lets say we want to read the contents of some file and want to operate on these contents using C program. In this case, we first need to read the complete file into buffer and work on to this character buffer to do what we want to do.

Following c program, copied the contents of JSON file test_file.json into buffer and then dumps that buffer to stdout. [ You can do whatever you want which this bufffer ]

 $ vim read_file_to_buffer.c 
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

 * 'slurp' reads the file identified by 'path' into a character buffer
 * pointed at by 'buf', optionally adding a terminating NUL if
 * 'add_nul' is true. On success, the size of the file is returned; on
 * failure, -1 is returned and ERRNO is set by the underlying system
 * or library call that failed.
 * WARNING: 'slurp' malloc()s memory to '*buf' which must be freed by
 * the caller.
long slurp(char const* path, char **buf, bool add_nul)
    FILE  *fp;
    size_t fsz;
    long   off_end;
    int    rc;

    /* Open the file */
    fp = fopen(path, "r");
    if( NULL == fp ) {
        return -1L;

    /* Seek to the end of the file */
    rc = fseek(fp, 0L, SEEK_END);
    if( 0 != rc ) {
        return -1L;

    /* Byte offset to the end of the file (size) */
    if( 0 > (off_end = ftell(fp)) ) {
        return -1L;
    fsz = (size_t)off_end;

    /* Allocate a buffer to hold the whole file */
    *buf = malloc( fsz+(int)add_nul );
    if( NULL == *buf ) {
        return -1L;

    /* Rewind file pointer to start of file */

    /* Slurp file into buffer */
    if( fsz != fread(*buf, 1, fsz, fp) ) {
        return -1L;

    /* Close the file */
    if( EOF == fclose(fp) ) {
        return -1L;

    if( add_nul ) {
        /* Make sure the buffer is NUL-terminated, just in case */
        *buf[fsz] = '\0';

    /* Return the file size */
    return (long)fsz;

 * Demonstrates a call to 'slurp'.
int main(void) {
    long  file_size;
    char *buf;

    file_size = slurp("./test_file.json", &buf, false);
    if( file_size < 0L ) {
        perror("File read failed");
        return 1;

    (void)fwrite(buf, 1, file_size, stdout);

    /* Remember to free() memory allocated by slurp() */
    free( buf );
    return 0;
 $ gcc -o read_file_to_buffer read_file_to_buffer.c 
 $ ./read_file_to_buffer 

Reference :

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