C program to read the contents of a file into character buffer

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 &gt; (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 */
    rewind(fp);
 
    /* Slurp file into buffer */
    if( fsz != fread(*buf, 1, fsz, fp) ) {
        free(*buf);
        return -1L;
    }
 
    /* Close the file */
    if( EOF == fclose(fp) ) {
        free(*buf);
        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", &amp;buf, false);
    if( file_size &lt; 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 : https://stackoverflow.com/questions/3747086/reading-the-whole-text-file-into-a-char-array-in-c