char big_buffer[BIG_SIZE];
char small_buffer[LINE_SIZE];
unsigned used = 0;

big_buffer[0] = '\0'; // initialize the big buffer to an empty string

// read a line data from the child program
while (fgets(small_buffer,LINE_SIZE,your_pipe)) {
    // check that it'll fit:
    size_t len = strlen(small_buffer);
    if (used + len >= BIG_SIZE)
        break;

    // and add it to the big buffer if it fits
    strcat(big_buffer,small_buffer);
    used += strlen(small_buffer);
}

如果您想要更精细,可以动态分配空间,并尝试根据需要增加空间以保持您获得的输出量.这将是一条更好的路线,除非你至少知道孩子可能产生多少输出.

编辑：鉴于您使用的是C,动态大小的结果实际上非常简单：

char line[line_size];
std::string result;

while (fgets(line,line_size,your_pipe))
     result += line;