Skip to content

Formatted I/O, Line Input, Scanf - C

read a file from stdin and put it to both stdout & stderr

int main (int argc, char *argv[]){
    char *fname = NULL;
    if (argc>1){
        return 1;
    FILE *file = fopen(fname, "r");

    int input;
    while((input=fgetc(file))!= EOF){

If reading a binary file, it is better to add "rb" flag for windows OS.

end of file ctrl+D

end program ctrl_C

stderr line buffer

stdout file buffer(???)


void rewind(FILE *stream);

changing location of a file to the beginning

fseek: move to some location

getting location

long ftell(FILE * stream);
off_t ftello(FILE *stream);

if is larger than a long in size, then:

int fgetpos(FILE *restrict stream, fpos_t *restrict pos);
int fsetpos(FILE *stream, const fpos_t *pos);

need to check man page for different systems

df: space of hard drive

file: blocks of pointers point to a chunk of data blocks of actual disk

Format Printing

printing to a string

int sprintf(char *str, const char *format, /* args */ );
  • return number of bytes that would have been written
  • writes at most size-1 bytes (leave space for null byte)

line based input

char *fgets(char *str, int size, FILE *stream);
  • stores in str
  • stops at -n, EOF
  • do not use gets


  • read line using fgets(), parses data using sscanf() from line
  • character ranges(check textbook)


void * realloc(void *ptr, size_t size);
  • change previous allocated block ptr be changed to size size(larger or smaller & copy old data as needed)
  • app: connect strings


ssize_t getline(char ** linep, size_t *linecapp, FILE * stream);
  • dynamically allocated, but must free later
  • not part of c standard

Standard IO buffering

  • unbuffered such as stderr
  • line-buffered such as stdout
  • block-buffered such as files

Could use fflush(fp) to flush the buffer to file ptr fp.

Could use setbuf(fp, NULL) to turn buffering off.