<stdio.h>

<stdio.h> provides buffered file and console I/O: printf, scanf, fopen, fclose, and stream operations. Use snprintf instead of sprintf to avoid buffer overflows; always check return values.

Compile with -lm if using math functions; I/O functions link with the C library automatically.

This example opens a file, writes formatted data, seeks, and reads it back.

// compile: gcc -o studioexample studioexample.c
// run: ./studioexample
// description: write to file, seek to start, read back line by line
 
#include <stdio.h>
 
int main() {
    FILE* f = fopen("/tmp/stdio.txt", "w+");
    if (!f) { perror("fopen"); return 1; }
 
    fprintf(f, "line 1\n");
    fprintf(f, "line 2\n");
    fprintf(f, "line 3\n");
 
    rewind(f);
 
    char buf[64];
    while (fgets(buf, sizeof(buf), f))
        printf("read: %s", buf);
 
    fclose(f);
    return 0;
}

File operations:

• fopen(path, mode): open file (“r”, “w”, “a”, “r+”, “rb”, “wb”, etc.)
• fclose(file): close file (flushes and releases)
• fflush(file): flush output buffer without closing
• rewind(file): seek to beginning
• fseek(file, offset, whence): seek to position (SEEK_SET, SEEK_CUR, SEEK_END)
• ftell(file): current file position
• remove(path): delete file
• rename(old, new): rename file

Formatted output (to stdout, file, or buffer):

• printf(fmt, ...): formatted output to stdout
• fprintf(file, fmt, ...): formatted output to file
• sprintf(buf, fmt, ...): formatted output to buffer (unsafe, use snprintf)
• snprintf(buf, size, fmt, ...): formatted output to buffer (safe, no overflow)
• vprintf(fmt, args), vfprintf(f, fmt, args), vsnprintf(buf, size, fmt, args): variadic versions

Formatted input (from stdin or file):

• scanf(fmt, ...): formatted input from stdin
• fscanf(file, fmt, ...): formatted input from file
• sscanf(str, fmt, ...): formatted input from string
• vscanf(fmt, args), vfscanf(f, fmt, args), vsscanf(s, fmt, args): variadic versions

Text I/O (line-oriented):

• fgets(buf, size, file): read up to size-1 characters or newline
• fputs(str, file): write string (without null terminator)
• getchar(): read character from stdin
• putchar(c): write character to stdout
• fgetc(file): read character from file
• fputc(c, file): write character to file

Binary I/O:

• fread(buf, size, count, file): read count objects of size bytes
• fwrite(data, size, count, file): write count objects of size bytes

Error handling:

• perror(msg): print system error message to stderr
• ferror(file): check error flag
• clearerr(file): clear error flag
• eof(file): check end-of-file flag
• fseek(file, offset, whence): seek to position
• rewind(file): seek to start of file
• perror(msg): print error message with errno